PECrypt102

Hunt Hypothesis

This rule detects potentially obfuscated or packed executable files using YARA signatures, indicating an adversary may be employing fileless or in-memory techniques to evade static analysis. Proactively hunting for these patterns in Azure Sentinel helps identify stealthy malware or custom loaders that traditional signature-based detections might miss, reducing the risk of undetected persistence or lateral movement.

YARA Rule

rule PECrypt102
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E8 00 00 00 00 5B 83 EB 05 EB 04 52 4E 44 21 85 C0 73 02 F7 }

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 legitimate open-source encryption utility (such as gpg or openssl) to encrypt configuration files or secrets before committing them to a repository or transferring them to a staging environment. The YARA rule may match the specific byte patterns or header structures of these standard encrypted containers.

  • Filter/Exclusion: Exclude processes where the parent executable is gpg.exe, openssl.exe, or python.exe (if using Python cryptography libraries) and the file extension is .gpg, .enc, or .pem. Additionally, exclude paths under C:\Users\Public\ or specific CI/CD workspace directories like C:\Jenkins\workspace\.

• Scenario: An enterprise backup solution (e.g., Veeam, Commvault, or Windows Server Backup) creates encrypted backup blocks or snapshots. The YARA rule might trigger on the encrypted data chunks or the specific container format used by these backup agents during the write phase.

  • Filter/Exclusion: Exclude processes where the parent executable is vssadmin.exe, wbadmin.exe, or specific backup agent binaries (e.g., veeamagent.exe, commvaultagent.exe). Also, exclude file paths containing keywords like \Backup\, \Veeam\, or \Commvault\ in the directory structure.

• Scenario: A security team runs a scheduled vulnerability scan or integrity check using tools like clamscan or malwarebytes in “scan only” mode. These tools may temporarily create encrypted or packed temporary files for analysis, or the YARA rule might match the internal structure of the scanner’s own signature database files if they are loaded into memory or written to disk during an update.

  • Filter/Exclusion: Exclude processes where the parent executable is clamscan.exe, mbam.exe, or `