PeCompact 2xx Slim Loader BitSum Technologies additional

Hunt Hypothesis

This rule identifies the presence of BitSum Technologies’ PeCompact 2xx Slim Loader, a common executable compression tool that adversaries frequently use to reduce the size of malware payloads and evade basic static analysis. Proactively hunting for this specific loader in Azure Sentinel allows the SOC to detect potentially obfuscated binaries early in the kill chain, ensuring that compressed executables are inspected for hidden malicious code before they can execute or spread across the environment.

YARA Rule

rule PeCompact_2xx_Slim_Loader_BitSum_Technologies_additional: PEiD
{
    strings:
        $a = { B8 ?? ?? ?? 02 50 64 FF 35 00 00 00 00 64 89 25 00 00 00 00 33 C0 89 08 50 45 43 6F 6D 70 61 63 74 32 00 }
    condition:
        $a 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 uses BitSum Technologies PECompact 2.20 to compress a custom C++ application or a large .NET assembly to reduce its disk footprint and improve load times before deployment to a staging environment.
  • Filter/Exclusion: Exclude files located in specific development or build directories (e.g., C:\Builds\, C:\Src\, D:\Projects\) or exclude processes initiated by known build tools like MSBuild, Jenkins, or TeamCity agents.
• Scenario: An IT administrator manually compresses a legacy line-of-business application executable (e.g., LegacyApp.exe) using the PECompact GUI or command-line tool to optimize performance on older endpoints, saving the compressed file to a shared network drive or local C:\Tools\ directory.
  • Filter/Exclusion: Exclude files with extensions .exe or .dll located in administrative tool directories (e.g., C:\Tools\, C:\Admin\, C:\Apps\) or exclude events where the source process is explorer.exe or cmd.exe interacting with paths containing Tools or Admin.
• Scenario: A scheduled maintenance job (e.g., a Windows Task Scheduler task named Nightly_Compress_Apps) runs a PowerShell script or batch file that invokes pecompact.exe to compress a set of application binaries as part of a disk space optimization routine.
  • Filter/Exclusion: Exclude events where the parent process is schtasks.exe, taskschd.msc, or powershell.exe and the command line contains pecompact or pecompact. Additionally, exclude files modified within the last 24 hours if the task runs daily.
• Scenario: A software vendor or internal QA team