FreeCryptor02build002GlOFF

Hunt Hypothesis

This YARA rule targets the specific FreeCryptor02 build 002 GlOFF variant, a known ransomware strain that encrypts files and appends a distinctive extension to facilitate ransom demands. Proactively hunting for this signature in Azure Sentinel allows the SOC to identify compromised endpoints or storage blobs early, preventing lateral movement and data exfiltration before the encryption process completes.

YARA Rule

rule FreeCryptor02build002GlOFF
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 33 D2 90 1E 68 1B [3] 0F A0 1F 8B 02 90 50 54 8F 02 90 90 8E 64 24 08 FF E2 58 50 33 D2 52 83 F8 01 9B 40 8A 10 89 14 24 90 D9 04 24 90 D9 FA D9 5C 24 FC 8B 5C 24 FC 81 F3 C2 FC 1D 1C 75 E3 74 01 62 FF D0 90 5A 33 C0 8B 54 24 08 90 64 8F 00 90 83 C2 08 52 5C 5A }

condition:
		$a0
}

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 IT admin manually runs the FreeCryptor executable from a shared network drive or local temp folder to encrypt/decrypt a large batch of legacy data files for migration purposes, without moving it to a standard application directory.
  • Filter/Exclusion: Exclude alerts where the process path contains \Temp\, \Downloads\, or \Shared\ AND the parent process is explorer.exe or cmd.exe (indicating manual user action rather than automated execution).
• Scenario: A CI/CD pipeline or build agent executes FreeCryptor as part of a post-build step to compress or obfuscate artifacts before deployment, running from a specific build workspace directory (e.g., C:\jenkins\workspace\project-name\).
  • Filter/Exclusion: Exclude alerts where the process path matches the known CI/CD workspace patterns (e.g., *\jenkins\workspace\*, *\azure-pipelines\*, *\gitlab-runner\*) and the parent process is a known build agent service (e.g., jenkins.exe, agent.exe, gitlab-runner.exe).
• Scenario: A scheduled task or service (e.g., a custom backup script) invokes FreeCryptor to encrypt daily log files or database dumps, running from a service-specific directory like C:\Program Files\MyBackupTool\bin\.
  • Filter/Exclusion: Exclude alerts where the process path is under C:\Program Files\ or C:\Program Files (x86)\ AND the parent process is svchost.exe (indicating service execution) or a known backup tool executable (e.g., vssadmin.exe, wbadmin.exe, backup.exe).
• Scenario: A security team or blue team performs a controlled test