Tài liệu Snort rules application docx

Analyze rule options next In this section you will learn how to analyze simple rules.. Destination defined by variable • $HOME_NET = your network alert UDP $EXTERNAL any -> $INTERNAL 313

Snort Rules: Application

Paul Ritchey,Jacob and Sundstrom, Inc

pritchey@jasi.com

V1.0.0

Welcome to the class titled ‘Snort Rules: Application’ The purpose of this class is to take the material you learned in the previous section, ‘Snort Rules: Syntax and Keywords’ This section will take those individual keywords, values and syntax to form complete rules You will also learn how

to analyze existing rules piece by piece to determine what the rule is looking for

The first half of this presentation will examine rules of increasing complexity You will be taught how to analyze an existing rule to determine what it is looking for This ability is key to

understanding how to piece together a complete rule from scratch that matches the signature of an attack

The second half of the presentation will ask you to write rules from scratch of increasing difficulty The process of creating these rules will be covered in a step by step process This will show you a possible methodology you can use when creating rules on your own

The very last section will tie together everything you have learned so far, showing you a few of the options available for Snort output

This presentation covers Snort version 1.7 If you are using a newer version of Snort, please remember that new features may have been added or existing features may have been modified after this presentation was assembled

Rule Analysis: Simple Rules

This section will show you how to analyze simple rules, step by step The analysis skills learned here will be built upon in later sections to analyze rules of increasing difficulty This will help you later when you will be required to write rules from scratch

Rule Analysis: Simple Rules

! Signature based on rule header.

set and www.whitehats.com.

! Analyze rule header first

• Determine source and destination addresses and ports

• Snort uses this section first.

! Analyze rule options next

In this section you will learn how to analyze simple rules The rules were chosen because they do not incorporate packet attributes which can make some rules difficult to analyze These are real life rules, taken directly from the rule set available from the snort.org web site and www.whitehats.com This means that it’s possible to do further research on the exploits that the rules are designed to detect to fully round out your understanding of rules

This section will start with teaching you how to analyze rules based on a logical approach The first step is to analyze the rule header This determines what hosts, ports, protocols and traffic flow must

be involved before Snort even starts to examine the rest of the rule – this allows Snort to quickly determine if it should completely analyze the rule against the options section, saving valuable time

Later sections will combine the analysis of the rule header with the options section for more

complicated rules

Further information on this particular trojan can be obtained any of the major online security web sites In depth analysis of this trojan is beyond the scope of this course.

Simple Rule #1:

Back Orifice (cont.)

! Will ‘alert’ when triggered.

! Applies only to UDP traffic.

! Source defined by variable

• $EXTERNAL = !$HOME_NET

! Destination defined by variable

• $HOME_NET = your network

alert UDP $EXTERNAL any -> $INTERNAL 31337 \

(msg: "IDS188/trojan-probe-back-orifice";)

For this simple rule, the contents of the signature is completely contained in the rule header This rule, when it is triggered, will execute the action ‘alert’ Alert means Snort will write an entry to the alert file and an entry to the logs unless they are overridden by command line options or other means

This rule only applies to UDP traffic If snort the traffic Snort is examining is from another protocol, this rule will not be tested against them

The destination address is defined as a variable, HOME_NET This variable is typically defined at the top of the rules file being used, and is set to the addresses Snort is monitoring In this particular rule, the source address is also defined as a variable, named EXTERNAL Typically, this is set to

!$HOME_NET, meaning that the source address should be outside of the network address space Snort is monitoring

The UDP packet can, however, be originating from any of the possible ports on the source host, but

Simple Rule #1:

Back Orifice (cont.)

! No packet attributes are examined.

! Only includes message.

! Low likelihood of occurrence.

! High likelihood of false-positives.

alert UDP $EXTERNAL any -> $INTERNAL 31337 \

(msg: "IDS188/trojan-probe-back-orifice";)

Examining the rule options section, it is seen that the only option being used is the message option This option provides a string that is used to tag alert and log entries, making it easier to determine what a log or alert entry represents

This rule is very simple The only thing limiting the rule down to a specific subset of UDP traffic is the destination port Since no packet attributes or options are specified, it is very likely that detects, although not often, may very well be false-positives Any traffic, such as streaming audio or video, that happens to be destined for destination port 31337 will trigger this rule Care must be taken when analyzing any available data to validate that the packet was truly a probe for Back Orifice or the master program contacting a Back Orifice client

The next simple rule we will examine is one that detects Deep Throat trojans Deep Throat is another trojan that can be accidentally installed by users who unknowingly execute attachments or download the software by accident Once installed, the trojan opens a port that allows remote hosts

to control the infected machine

Simple Rule #2:

Deep Throat (cont.)

! Examine the rule header:

! Will ‘alert’ when triggered.

! Applies only to UDP traffic.

! Source specified as ‘any’

•‘Any’ matches all possible IP addresses, including internal addresses

! Destination defined by variable

•$HOME_NET = internal network

alert udp any 2140 -> $HOME_NET 60000 \

(msg:"IDS106 - BACKDOOR SIGNATURE - \

DeepThroat 3.1 Server Active on Network";)

For this simple rule, the contents of the signature is again completely contained in the rule header This rule, when it is triggered, will execute the action ‘alert’ Alert means Snort will write an entry to the alert file and an entry to the logs unless they are overridden by command line options or other means

This rule only applies to UDP traffic If snort the traffic Snort is examining is from another protocol, this rule will not be tested against them

Now the rule deviates from the previous example Instead of specifying a variable for the source IP address, the keyword ‘any’ is specified This means that the packet can originate from any possible IP address, there are no restrictions However, the packet must originate from a specific port – 2140 The packet must be destined for the network the variable HOME_NET is set to, and to the specific port

6000 If the packet meets all of the above criteria, it will trigger the rule and will be logged to the alert file and logs with the message specified in the rule options section

I would like take a second to discuss the keyword ‘any’ that was specified for the source address Snort

is typically installed on a machine that resides in a ‘DMZ’ The DMZ sites outside of your internal network, and sees all traffic in bound from the internet to your network, or outbound from your network

to the internet It does not and should not see your internal traffic Because of this, it would have been just as effective to replace the keyword ‘any’ with !$HOME_NET

Simple Rule #2:

Deep Throat (cont.)

! Examine rule options.

! No packet attributes are examined.

! Only includes message.

! Possibility of false-positives:

! Low likelihood of occurrence.

! Likelihood of detect being a false-positives.

alert udp any 2140 -> $HOME_NET 60000 \

(msg:"IDS106 - BACKDOOR SIGNATURE - \

DeepThroat 3.1 Server Active on Network";)

Examining the rule options section, we again that this rule like the previous example is only specifying the message option This option provides a string that is used to tag alert and log entries, making it easier to determine what a log or alert entry represents

This rule is very simple The only real limiting factors are the source and destination ports Both ports are ephemeral ports, meaning they are out of the reserved range Although unlikely, it’s possible that this port combination could be used during the course of a valid connection, and because there are no other criteria for the rule false-positive detects may be made Most virus software should be capable of detecting this trojan if properly installed and used regularly This increases the chances that a detect is a false-positive so care must be taken to fully resolve any detects

Rule Analysis: Complex Rules

In this section the rules presented for analysis are a little more complicated than the previous examples Essentially they provide additional information about packets that are considered hostile beyond source and destination IPs and ports

Rule Analysis: Complex Rules

!Signature based on rule header.

!Signature also based on rule options.

set and www.whitehats.com.

!Analyze rule header first

!Analyze rule options next

• Specifies specific packet attributes

• Can increase accuracy – decrease false positives

This section concentrates on analyzing more complicated rules – those containing packet attributes

in the rule options section In these rules, the signature doesn’t just consist of the contents of the rule header It consists of the rule header and additional information specified in the rule options

This section will continue to build on the rule analysis technique that was used in the first section Interpretation of the rule option section with different kinds of packet attributes will be introduced here By adding packet attributes (such as TCP flags) to the rule options section, it’s possible to make rules more accurate, which can potentially reduce the number of false positives

The example rules used in this section are real world rules They have been taken from the rule sets available at the www.snort.org web site and from the www.whitehats.com web site

The rule we are going to examine next is one that detects the NetMetro trojan NetMetro is another trojan that when installed allows remote control of the infected machine Again, this trojan like any other can be accidentally installed by executing attachments to email messages, or downloading the trojan as it masquerades as a useful utility or game Most virus detection software should detect this trojan as long as the signatures are properly maintained.

Complex Rule #1:

NetMetro (cont.)

! Will ‘alert’ when triggered.

! Applies only to TCP traffic.

! Source defined by variable

• $EXTERNAL_NET = !$HOME_NET

! Destination defined by variable

• $HOME_NET = your network

alert tcp $EXTERNAL_NET 5031 -> $HOME_NET !53:80 \

(msg:"IDS79 - BACKDOOR SIGNATURE - NetMetro Incoming

Traffic"; flags:PA;)

This rule when triggered will alert – meaning it will create an entry in the alerts file and create a log file, unless these options are overridden by command line options It also only applies to TCP traffic that meets the criteria of the rest of the signature

The source address is specified by the variable EXTERNAL_NET In most cases

EXTERNAL_NET is set to !$HOME_NET, which means that the source address can be any IP address except the IPs belonging to your network The destination address is specified by the variable HOME_NET This variable is set to the IP addresses your sensor is to monitor Both of these variables are typically defined at the top of a rules file, but may also be set by command line options

The source port the traffic must originate from is port 5031 If the source port is anything but 5031,

it will not match the rule header information and this rule will not be triggered The destination port setting is more interesting It specifies that the destination port can be any port except ports 53 through 80, inclusive

Complex Rule #1:

NetMetro (cont.)

! No other packet attributes examined.

! Low likelihood of occurrence.

! High likelihood of being false positive.

alert tcp $EXTERNAL_NET 5031 -> $HOME_NET !53:80 \

(msg:"IDS79 - BACKDOOR SIGNATURE - NetMetro Incoming\

Traffic"; flags:PA;)

This rule is the first example of packet attributes being used in the rule options section The attribute being tested is the TCP flags setting In this case, the TCP flags PUSH and ACK must be set Other flags, such as SYN, FIN, URG and the two reserved bits must NOT be set No other packet attributes are examined beyond the TCP flag setting

For this particular rule, there is a low likelihood of false positives, although they will happen The false positives are limited because the source port must exactly match 5031, and the destination port must be outside the specified range The addition of packet attributes (in this case TCP flags) to the rule options section aids in reducing the possibility of false positives because it helps to narrow the possibility of matches somewhat

To rule out the possibility of a detect being a false positive, additional data possibly beyond what Snort provides may need to be examined For example, if an outside user telnets in to a server in your network, it’s possible this rule may be triggered The source port 5031 is an ephemeral port, meaning that is not a reserved port and available for anyone and any application to use If the port 5031 is used

by the person connecting to your telnet server, the rule will be triggered as soon as the TCP three way handshake is completed and the first packed with a payload is sent inbound to your network Telnet runs on port 23, outside the range specified by the destination port setting that specifies what ports it cannot be

The second difficult rule to be examined detects a particular tool used for scanning This particular scanner can allow an attacker to easily determine what services are available on a host Combined with the ability to determine the OS, and the hacker now has enough information to launch an effective attack.

For this scanner certain packet attributes are hard coded in the original source code This allowing

an accurate rule to be written that can easily detect scans from this software It can also allow the rule to be tuned to help eliminate false positives, increasing the accuracy

Complex Rule #2:

Myscan (cont.)

! Will ‘alert’ when triggered.

! Applies only to TCP traffic.

! Source defined by variable

• $EXTERNAL_NET = !$HOME_NET

! Destination defined by variable

• $HOME_NET = your network

alert tcp $EXTERNAL_NET 10101 -> $HOME_NET any \

(msg: "IDS439 - Scan -myscan"; ttl: >220; \

ack: 0; flags: S;)

This rule when triggered will alert – meaning it will create an entry in the alerts file and create a log file, unless these options are overridden by command line options It also only applies to TCP traffic that meets the criteria of the rest of the signature

The source address is specified by the variable EXTERNAL_NET In most cases

EXTERNAL_NET is set to !$HOME_NET, which means that the source address can be any IP address except the IPs belonging to your network The destination address is specified by the variable HOME_NET This variable is set to the IP address range your sensor is to monitor Both of these variables are typically defined at the top of a rules file, but may also be set by command line options

The source port the traffic must originate from is port 10101 If the source port is anything but

10101, it will not match the rule header information and this rule will not be triggered The

destination port can be anything, specified by the keyword ‘any’ This means the rule does not care what port is used on the destination host

Complex Rule #2:

Myscan (cont.)

! Examine the rule options:

! Time to live value must be greater than 220.

! Acknowledgement number must be zero (0).

! TCP flag SYN must be set.

! Likelihood of false positives:

! Low likelihood of occurrence.

! Low likelihood of being false positive.

alert tcp $EXTERNAL_NET 10101 -> $HOME_NET any \

(msg: "IDS439 - Scan -myscan"; ttl: >220; \

For this rule, there is a low likelihood that the rule will be triggered, but a very high likelihood that if

it is triggered that it is NOT a false positive There are many key items that lead to this conclusion and show that this rule is a very well written one The next slide will show you the individual parts that combined together make this happen

! Cannot normally be set to zero (0).

! Rule vulnerable to mutations.

alert tcp $EXTERNAL_NET 10101 -> $HOME_NET any \

(msg: "IDS439 - Scan -myscan"; ttl: >220; \

ack: 0; flags: S;)

The first item that helps tune this rule is the specification of a specific port for the source port By specifying a specific value, only source addresses using that specific port might cause a trigger Because the source port is such a high number, it is very unlikely – but possible, that source port will be used Ephemeral ports, meaning the non-reserved ports, start at 1024 and go up They are typically used in sequence, so for a source address to reach 10,101, it must have made many connections to other machines

The second item that helps tune this rule is the time to live value Most operating systems specify a value much less than 220 when the packet is created Only the Solaris 2.x operating system sets the time to live attribute to a value greater than 220 All other operating systems use values much less than 220

The last item that contributes to the rule’s tuning is the acknowledgement attribute value The rule specifies that this attribute must be set to the value zero (0) Under normal conditions, the

acknowledgement number can never be zero Only in a crafted packet will this value ever be used

All of the above combine to make this a finely tuned rule that will not false positive very often However it does depend on the above settings in the crafted packet not to be changed This makes it vulnerable to mutations of the scanning utility The source code for this utility is freely available, and

Rule Analysis: Advanced Rules

This section provides analysis of advanced rules – those using more sophisticated packet attributes to examine the packet’s payload These rules are the most difficult to write because they require close analysis of an attack’s signature and of the source code of the attack application if available

These types of rules also have the lowest likelihood of false positives because of the completeness of the examination of the packets They are also the easiest to avoid triggering by making slight alterations in the application’s source code

Rule Analysis: Advanced Rules

!Signature based on rule header.

!Signature also based on rule options.

set and www.whitehats.com.

!Analyze rule header first

!Analyze rule options next

• Specifies specific packet attributes

• Can increase accuracy – decrease false positives

This section concentrates on analyzing more complicated rules – those containing packet attributes

in the rule options section In these rules, the signature doesn’t just consist of the contents of the rule header It consists of the rule header and additional information specified in the rule options

This section will continue to build on the rule analysis technique that was used in the first section Interpretation of the rule option section with different kinds of packet attributes will be introduced here By adding packet attributes (such as TCP flags) to the rule options section, it’s possible to make rules more accurate, which can potentially reduce the number of false positives

The example rules used in this section are real world rules They have been taken from the rule sets available at the www.snort.org web site and from the www.whitehats.com web site

Advanced Rule #1:

Wu-FTP Exploit

! Exploits a bug in wu-ftp daemon.

! Allows instant root access.

! alert tcp $EXTERNAL_NET any -> $HOME_NET 21 \(msg: "IDS458 - FTP wuftp260-tf8"; flags: PA; \content: "|31C0 31DB 31C9 B046 CD80 31C0 31DB \

4389 D941 B03F CD80|";)

The first advanced rule we will examine is one that exploits a bug in an ftp daemon provided by www.wu-ftpd.org that is used as a replacement for many native ftp daemons on some flavors of Unix, as well as coming native in many Linux distributions In this case the exploit is known as the wuftp2600.c exploit which was originally distributed in a broken form If the exploit is successful, the attacker is instantly granted root access on a high numbered port that is opened up

Advanced Rule #1:

Wu-FTP Exploit (cont.)

! Will ‘alert’ when triggered.

! Applies only to TCP traffic.

! Source defined by variable

• $EXTERNAL_NET = !$HOME_NET

! Destination defined by variable

• $HOME_NET = your network

(msg: "IDS458 - FTP wuftp260-tf8"; flags: PA; \

content: "|31C0 31DB 31C9 B046 CD80 31C0 31DB \

43 89D941 B03F CD80|";)

This rule when triggered will alert – meaning it will create an entry in the alerts file and create a log file, unless these options are overridden by command line options It also only applies to TCP traffic that meets the criteria of the rest of the signature

The source address is specified by the variable EXTERNAL_NET In most cases

EXTERNAL_NET is set to !$HOME_NET, which means that the source address can be any IP address except the IPs belonging to your network The destination address is specified by the variable HOME_NET This variable is set to the IP addresses your sensor is to monitor Both of these variables are typically defined at the top of a rules file, but may also be set by command line options

The source port is set to the keyword ‘any’, meaning that the TCP packet can originate from any possible port on the source host However, the packet must be destined for port 21 Port 21 is a well known reserved port that is used to provide FTP services

Advanced Rule #1:

Wu-FTP Exploit (cont.)

! Examines payload for specific values.

! Low likelihood of occurrence.

! Low likelihood of being false positive.

alert tcp $EXTERNAL_NET any -> $HOME_NET 21 \

(msg: "IDS458 - FTP wuftp260-tf8"; flags: PA; \

content: "|31C0 31DB 31C9 B046 CD80 31C0 31DB \

4389 D941 B03F CD80|";)

For this rule, two packet attributes are examined in order to detect the exploit The first attribute is the TCP flag settings For this rule, the PUSH and ACK TCP flags must be set The second attribute specified examines the packet’s payload The examination of a packets payload is triggered by specifying the keyword ‘content’ In this example the content that is being searched for is given in hex values, which is denoted by the enclosing pipe (‘|’) symbols

This rule is tuned ever so slightly by the TCP flags attribute Only packets with a payload should be applied against this rule These packets will have the PUSH flag set indicating that data is being sent It’s possible to have other flag settings without the PUSH flag set and still have a payload, however these are typically other exploits which this rule doesn’t apply to and should have a

different rule written to detect them The exploit detected here works by initiating a proper TCP connection, more specifically an anonymous FTP session and initiating a buffer overflow

For this rule, detects will very rarely occur primarily because of the very specific content that is being searched for When detects do occur, it’s very likely that it is a positive detect The content value, although possible, is very unlikely to occur during a normal FTP session, hence this rule’s

! alert tcp $EXTERNAL_NET any -> $HOME_NET 80 \

(msg:"IDS265 - Web cgi cgitest"; \

content:"cgitest.exe|0d0a|user"; nocase; flags: AP; \

offset:4;)

The second advanced rule we will examine is a web based exploit The ‘cgitest.exe’ is a CGI that if

it is left installed on a particular web server can allow a remote attacker to execute arbitrary code on the web server The exploit works because of a buffer overflow vulnerability, which is one of the more lethal types of attacks an attacker can use The web daemon affected by this vulnerability runs

on Windows 95, which limits the possible ramifications of a successful attack that might exist on a Unix or Windows NT machine

Advanced Rule #2:

cgitest.exe Exploit (cont.)

! Will ‘alert’ when triggered.

! Applies only to TCP traffic.

! Source defined by variable

• $EXTERNAL_NET = !$HOME_NET

! Destination defined by variable

• $HOME_NET = your network

(msg:"IDS265 - Web cgi cgitest"; \

content: "cgitest.exe|0d0a|user"; nocase; \

flags: AP; offset:4;)

This rule when triggered will alert – meaning it will create an entry in the alerts file and create a log file, unless these options are overridden by command line options It also only applies to TCP traffic that meets the criteria of the rest of the signature

The source address is specified by the variable EXTERNAL_NET In most cases

EXTERNAL_NET is set to !$HOME_NET, which means that the source address can be any IP address except the IPs belonging to your network The destination address is specified by the variable HOME_NET This variable is set to the IP addresses your sensor is to monitor Both of these variables are typically defined at the top of a rules file, but may also be set by command line options

The source port is set to the keyword ‘any’, meaning that the TCP packet can originate from any possible port on the source host However, the packet must be destined for port 80 Port 80 is one of the most common ports used for web daemons If there are web daemons used on your network using alternative ports, the rule should be duplicated for each of the ports being used

Advanced Rule #2:

cgitest.exe Exploit (cont.)

! Examines payload for specific values.

! Low likelihood of occurrence.

! Low likelihood of being false positive.

alert tcp $EXTERNAL_NET any -> $HOME_NET 80 \

content: "cgitest.exe|0d0a|user"; nocase; \ flags: AP; offset:4;)

For this rule, two packet attributes are examined in order to detect the exploit The first attribute is the TCP flag settings For this rule, the PUSH and ACK TCP flags must be set The second attribute specified examines the packet’s payload The examination of a packets payload is triggered by specifying the keyword ‘content’ In this example the content that is being searched for is a

combination of two sections of ASCII data and one section of hex values This example shows how ASCII and hex values can be combined to form a payload signature, and can be interspersed between each other Note the use of the ‘nocase’ option This informs Snort that for the ASCII content being searched for, it can appear in any possible combination of upper and lower case letters possible.This rule is tuned ever so slightly by the TCP flags attribute Only packets with a payload should be applied against this rule These packets will have the PUSH flag set indicating that data is being sent It’s possible to have other flag settings without the PUSH flag set and still have a payload, however these are typically other exploits which this rule doesn’t apply to and should have a different rule written to detect them The exploit detected here works by initiating a proper TCP connection to a web server, and then executing the ‘cgitest.ext’ CGI on that server and causing a buffer overflow

The content attribute can be a very resource intensive attribute to use To help reduce the overhead of processing that must take place, it can be tuned by specifying the ‘offset’ and ‘depth’ options These options reduce the amount of a packet’s payload that must be inspected by Snort In this rule, only the

‘offset’ option is used This rule tells Snort to start examining the payload 4 bytes in, effectively ignoring the first 3 bytes This may not seem like a lot, but by ignoring 3 bytes of every packet on a very busy network can quickly add up

Writing Rules

In this section will demonstrate how to write a few rules from scratch of increasing difficulty A specification for a needed rule will be provided, followed by a possible solution Keep in mind that for some types of rules there may be several possible answers, all of which may be correct