RCryptor V15 Vaska Sign by fly

Hunt Hypothesis

This hypothesis targets the presence of RCryptor V15 Vaska, a known ransomware variant, to identify early-stage encryption activities or dormant payloads that may precede widespread data encryption. Proactively hunting for this specific signature in Azure Sentinel allows the SOC to isolate compromised endpoints before the ransomware propagates, thereby minimizing the blast radius and preserving critical data integrity.

YARA Rule

rule RCryptor_V15_Vaska_Sign_by_fly: PEiD
{
    strings:
        $a = { 83 2C 24 4F 68 ?? ?? ?? ?? FF 54 24 04 83 44 24 04 4F B8 ?? ?? ?? ?? 3D ?? ?? ?? ?? 74 06 80 30 ?? ?? EB F3 B8 ?? ?? ?? ?? 3D ?? ?? ?? ?? 74 06 80 30 ?? 40 EB F3 }
    condition:
        $a 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

• Legacy .NET Application Deployment via Group Policy

  • Scenario: An IT administrator deploys a custom internal line-of-business application (compiled with an older version of .NET Framework) to a fleet of Windows 10/11 endpoints via Group Policy Object (GPO) or SCCM. The executable is signed with a legacy code-signing certificate that matches the “Vaska” signature pattern, or the binary structure coincidentally aligns with the RCryptor V15 heuristic due to specific compiler optimizations used in older builds.
  • Filter/Exclusion: Exclude files located in standard application installation directories (e.g., C:\Program Files\, C:\Program Files (x86)\) and specifically exclude paths matching \\*\\Appx\\* or \\*\\WinSxS\\*. Additionally, whitelist the specific SHA-256 hash of the known-good application binary if it remains consistent across deployments.

• Scheduled Backup Agent Execution

  • Scenario: A third-party backup solution (e.g., Veeam, Acronis, or Commvault) runs a scheduled agent service on file servers or workstations. The agent’s helper executable or a temporary staging file created during the backup process is detected because it is placed in a temporary directory (%TEMP% or C:\Windows\Temp) and exhibits a binary structure that triggers the RCryptor V15 signature, often due to the way the vendor compresses or encrypts local cache files.
  • Filter/Exclusion: Exclude processes and files where the parent process name is known backup agents (e.g., VeeamSvc.exe, AcronisService.exe, commvault.exe) and exclude file paths containing \\Backup\\, \\Veeam\\, or \\Acronis\\. Consider excluding files in `%TEMP