VxVCLencrypted

Hunt Hypothesis

This rule identifies files containing VxVCL encryption, a technique often used by adversaries to obfuscate malicious payloads and evade static analysis during initial access or execution phases. Proactively hunting for these encrypted artifacts in Azure Sentinel allows the SOC to uncover stealthy threats that may have bypassed traditional signature-based detections, ensuring early visibility into potential compromise vectors.

YARA Rule

rule VxVCLencrypted
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 01 B9 [2] 81 34 [2] 46 46 E2 F8 C3 }
	$a1 = { 01 B9 [2] 81 35 [2] 47 47 E2 F8 C3 }

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: The vcl (Virtual Component Library) or similar proprietary encryption/obfuscation tools are used by DevOps teams to encrypt configuration files or secrets before deploying them to production environments via CI/CD pipelines (e.g., Jenkins, Azure DevOps).
  • Filter/Exclusion: Exclude processes initiated by known CI/CD service accounts or parent processes like java.exe, node.exe, or dotnet.exe when the file path contains /build/, /deploy/, or /ci/.
• Scenario: Administrative staff use command-line utilities like openssl enc or custom PowerShell scripts to encrypt sensitive data files (e.g., database dumps, API keys) for secure off-site backup or transfer, which may be scanned by the YARA rule if the encryption algorithm or header matches the VxVCLencrypted pattern.
  • Filter/Exclusion: Exclude files located in designated backup directories (e.g., C:\Backups\, D:\Archive\) or exclude processes where the command line contains arguments like -encrypt, enc, or aes-256.
• Scenario: Legacy application installers or self-extracting archives (SFX) created with tools like 7-Zip or WinRAR that use specific compression/encryption headers matching the YARA signature, often triggered during scheduled software updates or patch management cycles (e.g., SCCM, WSUS).
  • Filter/Exclusion: Exclude processes running under the context of patch management agents (e.g., ccmexec.exe, wuauserv.exe) or exclude files with extensions .7z, .rar, or .sfx if the parent process is a known installer or update service.
• Scenario: Development teams use local IDEs or build tools (e.g., Visual Studio, IntelliJ) that invoke native compilers or