EncryptPE2200481022005314WFS

Hunt Hypothesis

This detection identifies potential fileless or packed malware execution by leveraging a specific YARA signature to flag encrypted Portable Executable (PE) files that may conceal malicious payloads within the Azure Sentinel environment. A proactive hunt is essential because these encrypted binaries often evade traditional signature-based defenses, allowing adversaries to establish persistence and execute covert operations before broader indicators of compromise emerge.

YARA Rule

rule EncryptPE2200481022005314WFS
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 9C 64 FF 35 00 00 00 00 E8 7A }

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 EncryptPE2200481022005314WFS detection rule, along with suggested filters and exclusions:

• Scenario: Microsoft Office Document Protection via Group Policy

  • Context: When a user opens a sensitive Word or Excel document that has been wrapped with “Information Rights Management” (IRM) or encrypted by the organization’s GPO, the winword.exe or excel.exe process triggers the YARA rule as it dynamically loads and encrypts the PE headers of the document.
  • Suggested Filter: Exclude processes where the executable path contains \Microsoft Office\root\Office16\ AND the parent process is gpupdate.exe or csrss.exe.

• Scenario: Scheduled Antivirus Real-Time Scanning

  • Context: The enterprise endpoint protection suite (e.g., CrowdStrike Falcon or Microsoft Defender for Endpoint) performs a scheduled real-time scan on large PE files in the user’s “Downloads” folder. During this scan, the AV engine re-encrypts the file metadata to ensure integrity, mimicking the behavior detected by EncryptPE2200481022005314WFS.
  • Suggested Filter: Create an exclusion for process names matching C:\Program Files\CrowdStrike\fs_qr.exe or C:\Windows\System32\MsMpEng.exe, specifically when the file extension is .docx, .xlsx, or .pdf.

• Scenario: Automated Backup Agent Execution

  • Context: A nightly backup job using Veeam Agent for Microsoft Windows initiates. The agent scans and compresses local user profiles, which involves encrypting PE headers of active application binaries (like outlook.exe) before archiving them