RCryptorv13bVaska

Hunt Hypothesis

This YARA rule targets the RCryptor ransomware variant, which encrypts files and appends the .vaska extension to extort victims. Proactively hunting for this signature in Azure Sentinel allows the SOC to identify compromised endpoints or staging areas before the encryption process completes, minimizing potential data loss and operational disruption.

YARA Rule

rule RCryptorv13bVaska
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 61 83 EF 4F 60 68 [4] FF D7 }
	$a1 = { 61 83 EF 4F 60 68 [4] FF D7 B8 [4] 3D [4] 74 06 80 30 ?? 40 EB F3 }

condition:
		$a0 at pe.entry_point or $a1 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 2 string patterns in its detection logic.

• Source Rule

False Positive Guidance

• Scenario: A developer or sysadmin uses a custom-built internal tool or a specific version of a commercial encryption utility (e.g., a patched build of VeraCrypt or a proprietary BitLocker management script) that embeds the RCryptor engine for on-the-fly file encryption during backup or migration tasks.
  • Filter/Exclusion: Exclude processes where the parent executable is a known backup agent (e.g., VeeamAgent.exe, CommCell.exe) or where the command line contains arguments related to backup/migration (e.g., /backup, /migrate, /encrypt).
• Scenario: A scheduled maintenance job runs a legacy or custom-built PowerShell or Batch script that invokes a small C++/C# helper binary (compiled with the RCryptor library) to encrypt sensitive configuration files or API keys before they are stored in a secure vault or transferred to a central server.
  • Filter/Exclusion: Exclude executions originating from scheduled tasks with specific names (e.g., *EncryptConfig*, *SecureKeys*) or where the process path resides in a designated maintenance directory (e.g., C:\ProgramData\CorpTools\, C:\Scripts\Maintenance\).
• Scenario: A CI/CD pipeline or build server executes a build step that compiles a small native helper or test binary using a custom linker or compiler wrapper that statically links the RCryptor library for unit testing or obfuscation of test artifacts.
  • Filter/Exclusion: Exclude processes running under a service account associated with build agents (e.g., svc-build, ci-agent) or where the working directory is within a known build workspace (e.g., C:\jenkins\workspace\, C:\azure-pipelines\).
• Scenario: An application installer or