FSG v131

Hunt Hypothesis

This rule identifies potential fileless or script-based attack patterns associated with the FSG v131 YARA signature, which may indicate an adversary attempting to execute malicious code or establish a foothold within the environment. Proactively hunting for this indicator in Azure Sentinel allows the SOC to detect low-severity anomalies that could serve as early precursors to more complex intrusions or lateral movement activities.

YARA Rule

rule FSG_v131: PEiD
{
    strings:
        $a = { BB D0 01 40 00 BF 00 10 40 00 BE ?? ?? ?? ?? 53 BB ?? ?? ?? ?? B2 80 A4 B6 80 FF D3 73 F9 33 C9 }
    condition:
        $a 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 legitimate build script or containerization tool (e.g., docker build, packer build, or a custom Makefile wrapper) that temporarily writes a small, obfuscated helper script or binary to the system temp directory to perform a specific task (like checksum verification or dependency fetching). The YARA rule FSG_v131 may match the specific byte pattern or string signature of this transient helper.

  • Filter/Exclusion: Exclude processes originating from known CI/CD agent paths (e.g., C:\ProgramData\Jenkins\workspace\*, C:\agent\work\*) or exclude files located in standard temporary directories (%TEMP%, C:\Windows\Temp) if the parent process is a known build tool (e.g., docker.exe, packer.exe, make.exe).

• Scenario: An enterprise endpoint protection agent or a specific application updater (e.g., AdobeUpdater, MicrosoftUpdate, or a custom internal app installer) downloads a small, self-extracting archive or a temporary .cmd/.bat file to stage an update. The YARA rule might trigger on the specific header or string pattern of this staging file, especially if it uses a common compression or scripting format that matches the rule’s signature.

  • Filter/Exclusion: Exclude files with extensions .tmp, .part, or .download that are located in application-specific update directories (e.g., C:\Program Files\Adobe\Update\, C:\Windows\Installer\) and are created by known service accounts or processes (e.g., AdobeUpdateService.exe, msiexec.exe).

• Scenario: A system administrator uses a legitimate PowerShell one-liner or a small VBScript to perform a routine maintenance task, such as clearing event logs, restarting