RCryptor16cVaska

Hunt Hypothesis

This hunt hypothesis targets the presence of the RCryptor16cVaka malware family within Azure Sentinel to identify early-stage encryption activities that may precede ransomware deployment. Proactively hunting for this specific YARA signature allows the SOC team to detect low-severity indicators before they escalate into critical incidents, ensuring timely containment and reducing potential data loss.

YARA Rule

rule RCryptor16cVaska
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 8B C7 03 04 24 2B C7 80 38 50 0F 85 1B 8B 1F FF 68 [4] B8 [4] 3D [4] 74 06 80 30 ?? 40 EB F3 B8 [4] 3D [4] 74 06 80 30 ?? 40 EB F3 }

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

Here are 5 specific false positive scenarios for the RCryptor16cVaska detection rule, along with targeted exclusion strategies:

• Scenario: Enterprise Antivirus Real-Time Scanning of Compressed Archives

  • Context: The YARA rule triggers when the enterprise endpoint protection suite (e.g., CrowdStrike Falcon, Microsoft Defender for Endpoint, or Symantec) scans large .zip or .7z archives containing encrypted payloads in real-time. The heuristic behavior of these scanners mimics the encryption routines detected by RCryptor16cVaska.
  • Filter/Exclusion: Create an exclusion rule to ignore alerts where the parent process is a known security agent (e.g., csagent.exe, MsMpEng.exe, or rtvscan64.exe) and the file extension is .zip or .7z.

• Scenario: Automated Backup Jobs Executing via PowerShell

  • Context: Scheduled nightly backup tasks run by Veeam Agent for Windows or custom PowerShell scripts (e.g., Backup-Database.ps1) invoke compression and encryption modules. These jobs often spawn child processes that perform block-level encryption similar to the Vaska variant, causing a match on the YARA signature during peak maintenance windows.
  • Filter/Exclusion: Exclude alerts occurring between 02:00 AM and 06:00 AM (local time) where the command line arguments contain keywords like --backup, -compress, or specific backup tool identifiers, specifically targeting processes running under the SYSTEM or dedicated service accounts.

• Scenario: Development Build Pipelines in CI/CD Environments

  • Context: In DevOps environments using tools like Jenkins, GitLab Runner, or Azure DevOps Agents, build agents frequently compile and