FSGv110EngdulekxtMASM32

Hunt Hypothesis

This detection identifies potential malicious activity where a 32-bit process exhibits specific behavioral patterns defined by the FSGv110EngdulekxtMASM32 YARA signature within the Azure Sentinel environment. Although currently flagged with low severity, proactively hunting for this indicator allows the SOC team to validate false positives and uncover early-stage threats that may evolve into higher-risk incidents if left unmonitored.

YARA Rule

rule FSGv110EngdulekxtMASM32
{
      meta:
		author="malware-lu"
strings:
		$a0 = { EB 01 DB E8 02 00 00 00 86 43 5E 8D 1D D0 75 CF 83 C1 EE 1D 68 50 ?? 8F 83 EB 02 3D 0F 5A }

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 5 specific false positive scenarios for the FSGv110EngdulekxtMASM32 detection rule, including suggested filters and exclusions:

• Scenario: Microsoft Defender Antivirus Real-Time Protection Scanning

  • Context: The YARA rule may flag the legitimate MsMpEng.exe process when it performs real-time scanning of executable files in the %ProgramFiles% directory. This often triggers during peak business hours when users are actively downloading or installing software, causing high I/O activity that matches the rule’s signature patterns for MASM32 execution.
  • Suggested Filter/Exclusion: Create a process exclusion for C:\Program Files\Windows Defender\MsMpEng.exe within the detection logic to ignore events where the parent process is MsMpEng.exe.

• Scenario: Scheduled System Update Deployment via SCCM

  • Context: During nightly maintenance windows, Microsoft Endpoint Configuration Manager (SCCM) or Windows Update for Business executes scheduled tasks that invoke MASM32-based installers to patch system libraries. These automated jobs often spawn temporary processes with signatures similar to the rule’s target, leading to alerts when no human interaction is present.
  • Suggested Filter/Exclusion: Exclude events occurring between 01:00 and 05:00 (local time) where the parent process is ccmexec.exe or usoc_dla.exe, as these are known orchestrators for automated updates.

• Scenario: Enterprise Backup Agent Execution

  • Context: Veeam Backup & Replication or Commvault agents frequently utilize MASM32 components to compress and encrypt data blocks before transmission. When the backup agent scans large file systems, it triggers the detection rule as a potential anomaly due to the high volume of assembly language execution patterns in its compression engine.