VxXRCV1015

Hunt Hypothesis

This YARA rule targets specific malicious code patterns or memory artifacts associated with the VxXRCV1015 signature, indicating potential low-severity threat activity or known malware variants. Proactively hunting for this indicator in Azure Sentinel allows the SOC team to identify dormant or stealthy compromises that may not trigger high-fidelity alerts, ensuring early detection of subtle adversary presence within the environment.

YARA Rule

rule VxXRCV1015
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E8 [2] 5E 83 [2] 53 51 1E 06 B4 99 CD 21 80 FC 21 [5] 33 C0 50 8C D8 48 8E C0 1F A1 [2] 8B }

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 a custom Python script or PowerShell wrapper to invoke curl or wget for downloading specific artifacts (e.g., container images, model weights, or dependency libraries) from an internal artifact repository (like Artifactory, Nexus, or S3) during a CI/CD pipeline or local build process.
  • Filter/Exclusion: Exclude processes where the parent process is a known build tool (e.g., docker.exe, kubectl.exe, jenkins.exe, python.exe with specific arguments like -m pip or -m build) or where the destination path is within a standard build/cache directory (e.g., C:\Users\<User>\.cache\, C:\ProgramData\Jenkins\, /var/cache/).
• Scenario: An IT administrator performs a routine software deployment or patching task using a tool like SCCM, Intune, or a custom PowerShell script that downloads installer packages (.msi, .exe) from a shared network drive or internal web server to a temporary staging folder before execution.
  • Filter/Exclusion: Exclude events where the source URL/IP belongs to the internal corporate subnet (e.g., 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) or where the parent process is a known management agent (e.g., ccmexec.exe, IntuneAgent.exe, powershell.exe invoked by a scheduled task named “PatchManagement” or “SoftwareDistribution”).
• Scenario: A data analyst or ML engineer uses a Jupyter Notebook or a Python script to fetch datasets, API responses, or model checkpoints from an internal data lake or API gateway for analysis or training purposes.
  • **Filter/