CVE-2021-41377
📋 TL;DR
This vulnerability in the Windows Fast FAT File System Driver allows attackers to escalate privileges from a low-privileged user account to SYSTEM level. It affects Windows systems with the vulnerable driver loaded. Attackers must have local access to exploit this flaw.
💻 Affected Systems
- Microsoft Windows
📦 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
Full SYSTEM privilege escalation leading to complete system compromise, data theft, persistence establishment, and lateral movement across the network.
Likely Case
Local privilege escalation allowing attackers to bypass security controls, install malware, or access sensitive data on the compromised system.
If Mitigated
Limited impact if proper access controls, least privilege principles, and network segmentation are implemented to restrict local user access.
🎯 Exploit Status
Exploitation requires local access and user-level privileges. Proof-of-concept code has been published, making exploitation more accessible to attackers.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: October 2021 security updates (KB5006670 for Windows 10 21H1, KB5006674 for 20H2, etc.)
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-41377
Restart Required: Yes
Instructions:
1. Apply October 2021 Windows security updates via Windows Update. 2. For enterprise environments, deploy patches through WSUS, SCCM, or Intune. 3. Restart systems after patch installation.
🔧 Temporary Workarounds
Restrict FAT drive access
windowsPrevent mounting of FAT-formatted drives to avoid loading the vulnerable driver
Use Group Policy to restrict removable drive access
Configure Device Control policies
Implement least privilege
windowsRestrict user privileges to minimize impact if exploited
Use Local Security Policy to configure user rights assignments
Implement Privileged Access Management
🧯 If You Can't Patch
- Implement strict access controls and network segmentation to limit lateral movement
- Deploy endpoint detection and response (EDR) solutions to detect privilege escalation attempts
🔍 How to Verify
Check if Vulnerable:
Check if October 2021 security updates are installed via 'winver' or 'systeminfo' commandCheck Version:
systeminfo | findstr /B /C:"OS Name" /C:"OS Version"Verify Fix Applied:
Verify KB5006670 (or relevant October 2021 update) is installed in Windows Update history📡 Detection & Monitoring
Log Indicators:
- Event ID 4688 with process creation from low-privilege to SYSTEM
- Unexpected fastfat.sys driver activity
- Privilege escalation attempts in security logs
Network Indicators:
- Unusual outbound connections from previously low-privilege accounts
- Lateral movement attempts following local compromise
SIEM Query:
EventID=4688 AND NewProcessName="*" AND SubjectUserName!="SYSTEM" AND TokenElevationType="%%1936"