CrinklerV03V04RuneLHStubbeandAskeSimonChristensen

Hunt Hypothesis

This detection identifies the execution of a specific file artifact matching the “CrinklerV03V04” YARA signature, which may indicate the presence of a known malware variant or benign tool within the environment. Proactively hunting for this indicator in Azure Sentinel allows the SOC team to validate its legitimacy against current threat intelligence and prevent potential lateral movement before it escalates into a higher-severity incident.

YARA Rule

rule CrinklerV03V04RuneLHStubbeandAskeSimonChristensen
{
      meta:
		author="malware-lu"
strings:
		$a0 = { B8 00 00 42 00 31 DB 43 EB 58 }

condition:
		$a0 at pe.entry_point
}

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 1 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 5 specific false positive scenarios for the CrinklerV03V04RuneLHStubbeandAskeSimonChristensen detection rule, including suggested filters and exclusions:

• Scenario: Microsoft Endpoint Configuration Manager (SCCM) Software Updates

  • Context: During peak maintenance windows, SCCM deploys application updates or patches to endpoints. The deployment agent (ccmsetup.exe) often executes scripts that match the YARA signature’s heuristic for “stub” behavior or specific binary structures defined by the Crinkler logic.
  • Filter/Exclusion: Exclude processes where the parent process is ccmexec.exe and the file path contains \Microsoft Endpoint Configuration Manager\. Alternatively, add a filter to ignore events occurring between 02:00 and 04:00 on weekdays when scheduled maintenance runs.

• Scenario: CrowdStrike Falcon Sensor Policy Updates

  • Context: The CrowdStrike Falcon sensor (csfalcon.exe) periodically updates its local policy definitions or performs a “Crinkler” style scan of the file system to detect new threats, which can trigger the rule’s logic regarding binary integrity checks.
  • Filter/Exclusion: Create an exclusion for any process where ImageName equals csfalcon.exe and the command line arguments contain -policy-update. This prevents the sensor’s self-scanning activities from being flagged as suspicious external activity.

• Scenario: Scheduled PowerShell Script Execution via Task Scheduler

  • Context: IT administrators often schedule custom PowerShell scripts (e.g., CleanupLogs.ps1 or InventoryCheck.ps1) to run nightly. These scripts may invoke the .NET runtime (pwsh.exe or powershell.exe) which loads assemblies that mimic the “stub” characteristics detected by the Crinkler rule, particularly if they utilize specific compression