VirogenCryptv075

Hunt Hypothesis

This rule targets the VirogenCrypt ransomware variant, which utilizes specific encryption routines to lock victim files and demand payment. Proactively hunting for this signature allows the SOC to identify early-stage ransomware activity in Azure Sentinel, enabling rapid isolation of affected workloads before the encryption process completes and data is exfiltrated or lost.

YARA Rule

rule VirogenCryptv075
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 9C 55 E8 EC 00 00 00 87 D5 5D 60 87 D5 80 BD 15 27 40 00 01 }

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 IT administrator uses a commercial code obfuscation tool (e.g., Dotfuscator, Protege, or ConfuserEx) to protect proprietary .NET applications before deployment. The resulting binary contains packed or obfuscated sections that match the VirogenCryptv075 signature due to similar structural patterns in the PE header or code sections.

  • Filter/Exclusion: Exclude files located in specific build artifact directories (e.g., C:\Builds\Release\, D:\Artifacts\) or filter by file extension .dll/.exe where the parent process is a known build agent (e.g., vstest.console.exe, msbuild.exe, or dotnet.exe).

• Scenario: An enterprise endpoint protection suite (e.g., CrowdStrike Falcon, Carbon Black, or Trend Micro Apex One) injects its own driver or helper module into a legitimate application’s memory space or creates a temporary packed copy during a full system scan. The YARA rule may detect the packed state of the file on disk or in memory before it is fully unpacked by the AV engine.

  • Filter/Exclusion: Exclude detections where the parent process is a known EDR agent (e.g., FalconSensor.exe, CbService.exe, TmProxy.exe) or where the file path resides in the vendor’s specific temporary directory (e.g., C:\ProgramData\CrowdStrike\, C:\ProgramData\Trend Micro\).

• Scenario: A scheduled maintenance job runs a legitimate compression or archiving utility (e.g., 7-Zip, WinRAR, or PowerShell Compress-Archive) to package log files or configuration backups. If the YARA rule is scanning memory or temporary files, it may detect the compressed data