VxVCL

Hunt Hypothesis

This hypothesis targets the execution of VxVCL, a known variant of the VxVCL malware family, which adversaries often deploy to establish initial footholds or perform post-exploitation tasks within Azure environments. Proactively hunting for this indicator allows the SOC to identify low-severity, stealthy intrusions that may evade standard behavioral detections, ensuring early containment of persistent threats before they escalate in severity.

YARA Rule

rule VxVCL
{
      meta:
		author="malware-lu"
strings:
		$a0 = { AC B9 00 80 F2 AE B9 04 00 AC AE 75 ?? E2 FA 89 }

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

• Legitimate .NET Compiler Execution: The VxVCL YARA rule likely targets specific byte patterns or strings associated with the Visual C++ runtime or compiler artifacts (e.g., msvcr*.dll references or specific function signatures). In a development or build environment, legitimate execution of the Microsoft Visual C++ Build Tools (specifically cl.exe or link.exe) during CI/CD pipeline builds or local developer compilation tasks can trigger this rule.

  • Filter/Exclusion: Exclude processes where the parent process is cmd.exe, powershell.exe, or msbuild.exe and the command line contains arguments like /c, /t, or /link. Additionally, exclude executions from known build directories such as C:\Program Files\Microsoft Visual Studio\ or C:\Program Files (x86)\Microsoft Visual Studio\.

• Scheduled Backup or Maintenance Jobs: Many enterprise backup solutions (e.g., Veeam, Commvault) or maintenance scripts (e.g., disk cleanup, log rotation) use C++-based executables that link against the Visual C++ Redistributable packages. If the rule matches on the presence of specific C++ runtime symbols or import table entries, these scheduled tasks running under the Task Scheduler service (svchost.exe with TasksSched argument) can generate false positives.

  • Filter/Exclusion: Exclude events where the parent process is svchost.exe and the command line includes TasksSched. Alternatively, exclude specific known backup executables like VeeamBackup.exe or commvaultagent.exe if they are statically linked or dynamically load the C++ runtime in a way that matches the rule.

• Application Deployment via Group Policy: When deploying line-of-business applications via Group Policy Software Installation, the Windows Installer service (msiexec.exe) executes setup