PENinja

Hunt Hypothesis

The PENinja YARA rule detects the presence of the PENinja PowerShell framework, a tool frequently used by adversaries to execute in-memory payloads and perform post-exploitation tasks while maintaining a low profile. Proactively hunting for this signature in Azure Sentinel allows the SOC team to identify compromised endpoints or active attacker footholds before they can establish persistence or escalate privileges within the environment.

YARA Rule

rule PENinja: Packer PEiD
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 }

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 uses PEBunny or PEBunny64 (a standalone PE editor) to patch a specific binary (e.g., removing a license check or updating a version string) during a local build or testing phase.
  • Filter/Exclusion: Exclude processes where the parent process is an IDE (e.g., code.exe, idea64.exe, devenv.exe) or a build tool (e.g., msbuild.exe, dotnet.exe), or exclude if the target file path contains \bin\, \obj\, or \build\ directories.
• Scenario: An IT administrator uses HxD (Hex Editor) or 010 Editor to manually inspect or modify a PE header (e.g., adjusting the subsystem field or entry point) for a legacy application that needs to run on a newer OS version.
  • Filter/Exclusion: Exclude if the process name is hx.exe, 010editor.exe, or hexedit.exe, and the user is a member of the IT_Admins or DevOps security group.
• Scenario: A scheduled maintenance job runs a custom script that uses CFF Explorer (or its command-line equivalent) to strip debug information or resources from a newly compiled service executable before deployment to production.
  • Filter/Exclusion: Exclude if the process is launched by a scheduled task (taskschd.msi or svchost.exe with a specific service name) and the target file is located in a designated deployment folder (e.g., C:\Deploy\Staging\).
• Scenario: A security team member uses PE-sieve or PE-Bear to analyze a suspicious binary in a sandbox or analysis workstation, where they may use PE