PellesC300400450EXEX86CRTLIB

Hunt Hypothesis

This detection identifies 86-bit executable artifacts linked to the Pelles C runtime library, which may indicate the presence of legacy or custom-compiled tools often leveraged by adversaries for persistence and code execution within Azure Sentinel environments. Proactively hunting for these specific binaries allows the SOC team to distinguish between benign development artifacts and potential malicious implants that exploit standard C libraries to evade traditional signature-based detection.

YARA Rule

rule PellesC300400450EXEX86CRTLIB
{
      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 68 00 00 00 02 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

Here are 4 specific false positive scenarios for the PellesC300400450EXEX86CRTLIB detection rule, including suggested filters and exclusions:

• Scenario: Legacy Build Tool Execution

  • Context: The YARA rule flags the standard C runtime library (msvcr*.dll) associated with 32-bit executables. In an enterprise environment using PellesC or legacy build systems (e.g., older versions of MSBuild, Visual Studio 2015/2017), the compiler or linker often loads this specific CRT library during compilation tasks.
  • Suggested Filter: Exclude processes where the parent process is devenv.exe (Visual Studio) or msbuild.exe, and the file path contains \Program Files\Microsoft Visual Studio\ or \PellesC\.

• Scenario: Scheduled Database Maintenance Jobs

  • Context: Many database administration tools, such as SQL Server Agent running custom maintenance scripts or Oracle backup utilities, utilize 32-bit helper processes that rely on the standard C runtime. These scheduled jobs often trigger the rule during peak maintenance windows (e.g., early morning batch processing).
  • Suggested Filter: Create an exclusion for process names containing sqlagent, oracle.exe, or specific custom job executables located in \Program Files\Microsoft SQL Server\MSSQL*\MSSQL\Binn\ when the execution time falls within defined maintenance windows (e.g., 02:00–06:00).

• Scenario: Antivirus and Endpoint Protection Scanning

  • Context: Real-time protection engines like Symantec Endpoint Protection, McAfee, or CrowdStrike Falcon frequently spawn 32-bit scanning components that load the `