RpolycryptbyVaska2003071841

Hunt Hypothesis

This YARA rule targets the RpolycryptbyVaska2003071841 malware family, which is often associated with crypter-based payloads used to obfuscate malicious code and evade static analysis. Proactively hunting for this signature in Azure Sentinel allows the SOC to identify compromised endpoints or staged payloads early, reducing the dwell time of low-severity threats that may serve as initial access vectors for more complex attack chains.

YARA Rule

rule RpolycryptbyVaska2003071841
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 58 [7] E8 00 00 00 58 E8 00 [45] 00 00 00 [2] 04 }

condition:
		$a0
}

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

• Legacy Java Application Deployment: Enterprise Java applications (e.g., Tomcat, WebLogic) often use java.util.zip or custom encryption wrappers that may contain byte patterns matching the Rpolycrypt signature if they bundle specific cryptographic libraries or obfuscated JARs.
  • Filter/Exclusion: Exclude processes where the parent process is java.exe or javaw.exe, or exclude files located in standard Java application directories (e.g., C:\Program Files\Java\, C:\app\, or specific application root folders) if the file extension is .jar or .class.
• Third-Party .NET Obfuscators: Development teams using commercial .NET obfuscation tools (e.g., Dotfuscator, ConfuserEx, or SmartAssembly) to protect proprietary business logic may produce binaries with encryption headers or packed sections that trigger this YARA rule.
  • Filter/Exclusion: Exclude files with extensions .dll or .exe that are signed by known corporate code-signing certificates (e.g., “Corporate Code Signing CA”) and are located in the bin\ or obj\ directories of Visual Studio solution paths, or where the parent process is MSBuild.exe or dotnet.exe.
• Scheduled Backup Encryption Jobs: Enterprise backup solutions (e.g., Veeam, Commvault, or Acronis) that perform client-side encryption before uploading to the cloud may create temporary encrypted files or use encryption agents that match the Rpolycrypt pattern.
  • Filter/Exclusion: Exclude processes where the parent process is a known backup agent (e.g., VeeamBackup.exe, commvault_agent.exe, acronis_service.exe) or exclude files in temporary backup staging directories (e.g., C:\ProgramData\Veeam\Backup\, `C