FSGv131Engdulekxt

Hunt Hypothesis

This detection identifies specific file artifacts matching the FSGv131Engdulekxt signature, which may indicate early-stage adversary activity involving known malicious binaries or scripts within the environment. Proactively hunting for this pattern in Azure Sentinel allows the SOC team to validate potential low-severity indicators before they escalate into broader incidents, ensuring comprehensive coverage of subtle threat signals that automated rules might overlook.

YARA Rule

rule FSGv131Engdulekxt
{
      meta:
		author="malware-lu"
strings:
		$a0 = { BB D0 01 40 00 BF 00 10 40 00 BE [3] 00 53 BB [3] 00 B2 80 A4 B6 80 FF D3 73 F9 33 C9 FF D3 73 16 33 C0 FF D3 73 23 B6 80 41 B0 10 FF D3 12 C0 73 FA 75 42 AA EB E0 E8 46 00 00 00 02 F6 83 D9 01 75 10 E8 38 00 00 00 EB 28 AC D1 E8 74 48 13 C9 EB }

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 FSGv131Engdulekxt YARA rule, including suggested filters and exclusions:

• Microsoft Defender Antivirus Signature Update Service

  • Scenario: The MpCmdG.exe process (Microsoft Security Client) frequently executes new signature definitions during its scheduled update cycle. The YARA rule may flag the unpacking or execution of these compressed definition files as suspicious behavior due to their high entropy and dynamic loading patterns, which mimic malware evasion techniques.
  • Filter/Exclusion: Exclude processes where ProcessName equals MpCmdG.exe AND CommandLine contains UpdateService. Alternatively, add a whitelist for the specific file hash of the latest signature update package located in C:\ProgramData\Microsoft\Windows Defender\Platform.

• Sysmon Scheduled Log Collection Job

  • Scenario: A nightly scheduled task (Task Scheduler) runs a custom PowerShell script to aggregate logs from multiple endpoints. The script utilizes the .NET runtime and invokes System.Management.Automation assemblies, which can trigger the rule if it detects specific string patterns or memory allocation behaviors associated with the “Engdulekxt” signature logic.
  • Filter/Exclusion: Create an exclusion for processes spawned by TaskHost.exe where the parent process is svchost.exe. Specifically, filter out events where the CommandLine contains keywords like -ExecutionPolicy Bypass and references to internal log paths (e.g., \\fileserver\logs\).

• Jenkins CI/CD Pipeline Artifact Generation

  • Scenario: During automated build pipelines on a Windows-based build server, Jenkins agents compile code and generate deployment artifacts. The rule may trigger when the Java Virtual Machine (java.exe) or associated Maven/Gradle wrappers extract large temporary archives containing embedded scripts that match the YARA pattern’s