Crunchv40

Hunt Hypothesis

This rule detects the presence of the Crunchv40 YARA signature, which typically identifies specific malware families or tooling artifacts embedded in memory or disk. Proactively hunting for this indicator allows the SOC team to identify dormant or low-noise threats that may have evaded traditional signature-based detections, ensuring early containment before lateral movement occurs.

YARA Rule

rule Crunchv40
{
      meta:
		author="malware-lu"
strings:
		$a0 = { EB 10 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 E8 00 00 00 00 5D 81 ED 18 00 00 00 8B C5 55 60 9C 2B 85 E9 06 00 00 89 85 E1 06 00 00 FF 74 24 2C E8 BB 01 00 00 0F 82 92 05 00 00 E8 F1 03 00 00 49 0F 88 86 05 00 00 68 6C D9 B2 96 33 C0 50 E8 24 }

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 Ghidra or IDA Pro to reverse-engineer a proprietary DLL or executable for debugging or interoperability purposes. These tools often embed specific version strings, compiler artifacts, or structural patterns that match the Crunchv40 heuristic.
  • Filter/Exclusion: Exclude processes whose executable path contains \ghidra\ or \ida\, or filter by process name ghidra.exe and ida.exe if the rule triggers on file metadata rather than process behavior.
• Scenario: An IT administrator runs 7-Zip (7z.exe) or WinRAR (WinRAR.exe) to extract a large archive containing mixed file types (e.g., .exe, .dll, .bin) in a shared network drive or user temp folder. The extraction process may temporarily create files with ambiguous headers or trigger YARA matches on the extracted binaries if the rule scans file content.
  • Filter/Exclusion: Exclude file paths under C:\Users\*\AppData\Local\Temp\ or C:\ProgramData\ for specific extraction tools, or add an exclusion for parent processes 7z.exe, WinRAR.exe, and WinRAR.exe if the detection is process-based.
• Scenario: A security engineer or QA team runs Frida or x64dbg to attach to a running application for dynamic analysis or memory inspection. These debuggers often inject DLLs or modify process memory in ways that can mimic the patterns targeted by Crunchv40, especially if the rule looks for specific code obfuscation or packing signatures.
  • Filter/Exclusion: Exclude processes named frida-server.exe, frida.exe, x64dbg.exe, or x32dbg.exe, and consider excluding