FSGv120EngdulekxtMASM32TASM32

Hunt Hypothesis

This detection identifies potential malware execution or suspicious process activity on Windows systems based on specific YARA signature patterns matching known threat indicators. Proactive hunting for this behavior in Azure Sentinel is essential to uncover early-stage threats that may not yet trigger high-severity alerts but could indicate the initial foothold of a broader campaign requiring deeper investigation.

YARA Rule

rule FSGv120EngdulekxtMASM32TASM32
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 33 C2 2C FB 8D 3D 7E 45 B4 80 E8 02 00 00 00 8A 45 58 68 02 ?? 8C 7F EB 02 CD 20 5E 80 C9 16 03 F7 EB 02 40 B0 68 F4 00 00 00 80 F1 2C 5B C1 E9 05 0F B6 C9 8A 16 0F B6 C9 0F BF C7 2A D3 E8 02 00 00 00 99 4C 58 80 EA 53 C1 C9 16 2A D3 E8 02 00 00 00 9D CE }

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: Legitimate Microsoft Office 365 Click-to-Run updates trigger the rule when the OfficeClickToRun.exe process spawns child processes to download and apply patches in the background. The YARA signature likely matches a specific PE header or string sequence common in these update binaries.

  • Suggested Filter: Exclude alerts where the parent process is OfficeClickToRun.exe located within the standard installation path (e.g., C:\Program Files\Microsoft Office Root\Office16).

• Scenario: Scheduled antivirus definition updates from CrowdStrike Falcon or Microsoft Defender for Endpoint generate false positives. The rule detects the update service (DefenderService.exe or FalconSensor.exe) as it writes new signature databases to disk, matching the YARA pattern intended for malicious behavior.

  • Suggested Filter: Create an exclusion based on process names MsMpEng.exe, DefenderService.exe, and FalconSensor.exe running under the context of a scheduled task named “Antivirus Update” or similar service accounts (e.g., NT SERVICE\Wuauserv).

• Scenario: Enterprise deployment tools such as Microsoft Endpoint Configuration Manager (SCCM/MECM) or Intune execute application installations. When deploying internal line-of-business applications, the installer (InstallShield.exe or msiexec) extracts temporary binaries that match the detection logic’s heuristic for executable behavior.

  • Suggested Filter: Exclude alerts where the process path contains \ConfigMgr\ or \IntuneManagementExtension\, and the parent process is a known deployment agent like ccmsetup.exe.

• Scenario: Automated PowerShell scripts executed by IT administrators for routine maintenance (e.g., log rotation, user provisioning) invoke compiled .NET assemblies that contain similar code structures to the rule’s target. These scripts often run via powershell.exe or pwsh.exe in the context of