RCryptorV16dVaska

Hunt Hypothesis

This detection identifies the presence of the RCryptorV16dVaska ransomware variant by leveraging a specific YARA signature to catch early-stage infection indicators within endpoint processes. Proactively hunting for this behavior in Azure Sentinel is critical because low-severity alerts often precede full-scale encryption events, allowing the SOC team to isolate compromised assets and initiate containment before data loss occurs.

YARA Rule

rule RCryptorV16dVaska
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 90 61 61 80 7F F0 45 90 60 0F 85 1B 8B 1F FF 68 [4] B8 [4] 90 3D [4] 74 06 80 30 ?? 40 EB F3 B8 [4] 90 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 RCryptorV16dVaska detection rule, including suggested filters and exclusions tailored for an enterprise environment:

• Scenario: Microsoft Office Document Repair via Background Services

  • Context: When a user opens a corrupted .docx or .xlsx file, OfficeClickToRun.exe (Microsoft Office Click-to-Run) often spawns a temporary process to repair the document structure. This involves reading and rewriting binary headers that mimic the encryption/decryption patterns of RCryptor ransomware.
  • Filter/Exclusion: Exclude processes where the parent process is OfficeClickToRun.exe or WINWORD.EXE located in the standard installation path (C:\Program Files\Microsoft Office...). Additionally, filter out events where the file extension matches .docx, .xlsx, or .pptx.

• Scenario: Scheduled Antivirus Database Updates

  • Context: During nightly maintenance windows, enterprise antivirus solutions like CrowdStrike Falcon, SentinelOne, or Microsoft Defender download and unpack new definition signatures. The extraction of compressed signature archives triggers file creation and modification patterns that closely resemble the RCryptor staging behavior.
  • Filter/Exclusion: Create an exclusion for processes named CbsService.exe (CrowdStrike), SentinelOneAgent.exe, or MsMpEng.exe. Apply a time-based filter to exclude alerts generated between 02:00 and 04:00 local time, coinciding with the standard update window.

• Scenario: Automated Backup and Deduplication Jobs

  • Context: Enterprise backup agents such as Veeam Agent, Commvault, or Rubrik perform block-level deduplication on large datasets. This process involves reading source files, calculating hashes, and