ThreatFactor NSIA

h1. ThreatPattern Definitions

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ThreatPattern's similar in concept to the signatures used by the Snort IDS system.

h2. Definition Verb

The rules start with a action that indicates what the system should do with the rule. The action verbs are as follows:

|*Verb*|*Operation* |*Notes*|
|Eval |Causes the rule to be evaluated but no action taken. |Only valid when an Set option is used. Used to set a flag that may be used in another signature (allows the rules to maintain state).|
|Alert |Causes the rule to indicate that a match has been found. |Used only to determine if the site may be compromised.|
|Block |Causes the resource to be blocked (end user cannot access) |Useful when using a proxy to control access to the servers; this is not currently used yet.|

h2. Definition Name
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h2. Definition Body

The definition is composed of options that are used to make conclusions about the data provided. Below is a list of the various options:

h3. Meta-Data Options

The following options are used to provide information about the definition:

|*Option* |*Required*|*Value* |*Example* |*Description*|
| ID | Yes | <integer> | ID=10012441 | |
| Message | Yes | <string> | Message="Malicious Website Discovered" | |
| Version | | <integer> | Version=3 | |
| Reference | | <string> | url,threatfactor.com | Types of references are: url ,bugtraq, cve, nessus, arachnids, mcafee |
| Severity | | <string> | Severity="Medium" | Must be either low, medium or high and is required if the definition verb is alert or block |

h3. Data Analysis Options (Evaluators)

The following options are used to analyze the data:

|*Option* |*Required*|*Value* |*Example* |*Description*|
| String | | <string> | String=haxored | Looks for the given String value |
| Regex | | PCRE | Regex=/apple/i | Looks for the given Regex (in PCRE format) |
| Byte | | Bytes | Byte=90 90 90 | Looks for the given bytes |
| ContentType | | PCRE or <string> | ContentType=/text.*//i | Matches a given content type. The content type is inferred by the scanning engine based upon the file contents, HTTP headers and file extension. |
| URI | | <string> | URI="http://google.com" | |
| IgnoreCase | | | | Synonymous with "NoCase" |
| BasicEncoding | | <string> | | Causes the evaluator to skip character set encoding and treat the data as if it is raw bytes (or ASCII encoded) |
| IsDataAt | | <integer> | | Determines if data is at the given position (i.e. equates to false if the data stream ends before the value provided) |
| Offset | | <integer> | Offset=128 | Sets how much data should be skipped from the previous operator |
| Depth | | <integer> | Depth=1024 | Sets a maximum depth into the data that the definition will examine (relative to the beginning of the data stream)|
| Within | | <integer> | Within=512 | Sets how many characters or bytes to analyze for the previous option before giving up. This is oftentimes used to increase the performance of definitions by limiting the amount of data they have to analyze. |
| ByteTest | | operation | 4 digits >= 128 (hexadecimal) | Can accept options such as big-endian, little-endian, hexadecimal, absolute-value and can operate on both digits (character representations of a number) or bytes (integer values) |
| ByteJump | | operation | 4 bytes (big-endian, align-8) | Can accept options such as big-endian, little-endian, hexadecimal, absolute-value, octal, align-4 and align-8 and can operate on both digits (character representations of a number) or bytes (integer values)|

h3. Variable Options

The following options are used to set and read variables. Variables are values that are set by one definition and can be read by other definitions. For example, one definition could set a variable named "ActiveXFound" if an ActiveX control is found on a page and other definitions that look for specific ActiveX controls can check to determine if the variable "ActiveXFound" exist before bothering to analyze the data for a specific ActiveX control.

Below are the relevant variables:

|*Option* |*Required*|*Value* |*Example* |*Description*|
| Set | | <string> | Set="IframeFound"| Sets the given variable (allows rules to maintain state) |
| UnSet | | <string> | UnSet="IframeFound"| Unsets the given variable (allows rules to maintain state) |
| IfSet | | <string> | IfSet="ActiveX"| Makes the action dependent upon whether the variable exists |
| IfNotSet | | <string> | IfNotSet="ActiveX"| Opposite of above |
| Toggle | | <string> | Toggle="JavaScriptFound" | |

h2. How Definitions Analyze Data

A definition is considered a match when all of the evaluators match the data. For example the definition below will match on any combination of both "Foo" and "Bar" (in no particular order):

<pre>
Alert("Example.Test.FooBar"){
Message="Foo Bar Was found";
Severity="Low";
ID=1000001;
Version=1;
String="Foo";
String="Bar";
}
</pre>

Evaluators can be made relative to one another by using the Offset or Within options. The following definition will match when data contains "Bar" 4 or more characters after "Foo".

<pre>
Alert("Example.Test.FooBar"){
Message="Foo Bar Was found";
Severity="Low";
ID=1000001;
Version=1;
String="Foo";
String="Bar"; Offset=4;
}
</pre>

For performance reasons, you may want to place a limit on how many characters the definition will analyze. In the following example, we have added a limit to that causes the definition to only consider the 64 characters after "Foo" when looking for "Bar":

<pre>
Alert("Example.Test.FooBar"){
Message="Foo Bar Was found";
Severity="Low";
ID=1000001;
Version=1;
String="Foo";
String="Bar"; Offset=4; Within="64";
}
</pre>

Evaluators can be negated using the exclamation mark. The following definition matches when the data contains "Foo" but _does not_ include "Bar" within 64 characters:

<pre>
Alert("Example.Test.FooBar"){
Message="Foo Bar Was found";
Severity="Low";
ID=1000001;
Version=1;
String="Foo";
String!="Bar"; Within="64";
}
</pre>

h3. Mixed Mode Evaluators

NSIA will not allow evaluators that look for byte patterns to be used relative to evaluators looking for strings unless the BasicEncoding option is used. This is due to the fact that the number of bytes per character is highly variable depending on the charset involved; NSIA does not have a method to accurately account for this. Definitions with evaluators that look for data based on raw bytes and character are referred to as "mixed mode" evaluators and result in undefined behavior. Therefore, a definition such as the one below will be rejected:

<pre>
Alert("Example.Test.FooBar"){
Message="Foo Bar Was found";
Severity="Low";
ID=1000001;
Version=1;
Byte=46 6F 6F ;
String="Bar"; Offset=4;
}
</pre>

However, the following definition would be accepted since the evaluators are not relative to one another:

<pre>
Alert("Example.Test.FooBar"){
Message="Foo Bar Was found";
Severity="Low";
ID=1000001;
Version=1;
Byte=46 6F 6F ;
String="Bar";
}
</pre>

h3. Differences from Snort

NSIA PatternDefinitions function similarly to Snort but have some notable differences:

* NSIA does not have a distance option. Instead, the offset option should be used since it function equivalently.
* In Snort, the byte_test and byte_jump options can be specified as relative or absolute. In NSIA, you must follow the ByteJump or ByteTest operator with an Offset option that specifies whether the option is relative or absolute. Otherwise, NSIA treats it as having an absolute offset of 0 (beginning of the data).
* The Snort "rawbytes" option is "BasicEncoding" in NSIA.
* The ByteTest option in NSIA uses the following syntax as opposed to Snort's comma deliminated format: ByteTest="<number> <datatype> (<modifiers>)". Here is an example: ByteTest="2 bytes=65281(unsigned)"
* The Offset, Depth and Within options can be used with other options than just those that do string lookups. For example, these options can be used between Regex, ByteTest, ByteJump and String evaluators.

h2. Definition References
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