CVE-2026-20831
📋 TL;DR
A time-of-check time-of-use race condition in Windows Ancillary Function Driver for WinSock allows authenticated attackers to escalate privileges locally. This affects Windows systems with the vulnerable driver component. Attackers must already have some level of access to the system to exploit this vulnerability.
💻 Affected Systems
- Microsoft Windows
📦 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 22h2 by Microsoft
Windows 11 23h2 by Microsoft
Windows 11 24h2 by Microsoft
Windows 11 25h2 by Microsoft
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise with SYSTEM/administrator privileges, enabling full control over the affected system and potential lateral movement within the network.
Likely Case
Local privilege escalation from a standard user account to administrator/SYSTEM privileges, allowing installation of malware, disabling security controls, or accessing sensitive data.
If Mitigated
Limited impact due to proper privilege separation, application control policies, and restricted user permissions preventing initial access.
🎯 Exploit Status
TOCTOU race conditions require precise timing and may be challenging to exploit reliably. Requires local authenticated access first.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Check Microsoft Security Update Guide for specific KB numbers
Vendor Advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-20831
Restart Required: Yes
Instructions:
1. Open Windows Update Settings
2. Click 'Check for updates'
3. Install all available security updates
4. Restart system when prompted
🔧 Temporary Workarounds
Restrict user privileges
windowsImplement least privilege principle to limit initial access that could be escalated
Application control policies
windowsUse Windows Defender Application Control or AppLocker to restrict execution of unauthorized binaries
🧯 If You Can't Patch
- Implement strict access controls and monitoring for privileged accounts
- Deploy endpoint detection and response (EDR) solutions to detect privilege escalation attempts
🔍 How to Verify
Check if Vulnerable:
Check Windows Update history for the specific security update or use Microsoft's Security Update GuideCheck Version:
systeminfo | findstr /B /C:"OS Name" /C:"OS Version"Verify Fix Applied:
Verify the security update is installed via Windows Update history or systeminfo command showing the KB number📡 Detection & Monitoring
Log Indicators:
- Unusual process creation with elevated privileges
- Suspicious driver loading or modification
- Security event 4688 with elevated token
Network Indicators:
- Not applicable - local privilege escalation only
SIEM Query:
EventID=4688 AND NewProcessName CONTAINS 'cmd.exe' OR 'powershell.exe' AND SubjectLogonId!=0x3e7 AND TokenElevationType='%%1938'