PellesC280290EXEX86CRTLIB

Hunt Hypothesis

This rule identifies executable files compiled with the Pelles C compiler, a tool frequently utilized by threat actors to build lightweight, custom backdoors or payload droppers that may evade standard signature-based detections. Proactively hunting for these artifacts in Azure Sentinel allows the SOC to uncover potential low-and-slow implantations or staging activities that leverage non-standard toolchains to blend in with legitimate development environments.

YARA Rule

rule PellesC280290EXEX86CRTLIB
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 89 E5 6A FF 68 [4] 68 [4] 64 FF 35 [4] 64 89 25 [4] 83 EC ?? 83 EC ?? 53 56 57 89 65 E8 68 00 00 00 ?? E8 [4] 59 A3 }

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 small utility or test harness using Pelles C (a free, lightweight C compiler for Windows) on a workstation or build server to generate a 32-bit executable for legacy API testing.

  • Filter/Exclusion: Exclude files located in specific developer workspace directories (e.g., C:\Users\<User>\Projects\, C:\Builds\) or exclude processes where the parent process is a known IDE or compiler wrapper (e.g., cmd.exe, powershell.exe) and the file extension is .exe with a size less than 1MB, provided the file path does not match standard system directories like C:\Windows\System32.

• Scenario: An IT support technician runs a custom diagnostic script or a small utility compiled with Pelles C to troubleshoot network connectivity or registry issues on a user’s machine, often executed from a temporary folder or a shared network drive.

  • Filter/Exclusion: Exclude executions where the parent process is a standard administrative tool (e.g., mstsc.exe, wmic.exe, powershell.exe) and the file path contains keywords like temp, tmp, diagnostic, or tool in the directory name. Additionally, exclude if the file’s digital signature is valid or if the file is located in a designated “Tools” share on the file server.

• Scenario: A scheduled maintenance job or a third-party backup agent uses a small, statically linked C utility (compiled with Pelles C) to perform file system checks, log rotation, or data compression tasks in the background.

  • Filter/Exclusion: Exclude files that are executed by a known service account (e.g., NT AUTHORITY\SYSTEM, DOMAIN\svc_backup) and are located in standard application data or program files directories associated with the backup or