RCryptor20Vaska

Hunt Hypothesis

This hypothesis targets the execution or presence of the RCryptor20Vaska ransomware variant, which leverages specific encryption routines to compromise data integrity. Proactively hunting for this signature in Azure Sentinel allows the SOC to identify early-stage ransomware activity on endpoints before encryption completes, minimizing potential data loss and operational disruption.

YARA Rule

rule RCryptor20Vaska
{
      meta:
		author="malware-lu"
strings:
		$a0 = { F7 D1 83 F1 FF 6A 00 F7 D1 83 F1 FF 81 04 24 [4] F7 D1 83 F1 FF }

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 sysadmin uses the vaska command-line tool (a known RCryptor implementation) to encrypt a large log archive or backup file on a build server or CI/CD agent.
  • Filter/Exclusion: Exclude files located in specific directories (e.g., C:\Builds\, D:\Backups\) or exclude processes named vaska.exe or rcryptor.exe if they are running from a trusted application directory (e.g., C:\Tools\Crypto\).
• Scenario: A security team performs a periodic encryption stress test or benchmarking exercise using RCryptor to validate disk I/O performance and encryption throughput on new storage arrays.
  • Filter/Exclusion: Create a maintenance window exclusion for specific test hosts (e.g., TEST-SRV-01) or tag the test environment with a custom attribute (e.g., env=test) in the SIEM to suppress alerts during scheduled maintenance windows.
• Scenario: An automated backup script uses RCryptor to encrypt daily database dumps before transferring them to an off-site storage location, running as a scheduled task under a service account.
  • Filter/Exclusion: Exclude the specific service account (e.g., svc-backup) when the process is initiated by the Task Scheduler (taskschd.exe) or when the parent process is svchost.exe (if running as a Windows service).
• Scenario: A legacy application or custom internal tool bundles RCryptor as a dependency for encrypting configuration files or API keys at startup, running from a non-standard but trusted application directory.
  • Filter/Exclusion: Add an exclusion for the specific executable path (e.g., C:\Apps\LegacyTool\bin\rcryptor.dll or `C:\Apps\LegacyTool\rc