RCryptor15Vaska

Hunt Hypothesis

This hypothesis targets the presence of the RCryptor 15 Vaska ransomware variant, which is known for encrypting files and dropping specific artifacts on compromised hosts. Proactively hunting for this signature in Azure Sentinel allows the SOC team to identify early-stage infections or dormant payloads before they propagate, leveraging YARA-based detection to catch low-severity threats that might otherwise evade standard behavioral alerts.

YARA Rule

rule RCryptor15Vaska
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 83 2C 24 4F 68 [4] FF 54 24 04 83 44 24 04 4F B8 [4] 3D [4] 74 06 80 30 [2] EB F3 B8 [4] 3D [4] 74 06 80 30 ?? 40 EB F3 }

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 the vaska command-line tool (a known open-source RCryptor implementation) to encrypt sensitive configuration files or API keys on a build server or CI/CD agent before committing them to a repository or transferring them to a staging environment.

  • Filter/Exclusion: Exclude files located in specific development directories (e.g., C:\dev\builds\, /home/ci/) or exclude processes where the parent process is a recognized build tool (e.g., msbuild.exe, dotnet.exe, java.exe) and the file extension is .enc or .vask.

• Scenario: An IT administrator uses a custom PowerShell script or a batch job to encrypt legacy application data files during a scheduled maintenance window, utilizing the RCryptor algorithm via a third-party wrapper library (e.g., P-Invoke in .NET or a Python script using pyrcryptor).

  • Filter/Exclusion: Exclude events where the encryption process is initiated by a scheduled task (schtasks.exe) or a service (svchost.exe with specific service names) and the target file path resides in designated data archives (e.g., D:\DataArchive\, /var/backups/).

• Scenario: A security team performs a periodic integrity check or data-at-rest encryption audit where they use a specialized utility (e.g., AxCrypt, 7-Zip with RCryptor plugin, or a custom C++ tool) to encrypt test files to verify that the encryption/decryption pipeline is functioning correctly in the production environment.

  • Filter/Exclusion: Exclude files with temporary extensions (e.g., .tmp, .bak) or files located in a dedicated “Encryption Test” directory (e.g., C:\Temp\EncTest\) that are created and deleted within