RCryptorv15PrivateVaska

Hunt Hypothesis

This hunt hypothesis detects the presence of the RCryptor v15 Private Vaska ransomware variant by leveraging a specific YARA signature to identify its unique code patterns within Azure Sentinel workloads. Proactive hunting for this threat is essential to ensure early discovery and containment before encryption activities begin, thereby minimizing potential data loss and operational disruption even when initial severity indicators are low.

YARA Rule

rule RCryptorv15PrivateVaska
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 83 2C 24 4F 68 [4] FF 54 24 04 83 44 24 04 4F 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 4 specific false positive scenarios for the RCryptorv15PrivateVaska detection rule, including suggested filters and exclusions:

• Scenario: Legitimate Backup Encryption by Veeam or Commvault

  • Context: Enterprise backup agents (e.g., Veeam.Backup.Service.exe or commvault.cmd) frequently encrypt large data chunks during scheduled nightly jobs. The YARA rule may misinterpret the high-volume file I/O and cryptographic signatures of these backups as ransomware behavior, specifically matching the “Private Vaska” signature patterns often found in backup encryption modules.
  • Suggested Filter: Exclude processes running under specific service accounts (e.g., DOMAIN\VeeamBackupService) where the parent process is a known backup daemon and the file extension matches standard backup formats (.vbk, .bck, .vmdk).

• Scenario: Microsoft Office 365 ProPlus Auto-Save and Encryption

  • Context: When users save large documents with “Encrypt with Password” enabled or utilize OneDrive’s “Files On-Demand” feature, excel.exe or winword.exe trigger local encryption routines. The rule may flag the temporary creation of encrypted .tmp files followed by immediate deletion as a ransomware staging activity.
  • Suggested Filter: Whitelist the process image path for Microsoft Office executables (C:\Program Files\Microsoft Office...) and exclude events where the file extension is .tmp, .asd, or .docx located within user profile directories (e.g., C:\Users\[User]\AppData).

• Scenario: Antivirus Real-Time Scanning of Compressed Archives

  • Context: Endpoint Protection platforms like CrowdStrike Falcon or Microsoft Defender for Endpoint often scan and re-compress .zip or .7z archives created by