Detects VB.NET red/black-team tools via typelibguid

Hunt Hypothesis

This detection identifies adversary activity involving VB.NET-based red and black team tools by analyzing unique typelib GUIDs associated with known Lime RAT signatures. Proactively hunting for these indicators in Azure Sentinel allows the SOC to distinguish between legitimate security testing artifacts and potential malicious reconnaissance or command-and-control infrastructure established by threat actors leveraging similar .NET frameworks.

YARA Rule

rule HKTL_NET_GUID_Lime_RAT {
    meta:
        description = "Detects VB.NET red/black-team tools via typelibguid"
        reference = "https://github.com/NYAN-x-CAT/Lime-RAT"
        author = "Arnim Rupp"
        date = "2020-12-30"
    strings:
        $typelibguid0 = "e58ac447-ab07-402a-9c96-95e284a76a8d" ascii nocase wide
        $typelibguid1 = "8fb35dab-73cd-4163-8868-c4dbcbdf0c17" ascii nocase wide
        $typelibguid2 = "37845f5b-35fe-4dce-bbec-2d07c7904fb0" ascii nocase wide
        $typelibguid3 = "83c453cf-0d29-4690-b9dc-567f20e63894" ascii nocase wide
        $typelibguid4 = "8b1f0a69-a930-42e3-9c13-7de0d04a4add" ascii nocase wide
        $typelibguid5 = "eaaeccf6-75d2-4616-b045-36eea09c8b28" ascii nocase wide
        $typelibguid6 = "5b2ec674-0aa4-4209-94df-b6c995ad59c4" ascii nocase wide
        $typelibguid7 = "e2cc7158-aee6-4463-95bf-fb5295e9e37a" ascii nocase wide
        $typelibguid8 = "d04ecf62-6da9-4308-804a-e789baa5cc38" ascii nocase wide
        $typelibguid9 = "8026261f-ac68-4ccf-97b2-3b55b7d6684d" ascii nocase wide
        $typelibguid10 = "212cdfac-51f1-4045-a5c0-6e638f89fce0" ascii nocase wide
        $typelibguid11 = "c1b608bb-7aed-488d-aa3b-0c96625d26c0" ascii nocase wide
        $typelibguid12 = "4c84e7ec-f197-4321-8862-d5d18783e2fe" ascii nocase wide
        $typelibguid13 = "3fc17adb-67d4-4a8d-8770-ecfd815f73ee" ascii nocase wide
        $typelibguid14 = "f1ab854b-6282-4bdf-8b8b-f2911a008948" ascii nocase wide
        $typelibguid15 = "aef6547e-3822-4f96-9708-bcf008129b2b" ascii nocase wide
        $typelibguid16 = "a336f517-bca9-465f-8ff8-2756cfd0cad9" ascii nocase wide
        $typelibguid17 = "5de018bd-941d-4a5d-bed5-fbdd111aba76" ascii nocase wide
        $typelibguid18 = "bbfac1f9-cd4f-4c44-af94-1130168494d0" ascii nocase wide
        $typelibguid19 = "1c79cea1-ebf3-494c-90a8-51691df41b86" ascii nocase wide
        $typelibguid20 = "927104e1-aa17-4167-817c-7673fe26d46e" 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 21 string patterns in its detection logic.

References

• https://github.com/NYAN-x-CAT/Lime-RAT
• Source Rule

False Positive Guidance

Here are 5 specific false positive scenarios for the Detects VB.NET red/black-team tools via typelibguid rule, including suggested filters and exclusions:

• Scenario: Scheduled Backup Job Execution

  • Context: Enterprise backup solutions like Veeam Backup & Replication or Acronis Cyber Protect often utilize VB.NET components for their agent services. When a scheduled backup job triggers during business hours, the typelibguid of the underlying .NET assembly matches the rule’s signature.
  • Suggested Filter/Exclusion: Create an exclusion based on the specific process name (e.g., VeeamAgent.exe, AcronisAgent.exe) and restrict detection to non-business hours or exclude known backup service accounts (e.g., DOMAIN\BackupSvc).

• Scenario: Office 365 ProPlus Update Mechanism

  • Context: The Microsoft Office update engine (OfficeClickToRun.exe or Microsoft.Office.Updates.Service) frequently employs VB.NET libraries to manage patching and feature installation. These updates often generate network traffic and file system changes that trigger the YARA rule’s GUID detection logic.
  • Suggested Filter/Exclusion: Exclude processes running from the standard Office installation directory path (e.g., C:\Program Files\Microsoft Office\root\Office16\) or filter by the specific Publisher “Microsoft Corporation” in conjunction with the process name.

• Scenario: Internal HR/Payroll Portal Deployment

  • Context: Many enterprises host custom internal portals built on .NET Framework 4.x using VB.NET for employee self-service tasks (e.g., leave requests, payroll viewing). When users launch these web clients or background sync services run, the typelib GUIDs align with the detection rule.
  • Suggested Filter/Exclusion: Implement a