PCryptv351

Hunt Hypothesis

This detection identifies potential file encryption activities associated with ransomware or data exfiltration attempts by monitoring for specific YARA signatures indicative of cryptographic processes. A SOC team should proactively hunt for this behavior in Azure Sentinel to distinguish between legitimate administrative tasks and early-stage malicious encryption that could signal an impending ransomware attack, allowing for timely intervention before critical systems are compromised.

YARA Rule

rule PCryptv351
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 50 43 52 59 50 54 FF 76 33 2E 35 31 00 E9 }

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 PCryptv351 detection rule, including suggested filters and exclusions tailored for an enterprise environment:

• Scenario: Microsoft Office Document Macro Execution

  • Context: Users frequently open Excel workbooks or Word documents containing VBA macros that perform local data encryption (e.g., financial reports with encrypted cells) before saving. The YARA rule often flags the EXCEL.EXE or WINWORD.EXE process spawning a child process to handle these cryptographic operations as suspicious.
  • Filter/Exclusion: Create an exclusion for processes where the parent image path matches C:\Program Files\Microsoft Office* and the command line contains arguments related to macro execution (e.g., /m, /macro). Alternatively, exclude specific file hashes known to be legitimate internal templates used by the Finance department.

• Scenario: Scheduled Antivirus Cloud Scan Jobs

  • Context: Enterprise endpoint protection suites like CrowdStrike Falcon, SentinelOne, or Microsoft Defender for Endpoint run scheduled cloud-based heuristic scans during off-hours. These agents often utilize local cryptographic libraries to hash and encrypt telemetry data before transmission, triggering the PCryptv351 signature on the agent’s worker process (e.g., csfalcon.exe or MsMpEng.exe).
  • Filter/Exclusion: Implement a rule exclusion based on the specific service name of the EDR agent. For example, exclude events where the ProcessName is C:\Program Files\CrowdStrike\FalconSensor\csfalcon.exe and the CommandLine includes keywords like “cloud-scan” or “telemetry-upload”.

• Scenario: Automated Backup Encryption via Veeam or Commvault

  • Context: Scheduled backup jobs initiated by tools such as **Veeam Backup & Rep