FSGv110EngdulekxtMicrosoftVisualBasicMASM32

Hunt Hypothesis

This detection identifies potential Visual Basic MASM32 scripts that may indicate early-stage adversary activity involving custom or obfuscated automation tools within the environment. Proactive hunting is essential to validate these low-severity signals and distinguish legitimate administrative tasks from stealthy initial access attempts that could evolve into more critical threats if left uninvestigated.

YARA Rule

rule FSGv110EngdulekxtMicrosoftVisualBasicMASM32
{
      meta:
		author="malware-lu"
strings:
		$a0 = { EB 02 09 94 0F B7 FF 68 80 [2] 00 81 F6 8E 00 00 00 5B EB 02 11 C2 8D 05 F4 00 00 00 47 }

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 FSGv110EngdulekxtMicrosoftVisualBasicMASM32 detection rule, which targets Microsoft Visual Basic MASM32 executables:

• Scenario: Legacy HR Payroll Processing via VBScript Wrapper

  • Context: The enterprise runs a legacy payroll calculation engine built in the 90s using Microsoft Visual Basic. This application is invoked nightly by a scheduled task (Task Scheduler) that wraps the .exe in a VBScript to handle data ingestion before launching the main MASM32 executable.
  • Filter/Exclusion: Create an exclusion based on the specific file path and parent process. Exclude any Microsoft Visual Basic processes (e.g., vb6.exe, msvbvm50.dll) located in C:\Apps\LegacyPayroll\ where the parent process is svchost.exe or cscript.exe.
  • Filter Logic: Path contains "C:\Apps\LegacyPayroll" AND ParentProcess IN ("svchost.exe", "cscript.exe").

• Scenario: Automated Report Generation by Crystal Reports

  • Context: The finance department uses a scheduled job to generate monthly compliance reports. This job utilizes the Crystal Reports runtime engine, which internally invokes MASM32 Visual Basic components to render charts and export data to PDF. These jobs run every morning at 08:00 via cron or Windows Task Scheduler.
  • Filter/Exclusion: Exclude processes spawned by the specific Crystal Reports service account (svc-crystal) running within the known application directory.
  • Filter Logic: UserAccount == "svc-crystal" AND Path starts with "C:\Program Files (x86)\SAP BusinessObjects\CrystalReports\".

• **Scenario: Endpoint Security Agent