Crunchv5BitArts

Hunt Hypothesis

This rule detects the presence of the Crunchv5BitArts malware variant, which is often used for initial access or payload delivery in targeted attacks. Proactively hunting for this signature allows the SOC team to identify compromised endpoints early, preventing potential lateral movement or data exfiltration within the Azure Sentinel environment.

YARA Rule

rule Crunchv5BitArts
{
      meta:
		author="malware-lu"
strings:
		$a0 = { EB 15 03 00 00 00 06 00 00 00 00 00 00 00 00 00 00 00 68 00 00 00 00 55 E8 00 00 00 00 5D 81 ED 1D 00 00 00 8B C5 55 60 9C 2B 85 FC 07 00 00 89 85 E8 07 00 00 FF 74 24 2C E8 20 02 00 00 0F 82 94 06 00 00 E8 F3 04 00 00 49 0F 88 88 06 00 00 8B B5 E8 07 00 }

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

• Scenario: A developer or DevOps engineer uses a custom Python or C# script to process large batches of image assets (e.g., resizing, watermarking, or converting formats) for a marketing campaign or UI/UX update. The script may load the Crunch library or use specific bit-manipulation functions that match the YARA signature, especially if the script is compiled into a binary or executed via a wrapper.

  • Filter/Exclusion: Exclude processes where the parent process is a known development tool (e.g., python.exe, node.exe, dotnet.exe) or where the command line contains keywords like --resize, --convert, or image-process. Additionally, exclude files located in specific development directories (e.g., C:\Projects\, C:\Users\<user>\src\).

• Scenario: An automated scheduled task runs a legacy image processing utility (e.g., ImageMagick, GIMP, or a custom internal tool) to generate thumbnails for a content management system (CMS) or digital asset management (DAM) platform. The utility may use specific bit-art algorithms for compression or format conversion that trigger the rule.

  • Filter/Exclusion: Exclude processes initiated by Task Scheduler (svchost.exe with TasksSched service) or specific service accounts (e.g., svc-cms, svc-dam). Filter by process names known to be image processors (e.g., magick.exe, gimp-2.10.exe, thumbgen.exe) and exclude paths under C:\Program Files\ImageMagick\ or C:\Tools\.

• Scenario: A QA or testing team runs a performance benchmarking script that repeatedly generates and processes synthetic images to test the efficiency of the graphics pipeline or GPU drivers. This high-frequency, repetitive bit-manipulation