FSGv10

Hunt Hypothesis

This detection identifies potential file-based threats matching the FSGv10 signature pattern through YARA analysis within Azure Sentinel’s workload data. Proactive hunting for this behavior is essential to uncover early-stage indicators of compromise that may evade standard alerting thresholds due to their low severity classification, ensuring comprehensive coverage against emerging malware variants.

YARA Rule

rule FSGv10
{
      meta:
		author="malware-lu"
strings:
		$a0 = { BB D0 01 40 00 BF 00 10 40 00 BE [4] 53 E8 0A 00 00 00 02 D2 75 05 8A 16 46 12 D2 C3 FC B2 80 A4 6A 02 5B }

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 FSGv10 detection rule, including targeted filters and exclusions suitable for an enterprise environment:

• Scenario: Legitimate deployment of Microsoft Defender Antivirus updates via Windows Update.

  • Context: The MpCmdRun.exe process frequently executes YARA scans on newly downloaded definition files in the C:\ProgramData\Microsoft\Windows Defender\Definition directory, which may contain signatures matching FSGv10 patterns.
  • Suggested Filter/Exclusion: Exclude processes where the executable path contains \Windows Defender\ and the parent process is usoc.dll.exe or TiWorker.exe. Alternatively, add a hash-based exclusion for known good update files in the definition folder.

• Scenario: Scheduled backup job execution using Veeam Backup & Replication.

  • Context: The nightly backup agent (Veeam.Backup.Service.exe) scans and archives large data sets containing embedded scripts or configuration files that trigger the FSGv10 logic due to specific byte patterns in metadata headers.
  • Suggested Filter/Exclusion: Create a process exclusion for Veeam.Backup.Service.exe running under the SYSTEM account, specifically when the file access path is within the backup repository directory (e.g., D:\BackupRepository\).

• Scenario: Automated software patching via SCCM (Microsoft Endpoint Configuration Manager).

  • Context: During the “Client Agent” update cycle, ccmsetup.exe or wuauserv.dll extracts temporary installation packages that include embedded PowerShell scripts matching the FSGv10 YARA rule criteria.
  • Suggested Filter/Exclusion: Exclude alerts where the parent process is CcmExec.exe and the file extension is .msi, .cab, or `.psd