MASM32 / TASM32

Hunt Hypothesis

This rule detects the execution of legacy 32-bit assembly compilers like MASM or TASM, which adversaries may use to compile custom shellcode or low-level payloads that bypass standard language-based detections. Proactively hunting for these rare compiler invocations in Azure Sentinel helps identify stealthy post-exploitation activities where attackers attempt to minimize their footprint by avoiding common scripting or interpreted languages.

YARA Rule

rule masm32_tasm32
{
	meta:
		author = "PEiD"
		description = "MASM32 / TASM32"
		group = "20"
		function = "0"
	strings:
		$a0 = { 6A ?? E8 ?? ?? ?? ?? A3 }
	condition:
		$a0
}

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 Driver Compilation by Vendor Support: A third-party hardware vendor (e.g., for specialized industrial scanners or legacy POS systems) provides a support engineer with a script to recompile a custom .asm driver using ml.exe (MASM) or tasm.exe (TASM) to apply a hotfix for a specific firmware bug.
  • Filter/Exclusion: Allow execution of ml.exe or tasm.exe when the parent process is a known vendor support tool (e.g., vendor_support_tool.exe) or when the working directory contains specific vendor-specific artifacts (e.g., driver_hotfix_v2.1\).
• Embedded Assembly in CI/CD for Performance-Critical Libraries: A development team uses a scheduled Jenkins or Azure DevOps job to compile a small, performance-critical C++ library that includes inline assembly or separate .asm files for SIMD optimizations, invoking ml.exe via a wrapper script.
  • Filter/Exclusion: Exclude ml.exe when the parent process is a build agent (e.g., jenkins-agent.exe, azure-devops-agent.exe) and the command line arguments include standard build flags (e.g., /c, /coff) or reference paths within the src\lib\asm\ directory structure.
• Legacy Application Maintenance by IT Ops: An IT Operations technician runs a legacy internal tool (e.g., a custom inventory management system from the early 2000s) that uses tasm.exe to reassemble a small utility module during a scheduled maintenance window to update a hardcoded configuration string.
  • Filter/Exclusion: Allow tasm.exe when the parent process is the specific legacy application executable (e.g., legacy_inv_mgmt.exe) and the execution occurs during the defined maintenance window (e.g., Sunday 02:0