CVE-2021-40476
📋 TL;DR
This vulnerability allows an attacker with limited AppContainer privileges to elevate to SYSTEM-level privileges on Windows systems. It affects Windows 10, Windows 11, and Windows Server 2016/2019/2022 systems where AppContainer sandboxing is used. The flaw is in the WSAQuerySocketSecurity function which fails to properly validate security contexts.
💻 Affected Systems
- Windows 10
- Windows 11
- Windows Server 2016
- Windows Server 2019
- Windows Server 2022
📦 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 8.1 by Microsoft
Windows Rt 8.1 by Microsoft
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise with SYSTEM privileges, allowing installation of malware, data theft, and persistence mechanisms.
Likely Case
Privilege escalation from AppContainer sandbox to SYSTEM, enabling lateral movement and bypassing security boundaries.
If Mitigated
Limited impact if proper network segmentation and least privilege principles are enforced, though local privilege escalation remains possible.
🎯 Exploit Status
Exploit requires ability to execute code within AppContainer context first. Proof-of-concept code is publicly available.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: October 2021 security updates (KB5006670 for Windows 10 21H1, KB5006674 for Windows Server 2022, etc.)
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-40476
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
Disable AppContainer features
windowsRemove or disable AppContainer sandboxed applications to eliminate attack surface
🧯 If You Can't Patch
- Implement strict network segmentation to limit lateral movement
- Enforce least privilege principles and monitor for unusual privilege escalation attempts
🔍 How to Verify
Check if Vulnerable:
Check Windows version and build number. Systems without October 2021 security updates are vulnerable.Check Version:
wmic os get caption, version, buildnumberVerify Fix Applied:
Verify Windows Update history shows October 2021 security updates installed, or check system build number is updated.📡 Detection & Monitoring
Log Indicators:
- Event ID 4688 with unusual parent-child process relationships
- Security logs showing privilege escalation attempts
- AppContainer process spawning SYSTEM-level processes
Network Indicators:
- Unusual outbound connections from previously isolated AppContainer processes
SIEM Query:
EventID=4688 AND (NewProcessName="*cmd.exe*" OR NewProcessName="*powershell.exe*") AND ParentProcessName contains "AppContainer"🔗 References
- http://packetstormsecurity.com/files/164942/Microsoft-Windows-WSAQuerySocketSecurity-AppContainer-Privilege-Escalation.html
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-40476
- http://packetstormsecurity.com/files/164942/Microsoft-Windows-WSAQuerySocketSecurity-AppContainer-Privilege-Escalation.html
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-40476
