FreeBASIC016b

Hunt Hypothesis

This YARA rule identifies the presence of FreeBASIC compiler artifacts, which adversaries may use to compile lightweight, obfuscated payloads for initial access or lateral movement. Proactively hunting for these signatures in Azure Sentinel allows the SOC to detect low-and-slow threats that traditional behavioral detections might miss, particularly in environments where custom scripting or niche development tools are not standard.

YARA Rule

rule FreeBASIC016b
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 89 E5 83 EC 08 C7 04 24 01 00 00 00 FF 15 [3] 00 E8 88 FF FF FF 89 EC 31 C0 5D C3 89 F6 55 89 E5 83 EC 08 C7 04 24 02 00 00 00 FF 15 [3] 00 E8 68 FF FF FF 89 EC 31 C0 5D C3 89 F6 55 89 E5 83 EC 08 8B 45 08 89 04 24 FF 15 [3] 00 89 EC 5D C3 8D 76 00 8D BC 27 00 00 00 00 55 89 E5 83 EC 08 8B 45 08 89 04 24 FF 15 [3] 00 89 EC 5D C3 90 90 90 90 90 90 90 90 90 90 }

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 build engineer compiles a large FreeBASIC application using the fbc compiler within a local development environment or a specific CI/CD agent that is not fully isolated. The resulting binary or intermediate object files may contain FreeBASIC-specific string markers or section headers that match the YARA signature.

  • Filter/Exclusion: Exclude processes or files located in known development directories (e.g., C:\dev\, C:\src\, C:\builds\) or exclude the fbc.exe and fbc.exe child processes if the rule targets the compiler itself. Alternatively, exclude files with the .fb extension or specific build output directories like C:\build\output\.

• Scenario: An IT administrator deploys a custom internal utility or script written in FreeBASIC to automate a niche task (e.g., a specific hardware diagnostic tool or a legacy API bridge) that is distributed via Group Policy or a software deployment tool (like SCCM or Intune) to multiple endpoints. The binary is executed from a standard system directory but retains the FreeBASIC signature.

  • Filter/Exclusion: Create an allowlist for the specific hash (SHA256) of the known internal utility binary. Alternatively, exclude execution from known deployment paths such as C:\Windows\Temp\ (if deployed there) or C:\Program Files\InternalTools\ if the tool is installed in a dedicated folder.

• Scenario: A legacy application or a specific plugin for a larger software suite (e.g., a media player plugin, a game mod, or a specialized data converter) is written in FreeBASIC. When the user launches the main application, the FreeBASIC component is loaded or executed as a child process or DLL, triggering the detection.

  • Filter/Exclusion: Exclude the specific