MSVisualCv8DLLhsmallsig2

Hunt Hypothesis

This hunt detects potential malicious or compromised Visual C++ 8 DLLs that exhibit small signature anomalies often associated with early-stage fileless attacks or legacy library exploitation. Proactively hunting for these specific artifacts in Azure Sentinel is critical to identify subtle indicators of compromise before they escalate into full-scale lateral movement, especially within environments relying on older Microsoft runtime components.

YARA Rule

rule MSVisualCv8DLLhsmallsig2
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 8B FF 55 8B EC 53 8B 5D 08 56 8B 75 0C 85 F6 57 8B 7D 10 0F 84 [2] 00 00 83 FE 01 }

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 5 specific false positive scenarios for the MSVisualCv8DLLhsmallsig2 detection rule, including suggested filters and exclusions:

• Scenario: Deployment of legacy internal line-of-business (LOB) applications packaged with Microsoft Visual C++ 2005 Redistributable.

  • Context: Many older enterprise tools (e.g., custom inventory scanners or specialized HR portals) rely on msvcp80.dll and mfc80.dll. When these applications are installed via SCCM or Intune, the small signature of these DLLs often triggers the rule.
  • Filter/Exclusion: Create a file hash exclusion for the specific hashes of C:\Windows\WinSxS\*\msvcp80.dll and C:\Windows\WinSxS\*\mfc80.dll. Alternatively, exclude the installation paths of known LOB applications (e.g., C:\Program Files\LegacyHRApp\bin\).

• Scenario: Execution of scheduled backup jobs using legacy agents.

  • Context: Backup solutions like Veeam Agent or older versions of Acronis TrueImage often utilize Visual C++ 8 runtime libraries for their background services. During the nightly maintenance window, these services load small DLLs that match the rule’s signature criteria.
  • Filter/Exclusion: Implement a time-based exclusion (e.g., 02:00 – 06:00 local time) specifically for processes running under the SYSTEM or NT SERVICE\VeeamAgent accounts. Additionally, exclude the process image path C:\Program Files\BackupAgent\bin\*.exe.

• Scenario: Automated patching cycles by Microsoft Endpoint Configuration Manager (SCCM).

  • Context: During monthly Windows Update deployments, SCCM agents often extract and