MSVisualCv8DLLhsmallsig1

Hunt Hypothesis

This detection identifies potentially malicious or outdated Visual C++ 2005 DLLs with small signatures that may indicate early-stage supply chain compromises or legacy component exploitation within Azure Sentinel workloads. Proactive hunting for these artifacts is essential to uncover stealthy adversary activity that often evades standard signature-based defenses, allowing the SOC team to assess and remediate vulnerable dependencies before they are leveraged in a broader attack campaign.

YARA Rule

rule MSVisualCv8DLLhsmallsig1
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 8B FF 55 8B EC 83 7D 0C 01 75 05 E8 [3] FF 5D E9 D6 FE FF FF CC CC CC CC CC }

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 MSVisualCv8DLLhsmallsig1 rule, including targeted filters and exclusions:

• Scenario: Automated Deployment via SCCM/Intune

  • Context: System Center Configuration Manager (SCCM) or Microsoft Intune pushes a silent installation of a legacy application (e.g., Adobe Acrobat Reader DC or older ERP clients) that includes the msvcp80.dll and msvcr80.dll libraries. The YARA rule triggers on the DLL signature during the extraction phase by the deployment engine (ccmsetup.exe).
  • Filter/Exclusion: Create an exclusion for processes spawned directly by ccmexec.exe or Microsoft.IntuneManagementExtension.exe. Alternatively, filter out alerts where the parent process is a known deployment tool and the file path resides within the standard Windows installation directory (e.g., C:\Program Files\Microsoft Deployment Toolkit\).

• Scenario: Scheduled Antivirus Definition Updates

  • Context: The enterprise endpoint protection suite (e.g., Microsoft Defender for Endpoint or Symantec Endpoint Protection) performs a scheduled daily update. During the update cycle, the AV engine extracts and validates new definition packages which often contain Visual C++ 8 runtime components, triggering the rule on the temporary extraction folder (C:\ProgramData\Microsoft\Windows Defender\Updates\).
  • Filter/Exclusion: Exclude file paths containing Windows Defender or Symantec directories. Additionally, add a time-based filter to suppress alerts during known maintenance windows (e.g., 02:00 – 04:00 local time) when these updates typically occur.

• Scenario: Legacy Reporting Tool Execution

  • Context: A business intelligence tool like Crystal Reports or an older internal reporting dashboard runs a nightly job to generate