EXELOCK 666 15 Hint DOS EP

Hunt Hypothesis

This YARA rule identifies executable files that have been packed or protected by the EXELOCK 666 variant, a technique often used by adversaries to obscure the entry point and hinder static analysis of malicious payloads. Proactively hunting for these signatures in Azure Sentinel allows the SOC team to detect potentially obfuscated binaries early, reducing the risk of undetected malware execution or persistence mechanisms within the environment.

YARA Rule

rule EXELOCK_666_15_Hint_DOS_EP: PEiD
{
    strings:
        $a = { BA ?? ?? BF ?? ?? EB ?? EA ?? ?? ?? ?? 79 ?? 7F ?? 7E ?? 1C ?? 48 78 ?? E3 ?? 45 14 ?? 5A E9 }
    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: Execution of legacy 16-bit Windows applications (e.g., wine wrappers, ntos16.exe, or old MFC-based tools like Notepad++ 32-bit builds) that retain a DOS MZ header for backward compatibility.
  • Filter/Exclusion: Exclude processes where the Image path contains \SysWOW64\ or \Windows\Sysnative\, or specifically exclude known legacy application binaries such as wine.exe, ntos16.exe, and wow64cpu.dll host processes.
• Scenario: Execution of cross-platform development tools or compilers (e.g., gcc, g++, clang, or msbuild.exe) that generate or execute hybrid binaries with DOS stubs for Windows compatibility.
  • Filter/Exclusion: Exclude processes originating from standard developer toolchains directories (e.g., C:\Program Files (x86)\Microsoft Visual Studio\, C:\msys64\, C:\cygwin64\) or match against specific executable names like cc1.exe, gcc.exe, and msbuild.exe.
• Scenario: Scheduled maintenance jobs or backup agents (e.g., VeeamBackup.exe, CommCell.exe, or AcronisBackup.exe) that use 32-bit helper executables to interact with legacy file systems or drivers.
  • Filter/Exclusion: Exclude processes where the CommandLine contains keywords like backup, restore, snapshot, or maintenance, and the Image path resides in vendor-specific directories (e.g., C:\Program Files (x86)\Veeam\, C:\Program Files (x86)\Commvault\).
• Scenario: Execution of Java Virtual Machine (JVM) or .