EXECryptorv151x

Hunt Hypothesis

This detection identifies potential file encryption activities associated with the EXECryptorv151x signature, which may indicate early-stage ransomware or data exfiltration attempts. Proactively hunting for this behavior in Azure Sentinel allows the SOC team to validate low-severity alerts before they escalate into critical incidents that could disrupt business operations.

YARA Rule

rule EXECryptorv151x
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E8 24 [3] 8B 4C 24 0C C7 01 17 ?? 01 ?? C7 81 B8 [7] 31 C0 89 41 14 89 41 18 80 A1 C1 [3] FE C3 31 C0 64 FF 30 64 89 20 CC C3 }

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 4 specific false positive scenarios for the EXECryptorv151x detection rule, including suggested filters and exclusions:

• Scenario: Microsoft Office Background Encryption Services

  • Context: The rule triggers when OneDrive.exe or Excel.exe performs background file synchronization or auto-save operations that involve temporary file locking and encryption of user documents. This is common in enterprise environments with “Files On-Demand” enabled.
  • Suggested Filter/Exclusion: Exclude processes where the parent image path contains \Microsoft Office\root\Office16\ (or Office15) AND the process name matches OneDrive.exe, Excel.exe, or Winword.exe. Additionally, exclude file paths ending in .tmp located within the user’s OneDrive sync folder.

• Scenario: Scheduled Antivirus Real-Time Scanning

  • Context: During peak business hours, enterprise endpoint protection suites like CrowdStrike Falcon or Symantec Endpoint Protection perform real-time scanning of newly created executables. The YARA rule may misinterpret the AV engine’s internal encryption of its own quarantine logs as a malicious cryptor activity.
  • Suggested Filter/Exclusion: Create an exclusion for process images matching C:\Program Files\CrowdStrike\csagent.exe or C:\Program Files (x86)\Symantec Endpoint Protection\Smc.exe. Add a condition to ignore alerts where the target file path is within the AV vendor’s installation directory.

• Scenario: Automated Backup and Archiving Jobs

  • Context: Scheduled tasks running tools like Veeam Agent for Windows or Acronis Cyber Protect often compress and encrypt local backup files (.vbk, .tib) before uploading them to storage. The rule detects the rapid creation of encrypted archives