PellesC300400450EXEX86CRTDLL

Hunt Hypothesis

This detection identifies potential malicious activity involving 32-bit executable or DLL artifacts associated with the Pelles C compiler runtime library, which may indicate the presence of custom-compiled tools or obfuscated malware components. SOC teams should proactively hunt for these indicators in Azure Sentinel to uncover low-severity anomalies that could represent early-stage adversary tooling or supply chain compromises often overlooked by standard signature-based defenses.

YARA Rule

rule PellesC300400450EXEX86CRTDLL
{
      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 ?? 53 56 57 89 65 E8 C7 45 FC [4] 68 [4] E8 [4] 59 BE [4] EB }

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

False Positive Scenarios for Rule: PellesC300400400EXEX86CRTDLL

• Scenario 1: Legacy Administrative Tool Execution

  • Context: The rule detects the loading of a specific CRT DLL (msvcrt.dll or similar) by an older, 32-bit administrative utility (e.g., Pelles C Compiler Suite, Visual Studio 2015/2017 legacy tools, or custom in-house deployment scripts). These tools often run as standalone .exe processes that dynamically load the CRT library during routine configuration checks.
  • Filter/Exclusion: Exclude process names matching PellesC.exe, vstest.console.exe, and any executable located within the C:\Program Files (x86)\Microsoft Visual Studio\* directory path, provided the parent process is a known administrative account (e.g., SYSTEM or domain admins).

• Scenario 2: Scheduled Backup Agent Initialization

  • Context: A scheduled task runs nightly to initialize the backup engine (e.g., Veeam Agent, Acronis Cyber Protect, or Commvault). During initialization, the agent’s 32-bit service component loads the CRT DLL to handle file I/O operations before the main backup job commences. This often triggers the rule during off-hours when no user activity is expected.
  • Filter/Exclusion: Create a time-based exclusion for this specific YARA signature between 01:00 and 05:00 local time, or exclude process paths containing \BackupAgent\, \Veeam\, and \Commvault\.

• Scenario 3: Antivirus Engine Real-Time Scanning

  • Context: The endpoint protection suite (e.g., CrowdStrike,