Detects c# red/black-team tools via typelibguid

Hunt Hypothesis

This hunt hypothesis targets adversaries leveraging C#-based red and black team tools that expose unique TypeLib GUIDs to evade standard signature-based detection. Proactively hunting for these specific identifiers in Azure Sentinel is critical because it allows SOC teams to distinguish between legitimate administrative activities and stealthy tool deployment often used during initial reconnaissance or lateral movement phases.

YARA Rule

rule HKTL_NET_GUID_Fenrir {
    meta:
        description = "Detects c# red/black-team tools via typelibguid"
        reference = "https://github.com/nccgroup/Fenrir"
        author = "Arnim Rupp"
        date = "2020-12-13"
    strings:
        $typelibguid0 = "aecec195-f143-4d02-b946-df0e1433bd2e" ascii nocase wide
    condition:
        (uint16(0) == 0x5A4D and uint32(uint32(0x3C)) == 0x00004550) and any of them
}

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.

References

• https://github.com/nccgroup/Fenrir
• Source Rule

False Positive Guidance

Here are 4 specific false positive scenarios for the rule “Detects c# red/blue-team tools via typelibguid” in an enterprise environment, along with suggested filters or exclusions:

• Scenario: Automated Patch Management Deployment

  • Context: Enterprise patch management solutions (e.g., Microsoft Endpoint Configuration Manager (MECM/SCCM) or Ivanti) frequently utilize C# agents to scan system registries and inventory software. During a scheduled nightly maintenance window, these agents query typelibguid entries to verify .NET framework versions and installed COM components across thousands of workstations.
  • Suggested Filter: Exclude the specific process paths associated with the patch management agent (e.g., C:\Program Files\Microsoft Configuration Manager\client\execmgr.exe) or whitelist the known typelibguid values used by the vendor’s inventory engine.

• Scenario: Enterprise Antivirus Real-Time Scanning

  • Context: Next-generation antivirus solutions (such as CrowdStrike Falcon, SentinelOne, or Microsoft Defender for Endpoint) often employ C# modules to perform deep heuristic analysis of running processes. When these tools inspect the memory space of a user’s application, they may trigger this rule by accessing type library GUIDs associated with standard Windows system libraries (e.g., Shell32.dll or oleaut32.dll) which are common targets for red/blue team tooling detection.
  • Suggested Filter: Create an exclusion based on the parent process ID (PPID) of the known security vendor’s service (e.g., exclude events where ParentImage matches C:\Program Files\CrowdStrike\csagent.exe).

• Scenario: Scheduled Software License Compliance Audits

  • Context: IT Asset Management tools like Snow Software or Flexera run scheduled C# scripts