FSGv13

Hunt Hypothesis

This detection identifies potential file-based threats matching the specific FSGv13 signature pattern across Azure Sentinel workloads to uncover early-stage malware or suspicious artifacts. Proactive hunting for this rule is essential because its low severity classification may cause it to be overlooked in standard alerting, requiring manual investigation to prevent silent lateral movement by adversaries leveraging known file signatures.

YARA Rule

rule FSGv13
{
      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 B2 80 A4 6A 02 5B FF 14 24 73 F7 33 C9 FF 14 24 73 18 33 C0 FF 14 24 73 21 B3 02 41 B0 10 FF 14 24 12 C0 73 F9 75 3F AA EB DC E8 43 00 00 00 2B CB 75 10 E8 38 00 }

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 FSGv13 detection rule, including suggested filters and exclusions tailored for an enterprise environment:

• Scenario: Legitimate deployment of security patches via Microsoft Endpoint Configuration Manager (SCCM) or Intune.

  • Context: The YARA rule triggers when the ccmexec.exe process launches a child process to download or install updates, mimicking the behavior pattern FSGv13 is designed to catch.
  • Filter/Exclusion: Exclude processes where the parent executable path matches C:\Program Files\Microsoft Intune Management Extension\IntuneManagementExtensionService.exe or C:\Windows\CCM\ccmexec.exe. Additionally, add a filter for file hashes known to be signed by “Microsoft Corporation” with a valid timestamp.

• Scenario: Scheduled antivirus scanning jobs running as a background service (e.g., Microsoft Defender Antivirus).

  • Context: During peak business hours or scheduled maintenance windows, the MsMpEng.exe process scans user directories and may spawn temporary worker processes that match the FSGv13 signature logic.
  • Filter/Exclusion: Create an exclusion for any detection where the parent process is MsMpEng.exe. Furthermore, implement a time-based filter to suppress alerts generated between 02:00 and 04:00 local time when scheduled scans typically occur.

• Scenario: Execution of automated backup scripts using enterprise tools like Veeam or Commvault.

  • Context: Backup agents often spawn multiple child processes to compress data streams, which can trigger the rule if the YARA signature matches specific binary structures used by these agents during active transfer phases.
  • Filter/Exclusion: Exclude process trees where the root parent is VeeamTransportService.exe or CommServe.exe.