PECompact2xxSlimLoaderBitSumTechnologies

Hunt Hypothesis

This detection identifies the execution of a slim loader utilizing BitSum Technologies’ PECompact compression, which adversaries often employ to obfuscate malicious payloads and evade signature-based defenses. Proactive hunting for this behavior in Azure Sentinel is essential because low-severity alerts from compressed loaders can mask sophisticated fileless attacks that bypass traditional static analysis until runtime.

YARA Rule

rule PECompact2xxSlimLoaderBitSumTechnologies
{
      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 }

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 PECompact2xxSlimLoaderBitSumTechnologies detection rule, including tailored filters and exclusions:

• Scenario: Scheduled Antivirus Definition Updates via Bitdefender GravityZone

  • Context: The enterprise uses Bitdefender GravityZone for endpoint protection. During nightly scheduled updates (e.g., 02:00 AM), the bdagent.exe or bdcore.exe processes utilize a SlimLoader PE header structure to inject updated virus definition signatures into memory without restarting the agent service.
  • Suggested Filter/Exclusion: Create an exclusion based on the parent process name and path.
    • Filter Logic: Exclude alerts where ParentProcessName is bdagent.exe OR bdcore.exe AND ParentPath contains \Program Files\Bitdefender\GravityZone\.

• Scenario: Deployment of Internal Tools using PowerShell Just-In-Time (JIT) Compilation

  • Context: The DevOps team runs a nightly PowerShell script (UpdateInternalTools.ps1) that compiles and loads internal micro-services. These scripts often invoke the .NET runtime which utilizes PECompact headers for optimized memory footprint during the JIT compilation of custom C# assemblies used by BitSum Technologies’ internal SDKs.
  • Suggested Filter/Exclusion: Exclude based on command line arguments and process lineage.
    • Filter Logic: Exclude if CommandLine contains -Command "UpdateInternalTools" AND ParentProcessName is powershell.exe. Additionally, whitelist the specific hash of the internal SDK assembly if known.

• Scenario: Automated Patching via Microsoft Endpoint Configuration Manager (SCCM)

  • Context: During the monthly patching window, SCCM deploys updates to the “BitSum Technologies” application suite. The deployment agent (`ccm