VxGotcha879

Hunt Hypothesis

This detection identifies potential malware or suspicious artifacts matching the specific signature defined by the VxGotcha879 YARA rule within Azure Sentinel workloads. Proactive hunting for this indicator is essential to uncover stealthy threats that may evade standard alerting thresholds due to their low severity classification, ensuring early identification of emerging risks before they escalate.

YARA Rule

rule VxGotcha879
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E8 [2] 5B 81 EB [2] 9C FC 2E [7] 8C D8 05 [2] 2E [4] 50 2E [6] 8B C3 05 [2] 8B F0 BF 00 01 B9 20 00 F3 A4 0E B8 00 01 50 B8 DA DA CD 21 }

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 VxGotcha879 detection rule, including suggested filters and exclusions:

• Scenario: Automated Patch Deployment via SCCM/Intune

  • Context: The System Center Configuration Manager (SCCM) or Microsoft Intune agent executes a scheduled software update task during business hours. This process often involves downloading and unpacking large payload archives, which triggers the YARA signature due to high-volume file I/O and temporary executable generation in the C:\Windows\CCM directory.
  • Filter/Exclusion: Create an exclusion for processes with the image path %SYSTEMDRIVE%\Program Files (x86)\Microsoft Configuration Manager\AdminConsole\* or %SYSTEMDRIVE%\Windows\CCM\*. Additionally, exclude file paths matching C:\Windows\Temp\MSI* during the specific maintenance window hours (e.g., 09:00–17:00).

• Scenario: Endpoint Protection Engine Scanning

  • Context: The native antivirus engine (e.g., Microsoft Defender Antivirus or CrowdStrike Falcon) performs a real-time scan on user documents. When the scanner extracts and analyzes compressed files (.zip, .rar) containing macros or scripts, it mimics the behavior patterns defined in VxGotcha879, leading to an alert when no actual threat exists.
  • Filter/Exclusion: Exclude processes where the ImageName matches MsMpEng.exe, C:\Program Files\CrowdStrike\FalconSensor\csfalcon.exe, or SophosAgent.exe. Furthermore, add a condition to ignore alerts if the parent process is identified as an antivirus service (e.g., ParentImageName contains “antivirus” or “security”).

• Scenario: PowerShell Script Execution for Inventory Audits