VxARCV4

Hunt Hypothesis

This rule detects the presence of the VxARCV4 malware family, which is often used for initial access or persistence through memory-resident execution. Proactively hunting for this signature allows the SOC to identify compromised hosts before the adversary establishes deeper footholds or begins lateral movement within the Azure environment.

YARA Rule

rule VxARCV4
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E8 00 00 5D 81 ED 06 01 81 FC 4F 50 74 0B 8D B6 86 01 BF 00 01 57 A4 EB 11 1E 06 }

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

• Scenario: A legacy Java-based enterprise application (e.g., an internal HR portal or legacy ETL tool) is deployed via a standard MSI or ZIP installer. The JAR files or native libraries within the package contain specific byte sequences or string patterns that match the VxARCV4 YARA signature due to the rule’s broad matching criteria for certain archive headers or compressed data structures.

  • Filter/Exclusion: Exclude files located in known application directories (e.g., C:\Program Files\InternalHRApp\, C:\opt\legacy-etransform\) or exclude file extensions .jar, .war, and .so if the rule is primarily targeting executable binaries. Alternatively, whitelist the specific SHA256 hashes of the known-good JAR files.

• Scenario: An IT administrator performs a manual backup of a critical database using a tool like pg_dump or mysqldump, which outputs a compressed .gz or .tar.gz archive to a network share. The compression algorithm (e.g., GZIP) produces a header or data block that coincidentally matches the byte pattern defined in VxARCV4, especially if the rule includes generic checks for compressed stream signatures.

  • Filter/Exclusion: Exclude files with extensions .gz, .tar.gz, .zip, or .7z located in backup directories (e.g., \\fileserver\backups\, C:\Backups\). Ensure the exclusion applies only to non-executable files (MIME type application/gzip or application/zip).

• Scenario: A developer or DevOps engineer runs a build pipeline on a Windows agent that uses 7-Zip or WinRAR to package a release artifact. The resulting archive contains a specific metadata field or embedded resource (such as a digital certificate or a specific icon resource)