CVE-2026-20857
📋 TL;DR
This vulnerability allows an authorized attacker to exploit an untrusted pointer dereference in the Windows Cloud Files Mini Filter Driver to elevate privileges locally. It affects Windows systems with the vulnerable driver component. Attackers need initial access to the system but can then gain higher privileges.
💻 Affected Systems
- Windows Cloud Files Mini Filter Driver
📦 What is this software?
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-level privileges, enabling installation of persistent malware, credential theft, and lateral movement across the network.
Likely Case
Local privilege escalation from a standard user account to administrator or SYSTEM privileges, allowing attackers to bypass security controls and maintain persistence.
If Mitigated
Limited impact if proper privilege separation, application control, and endpoint protection are in place, though successful exploitation still provides elevated access.
🎯 Exploit Status
Requires local access and some technical knowledge of driver exploitation. No public exploit code is known at this time.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Check Microsoft's monthly security updates for the specific KB number
Vendor Advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-20857
Restart Required: Yes
Instructions:
1. Apply the latest Windows security updates from Microsoft. 2. Verify the update installed successfully. 3. Restart the system as required.
🔧 Temporary Workarounds
Disable Cloud Files feature
windowsTemporarily disable the Cloud Files Mini Filter Driver if not needed
fltmc unload cldflt
🧯 If You Can't Patch
- Implement strict application control policies to prevent unauthorized code execution
- Use endpoint detection and response (EDR) solutions to monitor for privilege escalation attempts
🔍 How to Verify
Check if Vulnerable:
Check Windows Update history for the specific security update KB number mentioned in Microsoft's advisoryCheck Version:
systeminfo | findstr /B /C:"OS Name" /C:"OS Version"Verify Fix Applied:
Verify the security update is installed via Windows Update or by checking system version📡 Detection & Monitoring
Log Indicators:
- Event ID 4688 with suspicious process creation following initial access
- Driver loading events related to cldflt.sys
Network Indicators:
- Unusual outbound connections following local privilege escalation
SIEM Query:
EventID=4688 AND (NewProcessName="*\cmd.exe" OR NewProcessName="*\powershell.exe") | where ParentProcessName contains "explorer.exe"