MASM32

Hunt Hypothesis

This detection identifies potential adversary activity involving custom or obfuscated 32-bit assembly code that may evade standard signature-based defenses by leveraging low-level execution patterns often missed by high-level heuristics. A SOC team should proactively hunt for this behavior in Azure Sentinel to uncover stealthy, resource-efficient malware or rootkits that utilize MASM32 constructs to establish persistence and execute malicious logic without triggering conventional alerts.

YARA Rule

rule MASM32: Packer PEiD
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 6A ?? 68 00 30 40 00 68 ?? 30 40 00 6A 00 E8 07 00 00 00 6A 00 E8 06 00 00 00 FF 25 08 20 }

condition:
		$a0 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

Here are 4 specific false positive scenarios for the MASM32 detection rule in an enterprise environment, along with recommended filters or exclusions:

• Legacy Financial Reporting Tool Execution

  • Scenario: The finance department utilizes a legacy custom reporting application (e.g., “FinReport v4.0”) compiled using MASM32 assembly libraries to generate end-of-month tax documents. This tool is launched manually by analysts or via a scheduled task, triggering the YARA rule due to its embedded MASM32 signatures.
  • Filter/Exclusion: Add an exclusion for the specific executable path C:\Program Files\FinReport\bin\report_gen.exe and whitelist the associated SHA-256 hash in the detection logic.

• Automated Hardware Diagnostics Job

  • Scenario: A nightly scheduled job runs a hardware health check utility (e.g., “Dell Command | Update” or a custom BIOS updater) that includes low-level MASM32 components for memory and CPU stress testing. The rule triggers during the execution of these diagnostic scripts on all endpoint workstations.
  • Filter/Exclusion: Create a filter to exclude processes spawned by the specific scheduled task name Dell_Diagnostic_Nightly or exclude any process running under the user context SYSTEM within the C:\ProgramData\Dell\Tools directory.

• Development Environment Build Pipeline

  • Scenario: The internal DevOps team runs a CI/CD pipeline (e.g., Jenkins or Azure DevOps) that compiles legacy C++ modules using MASM32 assemblers during the “Build” phase. This occurs on build agents where the rule is active, causing alerts every time a pull request triggers a compilation job.
  • Filter/Exclusion: Exclude all processes running within the specific container or agent group `