PeCompact253DLLSlimLoaderBitSumTechnologies

Hunt Hypothesis

This rule identifies the presence of the PeCompact 2.53 DLL Slim Loader, a common tool used by adversaries to compress and pack malicious DLLs to evade static analysis and reduce file size. Proactively hunting for this indicator in Azure Sentinel allows the SOC to detect potential supply chain compromises or staged payloads that may be leveraging packed binaries to hide their true functionality within the environment.

YARA Rule

rule PeCompact253DLLSlimLoaderBitSumTechnologies
{
      meta:
		author="malware-lu"
strings:
		$a0 = { B8 [4] 50 64 FF 35 00 00 00 00 64 89 25 00 00 00 00 33 C0 89 08 50 45 43 32 00 00 08 0C 00 48 E1 01 56 57 53 55 8B 5C 24 1C 85 DB 0F 84 AB 21 E8 BD 0E E6 60 0D 0B 6B 65 72 6E 6C 33 32 }

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 custom C/C++ application using the PECompact 2.53 toolchain to reduce binary size for deployment on resource-constrained edge devices or IoT gateways. The resulting .dll or .exe file is copied to a shared network drive or deployed via a configuration management agent (e.g., Ansible, Puppet), triggering the YARA rule on the file system.

  • Filter/Exclusion: Exclude files located in specific build artifact directories (e.g., C:\Builds\, D:\Artifacts\) or files with a specific hash prefix if the build process is deterministic. Alternatively, whitelist the specific user account or service account performing the build/deployment if it is a known CI/CD pipeline identity.

• Scenario: An IT administrator manually updates a legacy line-of-business application that relies on a third-party DLL compiled with PECompact 2.53. The administrator downloads the updated DLL from a vendor portal and places it in the application’s bin or lib directory on a server, overwriting the previous version.

  • Filter/Exclusion: Create a file path exclusion for known legacy application directories (e.g., C:\Program Files\LegacyApp\bin\) or whitelist the specific file name if the DLL name is unique to that vendor. Ensure the exclusion is scoped to the specific server or group of servers running that legacy application.

• Scenario: A scheduled maintenance job (e.g., a PowerShell script run by a service account) performs a “clean and rebuild” of a local cache or temporary workspace where a developer has placed a test DLL compiled with PECompact 2.53. The file is created, scanned by the EDR agent, and then deleted, but the creation event triggers the detection.

  • Filter/Exclusion: Exclude files in temporary or cache directories