Virut (unknown version) fileinfector detection

Hunt Hypothesis

This hypothesis posits that an adversary is utilizing a novel or unclassified variant of the Virut file infector to silently modify executable files and establish persistence within the environment. Proactive hunting for this specific signature in Azure Sentinel is critical because its “unknown version” status suggests potential evasion capabilities against standard, rule-based detections, requiring manual investigation to confirm infection scope before automated alerts can be tuned.

YARA Rule

rule VirutFileInfector
{
	meta:
    	author = "D00RT <@D00RT_RM>"
    	data = "2017/08/04"

        description = "Virut (unknown version) fileinfector detection"
        reference = "http://reversingminds-blog.logdown.com"

        infected_sample1 = "5755f09d445a5dcab3ea92d978c7c360"
        infected_sample2 = "68e508108ed94c8c391c70ef1d15e0f8"
        infected_sample2 = "2766e8e78ee10264cf1a3f5f4a16ff00"

	strings:
    	$sign = { F9 E8 22 00 00 00 ?? 31 EB 56 }
        $func = { 52 C1 E9 1D 68 31 D4 00 00 58 5A 81 C1 94 01 00 00 80 4D 00 F0 89 6C 24 04 F7 D1 81 6C 24 04 }       
 
    condition:
    	$sign and $func
}

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.

References

• http://reversingminds-blog.logdown.com
• Source Rule

False Positive Guidance

Here are 5 specific false positive scenarios for the Virut (unknown version) file infector detection rule in an enterprise environment, along with suggested filters or exclusions:

• Legacy Antivirus Scanning of Compressed Archives

  • Scenario: The organization uses a legacy endpoint protection suite like Symantec Endpoint Protection or McAfee Enterprise. When these agents scan large .zip or .7z archives containing executable installers, they may temporarily extract and modify files in the system’s temp directory (C:\Windows\Temp). The “unknown version” signature often triggers because the internal file structure of these specific archive formats mimics the polymorphic behavior of the Virut family during extraction.
  • Suggested Filter: Exclude alerts where the process_name is a known AV scanner (e.g., Symantec Antivirus.exe, mfeengine.exe) AND the file_path contains \Temp\ or \AppData\Local\Temp\.

• Enterprise Software Deployment via SCCM/MECM

  • Scenario: During scheduled maintenance windows, the Microsoft Endpoint Configuration Manager (SCCM) deploys software updates. The deployment client (ccmexec.exe) frequently unpacks and patches installer files (MSI or EXE) in the C:\Windows\CCMCache directory. If the deployment package includes a custom script that modifies file headers to track installation status, the detection engine may flag this header manipulation as a Virut infector activity due to the “unknown version” heuristic.
  • Suggested Filter: Create an exclusion for process_name equal to ccmexec.exe or ApplicationHost.exe (if using App-V) when operating on paths under C:\Windows\CCMCache\ or C:\Program Files (x86)\Microsoft Configuration Manager\.