VxHymn1865

Hunt Hypothesis

This YARA rule targets specific memory patterns associated with the VxHymn1865 signature, which may indicate the presence of a low-severity web shell or script-based implant used for initial access or persistence. Proactively hunting for this signature in Azure Sentinel helps identify dormant or stealthy footholds that traditional network-based detections might miss, ensuring early discovery of potential compromise within the environment.

YARA Rule

rule VxHymn1865
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E8 [2] 5E 83 EE 4C FC 2E [4] 4D 5A [2] FA 8B E6 81 [3] FB 3B [5] 2E [5] 50 06 56 1E 0E 1F B8 00 C5 CD 21 }

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 runs a local Python script that utilizes the pycryptodome or cryptography library to generate or validate symmetric keys (e.g., AES-256) for a microservice configuration file. The YARA rule VxHymn1865 likely matches specific byte patterns associated with key scheduling constants or initialization vectors embedded in the compiled Python bytecode or the resulting binary artifact.

  • Filter/Exclusion: Exclude files located in standard development directories (e.g., C:\Users\*\Projects\, C:\dev\) or files with extensions .py, .pyc, and .so/.dll if the parent process is a known IDE (e.g., code.exe, idea64.exe) or Python interpreter (python.exe, python3.exe).

• Scenario: An IT administrator performs a routine backup verification using Veeam Backup & Replication or Commvault, where the agent creates temporary snapshot files or deduplication chunks on the agent host. These chunks may contain fragmented data from previously backed-up applications (like SQL Server or Exchange) that include cryptographic headers or specific binary signatures matching the VxHymn1865 pattern.

  • Filter/Exclusion: Exclude files in backup agent temporary directories (e.g., C:\ProgramData\Veeam\Backup\, C:\Program Files\Commvault\GAL\) or files owned by backup service accounts (e.g., vssadmin, commvault) when the parent process is a known backup daemon (e.g., vbrsrvr.exe, cvaux.exe).

• Scenario: A CI/CD pipeline (e.g., Jenkins, Azure DevOps, or GitLab Runner) executes a build step that compiles a C++ or Rust application with optimization flags