FSGv131

Hunt Hypothesis

This hypothesis targets the presence of specific file signatures or in-memory patterns associated with the FSGv131 YARA rule, which may indicate the execution of a low-severity tool or script used for initial access or post-exploitation activities. Proactively hunting for this indicator in Azure Sentinel allows the SOC team to identify potentially benign or overlooked artifacts that could serve as footholds for adversaries, ensuring that low-fidelity signals are not missed during routine monitoring.

YARA Rule

rule FSGv131
{
      meta:
		author="malware-lu"
strings:
		$a0 = { BB D0 01 40 00 BF 00 10 40 00 BE [4] 53 BB [4] B2 80 A4 B6 80 FF D3 73 F9 33 C9 }

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

• Scenario: A developer or DevOps engineer runs a local build script using dotnet build or msbuild on a workstation that has a shared network drive mapped to a source code repository. The YARA rule may match the temporary .dll or .exe artifacts generated in the bin/Debug or bin/Release folders if they contain specific string patterns or metadata associated with the FSGv131 signature.

  • Filter/Exclusion: Exclude files located in standard build output directories (e.g., *\bin\Debug\*, *\bin\Release\*, *\obj\*) or exclude processes named dotnet.exe, msbuild.exe, and vstest.console.exe when the file path contains these keywords.

• Scenario: An IT administrator performs a routine backup of a SQL Server database using sqlcmd or a custom PowerShell script that writes .bak or .dat files to a network share. If the backup file is opened or scanned by an AV/YARA engine while being written, or if the rule targets specific binary structures present in the backup metadata, it may trigger a false positive.

  • Filter/Exclusion: Exclude files with extensions .bak, .dat, .trn, or .mdf when the parent process is sqlservr.exe, sqlcmd.exe, or powershell.exe and the file path contains \Backup\ or \SQL\.

• Scenario: A security team runs a vulnerability scanner (e.g., Nessus, Qualys, or OpenVAS) that creates temporary probe files or stores scan results in a local cache directory. These files often contain embedded strings or binary headers that mimic the patterns targeted by FSGv131, especially if the rule looks for specific magic bytes or version strings.

  • **Filter/Exclusion