Microsoft C Hint DOS EP

Hunt Hypothesis

This hypothesis targets the presence of executable files exhibiting a Microsoft C Hint DOS Entry Point, a characteristic often associated with legacy or minimally packed binaries that adversaries may use to evade standard signature-based detection. Proactively hunting for these artifacts in Azure Sentinel helps identify low-severity, potentially overlooked malicious payloads or staging tools that leverage unusual entry point structures to blend in with legitimate system files.

YARA Rule

rule Microsoft_C_Hint_DOS_EP: PEiD
{
    strings:
        $a = { B4 30 CD 21 3C 02 73 ?? B8 }
    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

• Legacy C Compiler Binaries in Development Environments: Developers using older versions of Microsoft Visual C++ (e.g., VC6 or early VS2005) or standalone C compilers like gcc (when cross-compiling for Windows) may generate executables that retain the DOS header hint structure. This is common in CI/CD pipelines where build artifacts are executed directly on Windows agents.

  • Filter/Exclusion: Exclude paths under common development directories such as C:\dev\, C:\builds\, C:\jenkins\workspace\, or any directory containing node_modules, venv, or .git if the executable is a build artifact. Additionally, exclude executables owned by service accounts like jenkins-svc or build-agent.

• Third-Party Utility Tools with Static Linking: Many small, standalone Windows utilities (e.g., 7z.exe from older 7-Zip versions, curl.exe from specific distributions, or wget.exe) are compiled with older toolchains or static linking that may leave the DOS EP hint intact. These are often placed in C:\Program Files\ or C:\Tools\ by IT for administrative use.

  • Filter/Exclusion: Maintain a whitelist of known legitimate third-party tools by their full file path (e.g., C:\Program Files\7-Zip\7z.exe, C:\Tools\curl\curl.exe). Exclude these specific hashes or paths if they are not in a user-writable directory.

• Scheduled Maintenance Scripts Executing Compiled C Tools: IT operations teams often schedule maintenance jobs (via Task Scheduler) that run small C-based utilities for disk cleanup, log rotation, or network diagnostics (e.g., chkdsk.exe wrappers, custom logrotate equivalents). These executables, if compiled with older MSVC versions