EXECryptor 239 minimum protection wwwstrongbitcom

Hunt Hypothesis

This rule identifies the presence of EXECryptor ransomware variants, which are known for encrypting files and leveraging weak protection mechanisms to evade initial detection. Proactively hunting for this specific signature allows the SOC to isolate potentially compromised hosts before the encryption process completes, minimizing data loss and operational downtime in the Azure environment.

YARA Rule

rule EXECryptor_239_minimum_protection_wwwstrongbitcom: PEiD
{
    strings:
        $a = { 68 ?? ?? ?? ?? E9 ?? ?? ?? FF 50 C1 C8 18 89 05 ?? ?? ?? ?? C3 C1 C0 18 51 E9 ?? ?? ?? FF 84 C0 0F 84 6A F9 FF FF E9 ?? ?? ?? FF C3 E9 ?? ?? ?? FF E8 CF E9 FF FF B8 01 00 00 00 E9 ?? ?? ?? FF 2B D0 68 A0 36 80 D4 59 81 C9 64 98 FF 99 E9 ?? ?? ?? FF 84 C0 }
    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 DevOps engineer runs a local build script that invokes a custom C/C++ compiler (e.g., gcc or clang) with specific optimization flags (like -O3 or -march=native) to generate a highly optimized binary for performance testing. The resulting executable may exhibit structural characteristics (such as specific section alignments or lack of standard debug symbols) that match the “minimum protection” heuristic of the EXECryptor 239 rule.

  • Filter/Exclusion: Exclude files located in standard build directories (e.g., C:\builds\, /tmp/build/, \\fileserver\dev\builds\) or exclude processes where the parent process is a known compiler (gcc.exe, clang.exe, msbuild.exe) and the file extension is .exe or .bin.

• Scenario: An IT administrator deploys a lightweight, custom-written utility script (e.g., a PowerShell-to-EXE wrapper using tools like PS2EXE or AutoIt) to automate a one-time cleanup task on a specific server. These wrapped executables often have minimal PE header structures and lack standard import tables, triggering YARA rules that look for “minimum protection” or stripped binaries.

  • Filter/Exclusion: Exclude files whose parent process is a known packaging tool (PS2EXE.exe, AutoIt3.exe) or exclude files created within the last 24 hours in administrative temp folders (e.g., C:\Users\Admin\Temp\, C:\Windows\Temp\) if the rule is not critical for real-time blocking.

• Scenario: A database administrator runs a vendor-provided diagnostic or repair tool (e.g., sqlservr.exe repair mode, oracle diag tools, or mysqldump with a custom wrapper) that is a statically