Definitions¶

Definition Types¶

NSIA supports two types of definitions:

• ThreatPattern
• ThreatScript

ThreatScript	ThreatPattern
Written in ECMAScript / JavaScript	Written in a format similar to Snort
Somewhat complex to create	Simple and easy to create
Can auto-baseline tune itself	Must be ignored completely when it triggers as a false positive
Is stateful (remember things from previous scans)	Is stateless (cannot remember things from previous scans)
Are slower than ThreatSignatures	Are faster than ThreatScripts
Very flexible detection logic; can be used to detect nearly anything	Functionality limited to what regular expressions

Definition IDs¶

Custom definitions must have an ID of 1000000 or more; only official definitions can have IDs of less than 1000000.

Identifying Definition Errors¶

NSIA will parse definitions before they are saved in order to identify syntax and some semantic errors. Errors that are discovered during runtime are noted on the definition errors page (e.g. http://127.0.0.1:8080/Definitions/Errors) and in the event logs (e.g. http://127.0.0.1:8080/System/Eventlog).

Note that ThreatScript definitions will be flagged as having an error if they fail to complete within 10 seconds (see ScriptDefinition.MAX_SCRIPT_RUNTIME).

Creating a ThreatPattern¶

ThreatPattern's similar in concept to the signatures used by the Snort IDS system. Below is a sample rule:

Alert("Abuse.DataHiding.Rar_File_In_Jpeg"){
        Message="A JPEG file with a RAR file appended was observed";
        Severity="Low";
        ID=194;
        Version=1;
        BasicEncoding;
        Byte="FF D9";
        String="Rar!"; Offset=0;
        ContentType="image/jpeg";
        Reference=url,lifehacker.com/software/encryption/hide-files-in-jpeg-images-207905.php;
        Reference=url,schmidt.devlib.org/file-formats/rar-archive-file-format.html;
}

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.

The following is a list of the various options:

Option	Required	Value	Example	Description
ID	Yes	<integer>	10012441
Version		<integer>	3
String		<string>	haxored	Looks for the given String value
Regex		PCRE	/apple/i	Looks for the given Regex (in PCRE format)
Bytes		Bytes	90 90 90	Looks for the given bytes
Set		<string>		Sets the given variable (allows rules to maintain state)
UnSet		<string>		Unsets the given variable (allows rules to maintain state)
IfSet		<string>		Makes the action dependent upon whether the variable exists
IfNotSet		<string>		Opposite of above
Distance		<string>		Sets a maximum depth into the data that the definition will examine (from the
Offset		<string>		Sets how much data should be skipped from the previous operator
Depth
Within		<string>
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)
Reference		<string>	url,threatfactor.com
BasicEncoding		<string>		Causes the evaluator to skip character set encoding and treat the data as if it is raw bytes (or ASCII encoded)

Creating a ThreatScript¶