Hide PE 101 BGCorp

Hunt Hypothesis

This rule identifies obfuscated or hidden Portable Executable (PE) files linked to BGCorp, suggesting an adversary is employing file obfuscation techniques to evade static analysis and signature-based detection. Proactively hunting for these artifacts in Azure Sentinel allows the SOC to uncover stealthy persistence mechanisms or initial access payloads that may have slipped past standard endpoint monitoring.

YARA Rule

rule Hide_PE_101_BGCorp: PEiD
{
    strings:
        $a = { ?? BA ?? ?? ?? 00 B8 ?? ?? ?? ?? 89 02 83 C2 04 B8 ?? ?? ?? ?? 89 02 83 C2 04 B8 ?? ?? ?? ?? 89 02 83 C2 F8 FF E2 0D 0A 2D 3D 5B 20 48 69 64 65 50 45 20 62 79 20 42 47 43 6F 72 70 20 5D 3D 2D }
        $b = { BA ?? ?? ?? 00 B8 ?? ?? ?? ?? 89 02 83 C2 04 B8 ?? ?? ?? ?? 89 02 83 C2 04 B8 ?? ?? ?? ?? 89 02 83 C2 F8 FF E2 0D 0A 2D 3D 5B 20 48 69 64 65 50 45 20 62 79 20 42 47 43 6F 72 70 20 5D 3D 2D }
    condition:
        for any of ($*) : ( $ 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 2 string patterns in its detection logic.

• Source Rule

False Positive Guidance

• Scenario: Legitimate software installers or self-extracting archives (e.g., 7z.exe, Inno Setup installers, or NSIS scripts) that embed a PE header within a compressed or packed container, causing the YARA rule to match the underlying binary structure.
  • Filter/Exclusion: Exclude files with extensions .7z, .zip, .rar, or .exe located in standard installation directories (e.g., C:\Program Files\, C:\Temp\) if the parent process is a known installer (msiexec.exe, setup.exe, 7zG.exe).
• Scenario: Development and testing environments where developers use tools like UPX (Ultimate Packer for eXecutables) to compress binaries for size optimization or to test packing behavior, resulting in a PE file with a modified header that matches the “hidden” or obfuscated signature.
  • Filter/Exclusion: Exclude files in developer workspaces (e.g., C:\Users\<dev_user>\Projects\, C:\dev\) or when the parent process is a known IDE or build tool (devenv.exe, code.exe, msbuild.exe).
• Scenario: Scheduled maintenance jobs or backup agents (e.g., Veeam, Commvault, or custom PowerShell scripts) that create temporary PE-based executables or shadow copies in system temp directories (%TEMP%, C:\Windows\Temp) which may retain PE headers even when stored as data files.
  • Filter/Exclusion: Exclude files in %TEMP% or C:\Windows\Temp if the parent process is a known backup agent (VeeamAgent.exe, commvault.exe) or a scheduled task service (svchost.exe with specific service names).
• Scenario: Ant