FSGv110EngdulekxtMASM32TASM32

Hunt Hypothesis

This hunt hypothesis targets the execution of specific MASM32-based processes identified by the FSGv110 YARA signature to detect potential low-severity anomalies or early-stage adversary activity involving custom assembly modules. Proactively hunting for this behavior in Azure Sentinel allows the SOC team to validate legitimate usage patterns and identify subtle deviations that could indicate stealthy reconnaissance or the deployment of specialized tools before they escalate into higher-severity incidents.

YARA Rule

rule FSGv110EngdulekxtMASM32TASM32
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 03 F7 23 FE 33 FB EB 02 CD 20 BB 80 ?? 40 00 EB 01 86 EB 01 90 B8 F4 00 00 00 83 EE 05 2B }
	$a1 = { 03 F7 23 FE 33 FB EB 02 CD 20 BB 80 ?? 40 00 EB 01 86 EB 01 90 B8 F4 00 00 00 83 EE 05 2B F2 81 F6 EE 00 00 00 EB 02 CD 20 8A 0B E8 02 00 00 00 A9 54 5E C1 EE 07 F7 D7 EB 01 DE 81 E9 B7 96 A0 C4 EB 01 6B EB 02 CD 20 80 E9 4B C1 CF 08 EB 01 71 80 E9 1C EB }

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

Here are 4 specific false positive scenarios for the FSGv110EngdulekxtMASM32TASM32 detection rule, including suggested filters and exclusions:

• Scenario: Microsoft Defender Antivirus Real-Time Scanning

  • Context: The YARA rule may flag the MsMpEng.exe process when it scans or updates its engine definitions. As a core security component running with high privileges on all endpoints, its frequent memory modifications and network activity often mimic the behavioral signature of the threat this rule targets.
  • Filter/Exclusion: Create an exclusion for the executable path C:\Program Files\Windows Defender\MsMpEng.exe within the detection logic to ignore events where the parent process is svchost.exe or services.exe.

• Scenario: Scheduled Office 365 Click-to-Run Updates

  • Context: The Microsoft Office Click-to-Run service (officec2rclient.exe) frequently executes background tasks during business hours to update the MASM (Microsoft Application Security Management) components or install patches. These updates involve loading specific DLLs that match the rule’s heuristic for suspicious module loading.
  • Filter/Exclusion: Exclude events where the process name is officec2rclient.exe and the command line contains arguments related to “Update” or “ClickToRun”. Additionally, suppress alerts during known maintenance windows (e.g., 01:00–04:00 local time).

• Scenario: Enterprise Endpoint Management (SCCM/Intune) Deployment

  • Context: When System Center Configuration Manager (SCCM) or Microsoft Intune pushes application deployments, the ccmexec.exe process often spawns child processes to install software. These installations frequently involve loading 32-bit assembly modules that trigger the TASM32 component of