PEStubOEPv1x

Hunt Hypothesis

This rule identifies executable files where the entry point offset (OEP) is located within the PE header, a structural anomaly often exploited by packers or obfuscated malware to hide the true code execution flow. Proactively hunting for these irregularities in Azure Sentinel allows the SOC team to surface potentially stealthy or packed binaries that may bypass standard signature-based detections, enabling early investigation of suspicious artifacts before they establish a foothold in the environment.

YARA Rule

rule PEStubOEPv1x
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 40 48 BE 00 [2] 00 40 48 60 33 C0 B8 [3] 00 FF E0 C3 C3 }

condition:
		$a0
}

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 manually compiles a small C/C++ utility using a minimal linker configuration (e.g., gcc with -nostartfiles or specific linker scripts) that results in an entry point (OEP) located in the first section, which is a common characteristic of stubs or packed binaries.

  • Filter/Exclusion: Exclude files located in specific development directories (e.g., C:\dev\, C:\projects\, C:\builds\) or filter by file extension (.exe, .dll) only if the parent process is a known compiler/linker (e.g., cl.exe, link.exe, gcc.exe).

• Scenario: An enterprise application or plugin is installed via a custom installer that uses a self-extracting archive or a lightweight bootstrapper (e.g., NSIS, Inno Setup, or a custom C# bootstrapper) where the initial executable is a small stub that loads the main application. This is common for legacy line-of-business (LOB) applications or specific vendor tools like Adobe Creative Cloud components or Java-based wrappers.

  • Filter/Exclusion: Exclude known vendor paths (e.g., C:\Program Files\Adobe\, C:\Program Files (x86)\Java\, C:\Program Files\Microsoft\) or filter by known signed certificates from trusted vendors (e.g., Adobe, Oracle, Microsoft) if the rule does not already check for digital signatures.

• Scenario: A scheduled task or service startup script executes a small, custom-written PowerShell-to-EXE wrapper (using tools like PS2EXE or PowerShell-EXE) that has a minimal PE structure. These wrappers are often used to hide PowerShell commands in scheduled tasks for internal automation, such as disk cleanup scripts, log rotation, or database backup triggers.