MASMTASM sig4 h

Hunt Hypothesis

This rule detects the presence of the MASMTASM malware signature, which is associated with the execution of malicious shellcode or payload components often used for initial access or privilege escalation. Proactively hunting for this signature in Azure Sentinel allows the SOC team to identify compromised hosts or memory dumps before the adversary can establish persistence or initiate lateral movement within the environment.

YARA Rule

rule MASMTASM_sig4_h: PEiD
{
    strings:
        $a = { FF 25 ?? ?? ?? 00 FF 25 ?? ?? ?? 00 FF 25 ?? ?? ?? 00 FF 25 ?? ?? ?? 00 FF 25 ?? ?? ?? 00 FF 25 ?? ?? ?? 00 FF 25 ?? ?? ?? 00 FF 25 ?? ?? ?? 00 FF 25 ?? ?? ?? 00 FF 25 ?? ?? ?? 00 FF 25 ?? ?? ?? 00 }
        $b = { C3 FF 25 ?? ?? ?? 00 FF 25 ?? ?? ?? 00 FF 25 ?? ?? ?? 00 FF 25 ?? ?? ?? 00 FF 25 ?? ?? ?? 00 FF 25 ?? ?? ?? 00 FF 25 ?? ?? ?? 00 FF 25 ?? ?? ?? 00 FF 25 ?? ?? ?? 00 FF 25 ?? ?? ?? 00 FF 25 ?? ?? ?? 00 }
    condition:
        for any of ($*) : ( $ 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: A legacy .NET application or internal tool uses a custom-built, obfuscated assembly loader that mimics the structural patterns of the MASMTASM signature (e.g., specific method names, resource sections, or IL bytecode sequences) to perform JIT compilation or dynamic assembly loading.

  • Filter/Exclusion: Exclude processes where the parent is a known .NET host (e.g., dotnet.exe, csc.exe, msbuild.exe) or where the loaded module path resides in a trusted application directory (e.g., C:\Program Files\InternalTools\).

• Scenario: A developer or CI/CD pipeline executes a build step that compiles mixed-mode C++/C# assemblies using csc.exe or msbuild.exe, where the intermediate assembly contains specific metadata or IL patterns that match the MASMTASM_sig4_h heuristic due to the use of unsafe code blocks or specific P/Invoke signatures.

  • Filter/Exclusion: Exclude detections where the parent process is msbuild.exe, dotnet.exe, or csc.exe, and the file path of the detected assembly ends in .dll or .exe within a build artifact directory (e.g., bin\Debug, obj\Release).

• Scenario: An enterprise Java or .NET application uses a JIT compiler or dynamic code generation library (such as System.Reflection.Emit or a custom AOT compiler) that generates assemblies with method bodies or resource sections that structurally resemble the target signature, particularly in high-throughput microservices.

  • Filter/Exclusion: Exclude detections for processes running under a known application pool (e.g., w3wp.exe with a specific AppPool name) or where the assembly is loaded from a temporary runtime cache directory (e.g., `C:\Users\Public\Documents