Windows: XSL Script Processing

Rule ID

PH_Rule_SIGMA_635

Default Status

Enabled

Description

Extensible Stylesheet Language (XSL) files are commonly used to describe the processing and rendering of data within XML files. Rule detects when adversaries abuse this functionality to execute arbitrary files while potentially bypassing application whitelisting defenses. This rule is adapted from https://github.com/SigmaHQ/sigma/blob/master/rules/windows/process_creation/proc_creation_win_wmic_xsl_script_processing.yml

Severity

MITRE ATT&CK® Tactics

Defense Evasion

Defense Evasion consists of techniques that adversaries use to avoid detection throughout their compromise. Techniques used for defense evasion include uninstalling/disabling security software or obfuscating/encrypting data and scripts. Adversaries also leverage and abuse trusted processes to hide and masquerade their malware. Other tactics’ techniques are cross-listed here when those techniques include the added benefit of subverting defenses.

https://attack.mitre.org/tactics/TA0005

MITRE ATT&CK® Techniques

T1220

XSL Script Processing

Adversaries may bypass application control and obscure execution of code by embedding scripts inside XSL files. Extensible Stylesheet Language (XSL) files are commonly used to describe the processing and rendering of data within XML files. To support complex operations, the XSL standard includes support for embedded scripting in various languages. Adversaries may abuse this functionality to execute arbitrary files while potentially bypassing application control.

https://attack.mitre.org/techniques/T1220

Impacts

Server

Data Source

Windows Sysmon via FortiSIEM Agent

Detection

Correlation

Remediation Guidance

No remediation guidance specified

Time Window

If the following pattern or patterns match an ingested event within the given time window in seconds, trigger an incident.

300 seconds

Trigger Conditions

If the following defined pattern/s occur within a 300 second time window.

Filter

SubPattern Definitions

SubPattern Name: Filter

This is the named definition of the event query, this is important if multiple subpatterns are defined to distinguish them.

SubPattern Query

This is the query logic that matches incoming events

eventType="Win-Sysmon-1-Create-Process" AND (((procName REGEXP "\\wmic\.exe$" AND command REGEXP ".*-format.*|.*/format.*") AND (command NOT REGEXP ".*Format:List.*|.*Format:csv.*|.*Format:hform.*|.*Format:htable.*|.*Format:mof.*|.*Format:rawxml.*|.*Format:table.*|.*Format:value.*|.*Format:xml.*")) OR procName REGEXP "\\msxsl\.exe$")

Group by Attributes

This defines how matching events are aggregated, only events with the same matching attribute values are grouped into one unique incident ID

command,hostName,procName

Aggregate Constraint

This is most typically a numerical constraint that defines when the rule should trigger an incident

COUNT(*) >= 1

Incident Attribute Mapping

This section defines which fields in matching raw events should be mapped to the incident attributes in the resulting incident.

The available raw event attributes to map are limited to the group by attributes and the aggregate event constraint fields for each subpattern

command = Filter.command,
hostName = Filter.hostName,
procName = Filter.procName

FortiSIEM Rules