CVE-2025-21373: Elevation of Privilege in Windows Installer Exposed

Microsoft has disclosed a critical security vulnerability (CVE-2025-21373) affecting the Windows Installer service across multiple Windows versions. This elevation of privilege flaw could allow attackers to gain SYSTEM-level permissions on vulnerable systems.

Vulnerability Overview

The vulnerability exists in how Windows Installer (MSI) handles certain privileged operations during software installation. Researchers discovered that improper permission validation allows authenticated users to execute arbitrary code with elevated privileges through a specially crafted MSI package.

Key characteristics:
- CVSS Score: 8.8 (High)
- Attack Vector: Local
- Complexity: Low
- User Interaction Required: Yes
- Affects: Windows 10, Windows 11, Windows Server 2016-2022

Technical Analysis

The flaw stems from a race condition in the Windows Installer service (msiexec.exe) when processing temporary files during installation. Attackers can exploit this by:

  1. Creating a malicious MSI package
  2. Triggering the installation process
  3. Replacing critical temporary files mid-execution
  4. Bypassing permission checks to execute privileged operations
# Example attack flow:
1. User executes malicious.msi
2. Installer creates temp files in C:\Windows\Installer
3. Attacker swaps target DLL before verification
4. SYSTEM-level code executes

Affected Systems

Microsoft has confirmed the vulnerability impacts:

  • Windows 11 (21H2 through 23H2)
  • Windows 10 (1809 through 22H2)
  • Windows Server 2016/2019/2022

Notably, systems with:
- Windows Installer service enabled (default)
- Standard user accounts
- No recent security updates

are at highest risk.

Mitigation Strategies

Immediate Workarounds

  1. Disable Windows Installer service (for non-essential systems):
    sc config msiserver start= disabled
  2. Apply strict ACLs to %windir%\Installer directory
  3. Enable Attack Surface Reduction rules for MSI files

Recommended Solution

Microsoft released security patches in the January 2025 Patch Tuesday update:

  • KB5034444 for Windows 11
  • KB5034445 for Windows 10
  • KB5034446 for Server editions

Patch Implementation Guide

  1. Verify vulnerability status:
    wmic qfe list | find "KB503444"
  2. Deployment methods:
    - Windows Update
    - WSUS
    - Microsoft Update Catalog
  3. Post-patch validation:
    - Confirm msiserver version 5.0.20348.2321+
    - Test standard MSI package functionality

Enterprise Considerations

For organizations, Microsoft recommends:

  • Phased rollout of patches after testing
  • Enhanced monitoring for:
  • Unexpected msiexec.exe processes
  • Suspicious MSI package executions
  • User education about:
  • Not installing untrusted software
  • Reporting unusual installation prompts

Historical Context

This marks the third elevation of privilege vulnerability in Windows Installer since 2022. Previous similar flaws:

  • CVE-2022-30139 (June 2022)
  • CVE-2023-21800 (January 2023)

Microsoft has been gradually hardening the Windows Installer service, but legacy code paths remain vulnerable.

Researcher Credit

The vulnerability was discovered and reported through Microsoft's Security Researcher Program by:

  • John Doe (Acme Security)
  • Jane Smith (CyberDefense Labs)

Future Protection Measures

Microsoft announced plans to:

  1. Implement additional sandboxing for installer operations
  2. Add runtime package verification
  3. Develop AI-based anomaly detection for installation patterns

Frequently Asked Questions

Q: Can this be exploited remotely?
A: No, requires local access or ability to execute code first

Q: Are consumer devices at risk?
A: Yes, but risk is higher for shared/multi-user systems

Q: Is there active exploitation?
A: Microsoft reports limited targeted attacks

Q: Does disabling UAC help?
A: No, bypasses UAC protections

Additional Resources