CVE-2026-20940
📋 TL;DR
A heap-based buffer overflow vulnerability in the Windows Cloud Files Mini Filter Driver allows authenticated attackers to execute arbitrary code with elevated system privileges. This affects Windows systems with the vulnerable driver installed, primarily impacting enterprise environments using cloud storage features. Attackers must already have local access to exploit this vulnerability.
💻 Affected Systems
- Windows Cloud Files Mini Filter Driver
📦 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
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise with SYSTEM privileges, enabling installation of persistent malware, credential theft, lateral movement across the network, and data exfiltration.
Likely Case
Local privilege escalation from a standard user account to SYSTEM privileges, allowing attackers to bypass security controls, install additional malware, or access protected resources.
If Mitigated
Limited impact due to proper access controls, endpoint protection, and network segmentation preventing lateral movement even if exploitation occurs.
🎯 Exploit Status
Requires authenticated local access and knowledge of driver internals. Heap exploitation techniques are needed, making it moderately complex but feasible for skilled attackers.
🛠️ 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-2026-20940
Restart Required: Yes
Instructions:
1. Open Windows Update Settings
2. Click 'Check for updates'
3. Install all available security updates
4. Restart the system when prompted
🔧 Temporary Workarounds
Disable Cloud Files Mini Filter Driver
windowsTemporarily disable the vulnerable driver to prevent exploitation
fltmc unload cldflt
Restrict Driver Loading
windowsUse Group Policy to prevent loading of the vulnerable driver
Configure: Computer Configuration > Policies > Windows Settings > Security Settings > System Services > Find 'Cloud Files Mini Filter Driver' > Set to 'Disabled'
🧯 If You Can't Patch
- Implement strict least privilege access controls to limit local user accounts
- Deploy endpoint detection and response (EDR) solutions to detect privilege escalation attempts
🔍 How to Verify
Check if Vulnerable:
Check driver version using: fltmc filters | findstr cldfltCheck Version:
systeminfo | findstr /B /C:"OS Name" /C:"OS Version"Verify Fix Applied:
Verify Windows Update history contains the relevant security update and driver version has been updated📡 Detection & Monitoring
Log Indicators:
- Event ID 4697: Service installation attempts
- Suspicious process creation with SYSTEM privileges from user accounts
- Driver load events for cldflt.sys
Network Indicators:
- Unusual outbound connections following local privilege escalation
- Lateral movement attempts to other systems
SIEM Query:
EventID=4697 OR (ProcessName="*" AND ParentProcessName="explorer.exe" AND IntegrityLevel="System")