CVE-2025-21271
📋 TL;DR
This vulnerability in the Windows Cloud Files Mini Filter Driver allows attackers to gain SYSTEM-level privileges on affected systems. It affects Windows 10, 11, and Server versions where the driver is present. Successful exploitation requires an attacker to already have local access to the system.
💻 Affected Systems
- Windows 10
- Windows 11
- Windows Server 2019
- Windows Server 2022
📦 What is this software?
Windows 10 1809 by Microsoft
Windows 10 1809 by Microsoft
Windows 10 21h2 by Microsoft
Windows 10 22h2 by Microsoft
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise with SYSTEM privileges, enabling installation of persistent malware, credential theft, and lateral movement across the network.
Likely Case
Local privilege escalation allowing attackers to bypass security controls, install additional tools, and maintain persistence on compromised systems.
If Mitigated
Limited impact due to layered security controls, but still provides foothold for attackers with initial access.
🎯 Exploit Status
Requires local access and ability to execute code; no public exploit available as of analysis.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: January 2025 security updates (KB5034441 for Windows 10, KB5034440 for Windows 11, etc.)
Vendor Advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21271
Restart Required: Yes
Instructions:
1. Apply January 2025 Windows security updates via Windows Update. 2. For enterprise environments, deploy updates through WSUS or SCCM. 3. Verify update installation and restart systems as required.
🔧 Temporary Workarounds
Disable Cloud Files Mini Filter Driver
allTemporarily disable the vulnerable driver to prevent exploitation
fltmc unload cldflt
🧯 If You Can't Patch
- Implement strict local privilege management and application control policies
- Monitor for suspicious process creation and privilege escalation attempts
🔍 How to Verify
Check if Vulnerable:
Check if cldflt.sys driver version is older than January 2025 updatesCheck Version:
fltmc filters | findstr cldfltVerify Fix Applied:
Verify KB5034441 (Win10) or KB5034440 (Win11) is installed and cldflt.sys version is updated📡 Detection & Monitoring
Log Indicators:
- Event ID 4697 (Service installation) for cldflt.sys
- Unexpected SYSTEM privilege escalation events
Network Indicators:
- None - local exploitation only
SIEM Query:
EventID=4697 AND ServiceFileName="*cldflt.sys*" OR ProcessName="*cldflt*"