CVE-2021-41357
📋 TL;DR
CVE-2021-41357 is a Win32k elevation of privilege vulnerability in Windows that allows authenticated attackers to gain SYSTEM-level privileges on affected systems. This affects Windows 10, Windows 11, and Windows Server 2019/2022 systems. Attackers need local access to exploit this vulnerability.
💻 Affected Systems
- Windows 10
- Windows 11
- Windows Server 2019
- Windows Server 2022
📦 What is this software?
Windows 10 2004 by Microsoft
Windows 10 20h2 by Microsoft
Windows 10 21h1 by Microsoft
Windows 11 21h2 by Microsoft
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise with SYSTEM privileges, enabling installation of malware, data theft, credential harvesting, and persistence mechanisms.
Likely Case
Privilege escalation from standard user to SYSTEM, allowing attackers to bypass security controls, install backdoors, and access sensitive system resources.
If Mitigated
Limited impact with proper privilege separation, application control policies, and endpoint protection that can detect privilege escalation attempts.
🎯 Exploit Status
Exploit code has been publicly released and is being actively used in attacks. Requires local authenticated access to execute.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: October 2021 security updates (KB5006670 for Windows 10 20H2, KB5006674 for Windows 11, etc.)
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-41357
Restart Required: Yes
Instructions:
1. Apply October 2021 Windows security updates via Windows Update. 2. For enterprise environments, deploy updates through WSUS or SCCM. 3. Restart systems after patch installation.
🔧 Temporary Workarounds
Restrict local user privileges
windowsLimit standard user accounts to prevent initial access required for exploitation
Enable Windows Defender Exploit Guard
windowsConfigure Exploit Guard to detect and block privilege escalation attempts
🧯 If You Can't Patch
- Implement strict application control policies to prevent unauthorized code execution
- Deploy endpoint detection and response (EDR) solutions to detect privilege escalation attempts
🔍 How to Verify
Check if Vulnerable:
Check Windows version and build number. Vulnerable builds include Windows 10 19042.1288, 19043.1288, 19044.1288 and earlier.Check Version:
wmic os get Caption, Version, BuildNumber, OSArchitectureVerify Fix Applied:
Verify Windows build number is at least: Windows 10 20H2 - 19042.1288, Windows 11 - 22000.258, Windows Server 2019 - 17763.2268, Windows Server 2022 - 20348.288📡 Detection & Monitoring
Log Indicators:
- Event ID 4688 with parent process anomalies
- Security log events showing privilege escalation
- Process creation from unusual parent processes
Network Indicators:
- No direct network indicators - local privilege escalation
SIEM Query:
EventID=4688 AND (NewProcessName LIKE '%win32k%' OR ParentProcessName LIKE '%win32k%') AND SubjectUserName NOT IN ('SYSTEM', 'LOCAL SERVICE', 'NETWORK SERVICE')