ThreatScript Definitions¶

ThreatScript Definitions are written in ECMAScript (basically the same as JavaScript). The ThreatScript definitions return a Result object which indicates whether a match was observed.

ThreatScript Example¶

Below is an example of a ThreatScript that triggers if the web-page has a form element.

/*
 * Name: Example.General.Has_Form_Tag
 * Version: 1
 * ID: 1000000
 * Message: Indicates if the page has as a form tag
 * Severity: Low
 */

importPackage(Packages.ThreatScript);
importPackage(Packages.HTTP);

function analyze( httpResponse, variables, environment ){

    var parser = httpResponse.getDocumentParser();
    var location = new URL( httpResponse.getLocation() );

    //Get a list of all script tags
    var tagNameFilter = new TagNameFilter("form");
    var nodesList = parser.extractAllNodesThatMatch(tagNameFilter); 
        if( nodesList.size() > 0 ){
         return new Result( true, "A form was detected" );
    }

    return new Result( false, "No forms detected" );
}

Analysis Function¶

ThreatScripts must provide an analyze function that takes 5 arguments:

Name	Type	Note
httpResponse	HttpResponseData	See source:trunk/src/net/lukemurphey/nsia/scan/HttpResponseData.java
variables	Variables	See source:trunk/src/net/lukemurphey/nsia/scan/scriptenvironment/Variables.java
environment	Environment	See source:trunk/src/net/lukemurphey/nsia/scan/ScriptDefinition.java#L605

Baseline Function¶

ThreatScripts may declare a baseline function that will allow the definition to be configured to baseline itself against the previous set of scan results. The baseline function is called by NSIA when a user presses the baseline method for a rule. The objective of the baseline function is to view the provided scan results and ignore the particular finding for the given resource in the future. For example, a definition that triggers when the hash of the web-page changes may define a baseline function that causes it to not trigger unless the web-page hashes changes to yet another value.

Below is an example:

function baseline( environment ){
    var previousValue = environment.get("LastObservedHash");

    if( previousValue != null && previousValue.getValue() != null ){
        environment.set("Hash", previousValue.getValue() );
    }

    return true;
}

Meta-Data¶

ThreatScripts must provide a meta-data that indicates the following information:

Name	Valid Input	Notes
Name	<category>.<sub_category>.<definition_name>
Version	integer	Should be incremented each time the definition is updated
ID	integer	Must be 1000000 or greater (only official definitions can be less than 1000000)
Message	message to be displayed when definition matches
Severity	Either: Low, Medium or High
Invasive	Either: True or False (this argument is optional)

This meta-data is provided in a comment as name-value pairs (see above ThreatScript example).

Definition Name¶

Definitions must have a name that is composed of three parts: a category, sub-category, and name. The part are separated by periods (like "category.subcategory.name"). The category and sub-category are important because the user can create exceptions and disable definitions by category or sub-category. Therefore, it is important that you use category names that makes sense; otherwise, the user may disable or enable your definition inadvertently.

In some cases a definition may be created for a specific site-group or rule. It is recommended that the definition be named such it is clear it is site-group or rule specific. Oftentimes, this done by appending an underscore to the category name (e.g. "_Acme.InformationLeak.ConfidentialDocumentFound").

Definition Severity¶

Definition severities are defined per the below table:

Severity	Definition
Low	An issue that may represent a problem
Medium	Reasonably likely of being an issue
High	High likelihood of being a problem and/or are an issue with substantial impact

Definition References¶

References can be defined to point users to additional information. The references are formatted with the prefix before the argument that is used to create the argument (e.g. "bid,39545" creates a link to BugTraq writeup for issue 39545).

Generic URLs can be created with the url prefix (e.g. "url,google.com" creates a link to Google.com).

The following types of references exist:

Name	Prefix	Links To
BuqTraq	bugtraq	http://www.securityfocus.com/bid
CVE	cve	http://cve.mitre.org/cve/
Nessus	nessus	http://www.nessus.org/plugins/index.php?view=all
Arachnids	arachnids	(URL is now defunct, don't use this anymore)
McAfee	mcafee	http://vil.nai.com/vil/default.aspx
URL	url
OSVDB	osvdb	http://osvdb.org/show/osvdb
USN	usn	http://www.ubuntu.com/usn/
Milw0rm	milworm	http://milw0rm.com/exploits
Secunia	secunia	http://secunia.com/advisories/
RHSA	rhsa	http://rhn.redhat.com/errata/
Microsoft KB	microsoft_kb	http://support.microsoft.com/kb/
Microsoft Security Bulletin	microsoft_bulletin	http://www.microsoft.com/technet/security/bulletin/

Available Packages¶

A series of packages are available to ThreatScripts in order to perform analysis.

Package	Class	Description
HTTP	URL	Same as java.net.URL
	TagNameFilter	See http://htmlparser.sourceforge.net/javadoc/org/htmlparser/filters/TagNameFilter.html
<default>	StringUtils	Provides a trim function for Strings, see source:/trunk/src/net/lukemurphey/nsia/scan/scriptenvironment/StringUtils.java
	Debug	Provides method that allows scripts to create log messages, see source:/trunk/src/net/lukemurphey/nsia/scan/scriptenvironment/Debug.java
ThreatScript	Result	Indicates the results of analysis, see source:/trunk/src/net/lukemurphey/nsia/scan/scriptenvironment.Result.java
	DataAnalysis	Provides functions useful for analysis, see source:/trunk/src/net/lukemurphey/nsia/scan/ScriptSignatureUtils.java

Debugging ThreatScripts¶

ThreatScripts can create event log messages by using the sendMessage() function in the Debug class. Simply call Debug.sendMessage with a string as an argument to create an event log message. The event log messages can be viewed in the event log for NSIA.

Generally, script created log messages are used only for debugging and should be disabled on rules you want to use in production.

General Notes When Writing Definitions¶

ThreatScript Maximum Runtime¶

ThreatScript definitions are forceably terminated by the scan engine if the script runs for longer than 10 seconds. Thus, it is important to write definitions that can complete within the timeframe alloted; otherwise, the definition will be flagged as having an error.

Maximum Data Size¶

The scan engine only provides the first 1 MB of the data observed to the scan engine. Therefore, do not design ThreatScripts that won't work if only the first 1 MB of a larger file is provided.