RCryptorv15Vaska

Hunt Hypothesis

This rule identifies the presence of the RCryptor ransomware variant, specifically version 15, which is known for encrypting files and deploying a Vaska-based payload. Proactively hunting for this signature in Azure Sentinel allows the SOC to detect early-stage ransomware infections or dormant payloads before they execute their encryption routines, minimizing potential data loss and operational disruption.

YARA Rule

rule RCryptorv15Vaska
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 83 2C 24 4F 68 [4] FF 54 24 04 83 44 24 04 4F }

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 is testing a custom encryption utility or a specific version of the RCryptor tool (v1.5) on a non-production build server or CI/CD agent to verify cryptographic output before deployment.
  • Filter/Exclusion: Exclude files located in specific development directories (e.g., C:\dev\builds\, D:\ci\artifacts\) or exclude processes initiated by known CI/CD service accounts (e.g., svc-cicd, build-agent) that are not part of the standard domain admin group.
• Scenario: A legacy line-of-business application or a specific vendor-provided utility (e.g., a data migration tool) bundles the RCryptor v1.5 binary or uses its specific encryption algorithm internally for data-at-rest protection, and the YARA rule matches the static binary or memory section during a routine integrity scan.
  • Filter/Exclusion: Maintain a whitelist of known vendor executables (e.g., VendorMigrationTool.exe, LegacyDataSync.dll) or exclude paths under vendor-specific installation directories (e.g., C:\Program Files\VendorApp\bin\) from YARA scanning if the application is signed by a trusted certificate.
• Scenario: An IT administrator is performing a manual disk cleanup or archival task on a file server, using a script that invokes the RCryptor v1.5 CLI tool to encrypt large backup sets before moving them to cold storage, triggering the detection on the file server rather than a workstation.
  • Filter/Exclusion: Exclude the detection if the host is a File Server (identified via AD group membership or WMI Role property) and the process name is rcryptor.exe or similar, specifically when the parent process is a script interpreter (e.g., powershell.exe, cmd.exe) initiated