MSLRHv032afakePECrypt102emadicius

Hunt Hypothesis

This hypothesis targets the execution of obfuscated or packed executables that utilize fake PE headers to evade static analysis, a common technique employed by initial access malware to hide its true functionality. Proactively hunting for these artifacts in Azure Sentinel allows the SOC team to identify stealthy payloads that may be establishing footholds or preparing for lateral movement before they trigger more obvious behavioral alerts.

YARA Rule

rule MSLRHv032afakePECrypt102emadicius
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E8 00 00 00 00 5B 83 EB 05 EB 04 52 4E 44 21 85 C0 73 02 F7 05 50 E8 08 00 00 00 EA FF 58 EB 18 EB 01 0F EB 02 CD 20 EB 03 EA CD 20 58 58 EB 05 E8 EB 04 40 00 EB FA E8 0A 00 00 00 E8 EB 0C 00 00 E8 F6 FF FF FF E8 F2 FF FF FF 83 C4 08 74 04 75 02 EB 02 EB 01 81 50 E8 02 00 00 00 29 5A 58 6B C0 03 E8 02 00 00 00 29 5A 83 C4 04 58 74 04 75 02 EB 02 EB 01 81 0F 31 50 0F 31 E8 0A 00 00 00 E8 EB 0C 00 00 E8 F6 FF FF FF E8 F2 FF FF FF }

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

• Scenario: A developer or DevOps engineer runs a custom build script that uses objdump or strings to analyze a compiled binary for debugging purposes, causing the YARA scanner to flag the temporary file or the analysis tool itself if it contains specific cryptographic constants or header structures.
  • Filter/Exclusion: Exclude processes named objdump, strings, or binutils from the YARA scan path, or exclude files located in specific developer workspaces (e.g., C:\Users\dev\build\) during non-business hours.
• Scenario: An enterprise antivirus or endpoint detection and response (EDR) agent performs a real-time scan on a newly downloaded installer package (e.g., setup.exe for Adobe Creative Cloud or Oracle Java) that uses standard PE encryption headers or obfuscation techniques common in commercial installers.
  • Filter/Exclusion: Whitelist known vendor installer directories (e.g., C:\Program Files\Adobe\, C:\Program Files\Java\) and specific executable names like setup.exe or install.exe when they are executed from standard application installation paths.
• Scenario: A scheduled Windows Task runs a maintenance script that invokes certutil or openssl to generate or verify digital certificates for internal services, creating temporary encrypted files or memory regions that match the “fakePE” or “crypt” patterns in the rule.
  • Filter/Exclusion: Exclude processes certutil.exe and openssl.exe when they are launched by the Task Scheduler service (svchost.exe with specific arguments) or when the working directory is within C:\Windows\System32\ or C:\ProgramData\.
• Scenario: A legitimate .NET application (e.g., a custom internal dashboard) loads a native C++ DLL that uses standard PE encryption