msvcrt Win7x86

Hunt Hypothesis

The hypothesis is that the detection of the msvcrt_Win7x86 YARA rule may indicate the presence of legacy Windows 7 x86-specific malware or obfuscated code leveraging Windows API functions. A SOC team should proactively hunt for this behavior in Azure Sentinel to identify potential targeted attacks or persistence mechanisms that may bypass modern detection controls.

YARA Rule

rule msvcrt_Win7x86 {
    meta:
        author = "Jaume Martin"
    condition:
        hash.md5(0, filesize) == "7713e5c5a48b020c9575b1b50f2e5e9e"
}

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 Microsoft Update Assistant
  Filter/Exclusion: process.name != "UpdateAssistant.exe" or process.parent.name == "svchost.exe"

• Scenario: Scheduled Task running a legitimate system maintenance script (e.g., schtasks.exe executing cleanmgr.exe)
  Filter/Exclusion: process.name == "cleanmgr.exe" and process.parent.name == "schtasks.exe"

• Scenario: Microsoft .NET Framework installation via the Microsoft Installer (MSI)
  Filter/Exclusion: process.name == "msiexec.exe" and command_line contains "dotnetfx" or "netfx"

• Scenario: System File Checker (SFC) scan initiated by an administrator
  Filter/Exclusion: process.name == "sfc.exe" or process.parent.name == "explorer.exe"

• Scenario: Antivirus or endpoint protection software performing a system scan (e.g., Windows Defender or Bitdefender)
  Filter/Exclusion: process.name contains "WindowsDefender" or process.name contains "Bitdefender"