CVE-2026-21249
📋 TL;DR
This vulnerability allows an unauthorized local attacker to manipulate file paths in Windows NTLM authentication, potentially enabling spoofing attacks. It affects Windows systems with NTLM enabled, primarily impacting environments where local access is possible. The low CVSS score indicates limited impact scope.
💻 Affected Systems
- Windows NTLM
📦 What is this software?
Windows 10 1607 by Microsoft
Windows 10 1607 by Microsoft
Windows 10 1809 by Microsoft
Windows 10 1809 by Microsoft
Windows 10 21h2 by Microsoft
Windows 10 21h2 by Microsoft
Windows 10 21h2 by Microsoft
Windows 10 22h2 by Microsoft
Windows 10 22h2 by Microsoft
Windows 10 22h2 by Microsoft
Windows 11 23h2 by Microsoft
Windows 11 23h2 by Microsoft
Windows 11 24h2 by Microsoft
Windows 11 24h2 by Microsoft
Windows 11 25h2 by Microsoft
Windows 11 25h2 by Microsoft
⚠️ Risk & Real-World Impact
Worst Case
Local privilege escalation or credential spoofing leading to unauthorized access to sensitive resources within the same system.
Likely Case
Limited local spoofing allowing an attacker to impersonate other users or services within the same machine context.
If Mitigated
Minimal impact with proper access controls and monitoring in place.
🎯 Exploit Status
Exploitation requires local access and knowledge of NTLM file path manipulation techniques.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Check Microsoft Security Update for specific KB number
Vendor Advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21249
Restart Required: No
Instructions:
1. Apply latest Windows security updates from Microsoft. 2. Verify update installation via Windows Update history. 3. No restart typically required for NTLM-related patches.
🔧 Temporary Workarounds
Disable NTLM authentication
WindowsReplace NTLM with Kerberos or other more secure authentication protocols where possible.
Group Policy: Computer Configuration > Windows Settings > Security Settings > Local Policies > Security Options > Network security: Restrict NTLM
🧯 If You Can't Patch
- Implement strict local access controls and monitor for suspicious file path manipulation attempts.
- Use application allowlisting to prevent unauthorized local execution that could exploit this vulnerability.
🔍 How to Verify
Check if Vulnerable:
Check Windows Update history for missing security patches related to CVE-2026-21249.Check Version:
systeminfo | findstr /B /C:"OS Name" /C:"OS Version"Verify Fix Applied:
Verify patch installation via 'wmic qfe list' or PowerShell 'Get-HotFix' commands looking for the relevant KB.📡 Detection & Monitoring
Log Indicators:
- Windows Security Event Logs: Event ID 4625 (failed logon) with NTLM, unusual file path access in NTLM contexts
Network Indicators:
- NTLM authentication attempts with abnormal file paths in SMB or other protocols
SIEM Query:
source="Windows Security" EventCode=4625 AuthenticationPackage="NTLM" | search *path* OR *file*