RCryptor 15 by Vaska UsAr sign individual version 21032007 2215

Hunt Hypothesis

This hypothesis targets the presence of the RCryptor 15 ransomware variant, specifically the individual version signed by Vaska UsAr, which indicates a potential deployment of a known ransomware strain within the environment. Proactively hunting for this signature allows the SOC to identify early-stage infections or dormant payloads before encryption occurs, mitigating the risk of data loss and operational disruption in Azure Sentinel.

YARA Rule

rule RCryptor_15_by_Vaska_UsAr_sign_individual_version_21032007_2215: PEiD
{
    strings:
        $a = { 83 2C 24 4F 68 40 A1 14 13 FF 54 24 04 83 44 24 04 4F B8 00 10 14 13 3D 24 C0 14 13 74 06 80 30 2B 40 EB F3 B8 8C 20 18 13 3D B9 27 18 13 74 06 80 30 19 40 EB F3 E8 00 00 00 00 C3 }
    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 Custom Application Deployment: An internal line-of-business (LOB) application developed in-house years ago was compiled using an older version of the RCryptor packer to reduce binary size or protect IP. During a routine patch cycle or re-deployment via SCCM/Intune, the packed executable is written to disk, triggering the signature.
  • Filter/Exclusion: Allowlist the specific executable path (e.g., C:\Program Files\InternalTools\LegacyApp\app.exe) and its SHA256 hash. Verify the parent process is a known deployment agent (e.g., ccmsetup.exe, intunemanagedsetup.exe).
• Third-Party Utility with Packed Binaries: A legitimate third-party utility, such as a specific version of 7-Zip, WinRAR, or a niche database tool (e.g., Oracle Client binaries), uses RCryptor for its installer or main executable. When an admin manually runs the installer or updates the tool, the YARA rule matches the binary on disk.
  • Filter/Exclusion: Exclude files located in standard vendor directories (e.g., C:\Program Files\7-Zip\, C:\Program Files (x86)\WinRAR\) if the parent process is a known installer (setup.exe, install.exe) or a standard user/admin shell.
• Scheduled Maintenance Job with Packed Script Runner: A scheduled task (e.g., DailyLogCleaner) executes a small, custom-written utility that was packed with RCryptor to obfuscate the logic. This utility runs nightly at 02:00 AM to clean up log files. The YARA rule triggers when the packed binary is loaded into memory or written to a temp directory.
  • Filter/Exclusion: Correlate with the scheduled task name (`DailyLogClean