RCryptorv20Vaska

Hunt Hypothesis

This hypothesis targets the presence of the RCryptor v20 Vaska ransomware variant, which is known for encrypting files and leaving specific artifacts that can be identified via YARA scanning. Proactively hunting for this signature in Azure Sentinel allows the SOC to detect early-stage infections or dormant payloads before the encryption process begins, reducing the potential impact on workloads.

YARA Rule

rule RCryptorv20Vaska
{
      meta:
		author="malware-lu"
strings:
		$a0 = { F7 D1 83 F1 FF 6A 00 F7 D1 83 F1 FF 81 04 24 ?? 02 00 00 F7 D1 83 F1 FF 59 BA 32 21 ?? 00 F7 D1 83 F1 FF F7 D1 83 F1 FF 80 02 E3 F7 D1 83 F1 FF C0 0A 05 F7 D1 83 F1 FF 80 02 6F F7 D1 83 F1 FF 80 32 A4 F7 D1 83 F1 FF 80 02 2D F7 D1 83 F1 FF 42 49 85 C9 75 CD 1C 4F 8D 5B FD 62 1E 1C 4F 8D 5B FD 4D 9D B9 [3] 1E 1C 4F 8D 5B FD 22 1C 4F 8D 5B FD 8E A2 B9 B9 E2 83 DB E2 E5 4D CD 1E BF 60 AB 1F 4D DB 1E 1E 3D 1E 92 1B 8E DC 7D EC A4 E2 4D E5 20 C6 CC B2 8E EC 2D 7D DC 1C 4F 8D 5B FD 83 56 8E E0 3A 7D D0 8E 9D 6E 7D D6 4D 25 06 C2 AB 20 CC 3A 4D 2D 9D 6B 0B 81 45 CC 18 4D 2D 1F A1 A1 6B C2 CC F7 E2 4D 2D 9E 8B 8B CC DE 2E 2D F7 1E AB 7D 45 92 30 8E E6 B9 7D D6 8E 9D 27 DA FD FD 1E 1E 8E DF B8 7D CF 8E A3 4D 7D DC 1C 4F 8D 5B FD 33 D7 1E 1E 1E A6 0B 41 A1 A6 42 61 6B 41 6B 4C 45 1E 21 F6 26 BC E2 62 1E 62 1E 62 1E 23 63 59 ?? 1E 62 1E 62 1E 33 D7 1E 1E 1E 85 6B C2 41 AB C2 9F 23 6B C2 41 A1 1E C0 FD F0 FD 30 20 33 9E 1E 1E 1E 85 A2 0B 8B C2 27 41 EB A1 A2 C2 1E C0 FD F0 FD 30 62 1E 33 7E 1E 1E 1E C6 2D 42 AB 9F 23 6B C2 41 A1 1E C0 FD F0 FD 30 C0 FD F0 8E 1D 1C 4F 8D 5B FD E0 00 33 5E 1E 1E 1E BF 0B EC C2 E6 42 A2 C2 45 1E C0 FD F0 FD 30 CE 36 CC F2 1C 4F 8D 5B FD }

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

• Legacy .NET Application Deployment: A common false positive occurs when deploying or updating legacy internal business applications (e.g., ERP modules or custom inventory tools) that were compiled using older versions of the .NET Framework or Mono runtime. These binaries may retain specific byte patterns or string constants that match the RCryptorv20Vaska heuristic, particularly if the application has not been recompiled in the last 5–7 years.

  • Suggested Filter: Exclude files located in specific application directories (e.g., C:\Program Files\LegacyERP\ or C:\AppData\Local\InternalTools\) if the executable name matches known legacy binaries (e.g., LegacyApp.exe, InventoryManager.dll) and the file hash is whitelisted in the CMDB.

• Third-Party SDK and Library Updates: Software development teams often integrate third-party SDKs (e.g., AWS SDK for .NET, Azure Identity, or specific financial data feeds) that may be distributed as pre-compiled DLLs. If these SDKs are built with specific obfuscation or encryption routines that align with the Vaska cryptor signature, the YARA rule may trigger during routine dependency updates via NuGet or package managers.

  • Suggested Filter: Exclude files within standard package cache directories (e.g., C:\Users\<user>\.nuget\packages\ or C:\Program Files\dotnet\sdk\) or specific vendor folders (e.g., C:\Program Files\AWS SDK\) where the file extension is .dll or .exe and the parent process is a known build tool (e.g., dotnet.exe, msbuild.exe).

• Scheduled Backup and Archiving Jobs: Enterprise backup solutions (e.g., Veeam, Commvault, or Windows Server Backup) sometimes create temporary encrypted or compressed archives of application