msvcrt WIN8x86

Hunt Hypothesis

The hypothesis is that the detection of the msvcrt_WIN8x86 YARA rule indicates potential exploitation of a known Windows runtime library, which may be used by adversaries to execute malicious code or evade detection. A SOC team should proactively hunt for this behavior in Azure Sentinel to identify and mitigate potential compromise of systems running Windows environments.

YARA Rule

rule msvcrt_WIN8x86 {
    meta:
        author = "Jaume Martin"
    condition:
        hash.md5(0, filesize) == "95490c2b284a9bb63f0ee49254ab727e"
}

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
• Source Rule

False Positive Guidance

• Scenario: Windows Update installation using Windows Update Agent
  Filter/Exclusion: process.name != "wuauserv.exe" or process.parent.name == "svchost.exe"

• Scenario: Scheduled Task running a legitimate script or batch file
  Filter/Exclusion: process.name != "schtasks.exe" or file.hash in (known legitimate script hashes)

• Scenario: Microsoft Visual C++ Redistributable installation
  Filter/Exclusion: process.name == "msiexec.exe" and file.name == "vcredist_x86.exe"

• Scenario: System file check (sfc /scannow) initiated by an administrator
  Filter/Exclusion: process.name == "svchost.exe" and command_line contains "sfc /scannow"

• Scenario: Microsoft .NET Framework installation or repair
  Filter/Exclusion: process.name == "msiexec.exe" and file.name contains "dotnetfx.exe" or file.name contains "netfx.exe"