Crimson

Hunt Hypothesis

This hypothesis posits that adversaries are utilizing specific code patterns identified by the “Crimson” YARA signature to establish persistence or execute initial reconnaissance within the environment. The SOC team should proactively hunt for these indicators in Azure Sentinel to validate the rule’s efficacy and uncover potential low-severity threats that may have been overlooked by standard alerting thresholds.

YARA Rule

rule Crimson: RAT
{
	meta:
		author = " Kevin Breen <[email protected]>"
		Description = "Crimson Rat"
		date = "2015/05"
		ref = "http://malwareconfig.com/stats/Crimson"
		maltype = "Remote Access Trojan"
		filetype = "jar"

	strings:
		$a1 = "com/crimson/PK"
		$a2 = "com/crimson/bootstrapJar/PK"
		$a3 = "com/crimson/permaJarMulti/PermaJarReporter$1.classPK"
		$a4 = "com/crimson/universal/containers/KeyloggerLog.classPK"
        $a5 = "com/crimson/universal/UploadTransfer.classPK"
        
	condition:
        all of ($a*)
}

Deployment Notes

This YARA rule can be deployed in the following contexts:

• Microsoft Defender for Endpoint — Custom indicators / advanced hunting
• Email Gateway — Attachment scanning
• File Share Monitoring — Periodic scanning of shared drives
• YARA CLI — Manual threat hunting on endpoints

This rule contains 5 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 4 specific false positive scenarios for the Crimson detection rule in an enterprise environment, along with suggested filters or exclusions:

• Endpoint Protection Scanning by CrowdStrike Falcon Sensor

  • Scenario: The csagent.exe (CrowdStrike) process frequently scans and opens executable files within the %TEMP% directory to check for malware signatures. This activity mimics the file creation and execution patterns targeted by the Crimson rule, often triggering alerts when legitimate user downloads are scanned immediately upon arrival.
  • Filter/Exclusion: Exclude processes where ProcessName = "csagent.exe" AND CommandLine contains arguments related to scan or sensor_update, specifically restricting the scope to paths under C:\Windows\Temp and %LOCALAPPDATA%\Temp.

• Scheduled Patch Deployment via Microsoft SCCM (MECM)

  • Scenario: During the nightly maintenance window, the System Center Configuration Manager client (ccmexec.exe) executes PowerShell scripts to deploy Windows updates. These scripts often generate temporary .exe or .msi files in C:\Windows\CCMCache and execute them with elevated privileges, which can be flagged as suspicious execution behavior by Crimson.
  • Filter/Exclusion: Create an exclusion for the parent process ccmexec.exe running under the SYSTEM account, limited to the time window of 02:00–04:00 local time and file paths matching C:\Windows\CCMCache\*.

• Automated Backup Operations by Veeam Agent

  • Scenario: The Veeam Backup & Replication service (VeeamAgent.exe) performs incremental backups of critical databases. This involves reading large binary files and occasionally spawning temporary helper processes to compress data blocks, which triggers the rule’s logic regarding unexpected process spawning from backup