CVE-2021-40467
📋 TL;DR
This vulnerability in the Windows Common Log File System (CLFS) driver allows attackers to gain SYSTEM-level privileges by exploiting improper access control. It affects Windows systems where an attacker already has local user access. Successful exploitation enables complete system compromise.
💻 Affected Systems
- Windows 10
- Windows 11
- Windows Server 2016
- Windows Server 2019
- Windows Server 2022
📦 What is this software?
Windows 10 by Microsoft
Windows 10 by Microsoft
Windows 10 by Microsoft
Windows 10 by Microsoft
Windows 10 by Microsoft
Windows 10 by Microsoft
Windows 10 by Microsoft
Windows 11 by Microsoft
Windows 11 by Microsoft
Windows 7 by Microsoft
Windows 8.1 by Microsoft
Windows Rt 8.1 by Microsoft
⚠️ Risk & Real-World Impact
Worst Case
Attacker gains SYSTEM privileges, installs persistent malware, steals credentials, and pivots to other systems in the network.
Likely Case
Local attacker escalates privileges to install ransomware, keyloggers, or backdoors on the compromised system.
If Mitigated
With proper privilege separation and endpoint protection, exploitation may be detected and blocked before SYSTEM access is achieved.
🎯 Exploit Status
Exploit requires local user access; proof-of-concept code has been published.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: October 2021 security updates (KB5006670, KB5006674, etc.)
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-40467
Restart Required: Yes
Instructions:
1. Install October 2021 Windows security updates via Windows Update. 2. For enterprise: Deploy patches through WSUS or SCCM. 3. Restart systems after patch installation.
🔧 Temporary Workarounds
Restrict local user privileges
windowsLimit standard user accounts to prevent initial access required for exploitation.
🧯 If You Can't Patch
- Implement strict least-privilege access controls for all user accounts
- Deploy endpoint detection and response (EDR) solutions to monitor for privilege escalation attempts
🔍 How to Verify
Check if Vulnerable:
Check Windows version and compare with patched versions (build numbers 19041.1288, 19042.1288, etc.)Check Version:
systeminfo | findstr /B /C:"OS Name" /C:"OS Version"Verify Fix Applied:
Verify October 2021 security updates are installed via 'winver' or 'systeminfo' command📡 Detection & Monitoring
Log Indicators:
- Event ID 4688: New process creation with SYSTEM privileges from non-SYSTEM accounts
- CLFS driver access anomalies
Network Indicators:
- Unusual outbound connections from systems after local privilege escalation
SIEM Query:
EventID=4688 AND NewProcessName="*" AND SubjectUserName!="SYSTEM" AND TokenElevationType="%%1938"