PEtitev12

Hunt Hypothesis

This rule detects the presence of a specific malicious or suspicious Portable Executable (PE) file signature, potentially indicating the execution of a low-level implant or tool used for initial access or persistence. Proactively hunting for this artifact allows the SOC to identify dormant or stealthy threats that may have bypassed traditional signature-based detections, ensuring early visibility into potential compromise within the Azure environment.

YARA Rule

rule PEtitev12
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 9C 60 E8 CA [3] 03 ?? 04 ?? 05 ?? 06 ?? 07 ?? 08 }

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 local build script that uses python -m venv or pip install to create a virtual environment, which may temporarily create small, unsigned Python bytecode files (.pyc) or helper scripts that match the “petite” PE characteristics.
  • Filter/Exclusion: Exclude files located in standard Python virtual environment directories (e.g., \\*\\venv\\, \\*\\.venv\\, \\*\\env\\) or files with the extension .pyc if the rule targets executable PEs, or exclude processes named python.exe or pip.exe from generating these artifacts.
• Scenario: An IT administrator deploys a lightweight, custom PowerShell wrapper script compiled to a small executable using tools like PS2EXE or PowerShell2EXE for a specific one-off admin task (e.g., disk cleanup, service restart), resulting in a small, unsigned PE file.
  • Filter/Exclusion: Exclude files in the C:\Windows\Temp\ or C:\Users\<Admin>\AppData\Local\Temp\ directories if their size is below a certain threshold (e.g., < 50KB) and they were created by a known admin account within the last 24 hours.
• Scenario: A CI/CD pipeline agent (e.g., Jenkins, Azure DevOps, or GitHub Actions runner) downloads and executes a small, unsigned helper binary or tool (such as a specific version of curl, wget, or a custom Go/Rust binary) for artifact fetching or log processing, which may appear as a “petite” PE.
  • Filter/Exclusion: Exclude files located in the CI/CD agent’s working directory (e.g., \\*\\workspace\\, `\*\agent