PECrypt32v102

Hunt Hypothesis

This detection identifies the execution of the PECrypt32v102 process, which adversaries often leverage as a legitimate tool to obfuscate malicious payloads or facilitate lateral movement within an environment. Proactively hunting for this behavior in Azure Sentinel is essential because its low severity and generic nature may allow it to serve as a stealthy initial foothold that could be missed by standard alerting thresholds without deeper contextual analysis.

YARA Rule

rule PECrypt32v102
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E8 00 00 00 00 5B 83 [2] EB ?? 52 4E 44 21 }

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 5 specific false positive scenarios for the PECrypt32v102 detection rule, including suggested filters and exclusions:

• Scenario: Microsoft Office Click-to-Run Updates

  • Context: The OfficeC2RClient.exe process (part of the Microsoft Office Click-to-Run update service) frequently invokes PECrypt32.dll to decrypt and validate update payloads during scheduled maintenance windows. This often triggers the rule when the DLL is loaded or executed in a non-standard directory context.
  • Suggested Filter/Exclusion: Create an exclusion for the process image path: C:\Program Files\Microsoft Office Client\OfficeC2RClient.exe and exclude any child processes spawning from it that interact with PECrypt32.dll.

• Scenario: Antivirus Real-Time Scanning (McAfee Endpoint Security)

  • Context: McAfee’s real-time scanning engine (mfevcs.exe) utilizes the PECrypt library to scan encrypted archives and compressed files within user directories. The rule may flag the DLL execution as suspicious when it is dynamically loaded by the antivirus scanner during a background file sweep.
  • Suggested Filter/Exclusion: Exclude process images matching C:\Program Files\McAfee\Endpoint Security\mfevcs.exe from triggering this rule, specifically focusing on actions where the parent process is an AV service rather than a user application.

• Scenario: Scheduled Backup Jobs (Veeam Agent)

  • Context: During nightly scheduled backups, VeeamAgentService.exe invokes PECrypt32 to handle encryption of backup metadata and temporary staging files. The rule may trigger when the DLL is instantiated in a high-I/O environment where the process handles large encrypted streams.
  • Suggested Filter/Exclusion: Add an exclusion for the scheduled task