CVE-2021-40476: Windows 10 Privilege Escalation Vulnerability

CVE-2021-40476 Overview

CVE-2021-40476 is a Windows AppContainer Elevation of Privilege vulnerability that affects a wide range of Microsoft Windows operating systems. This vulnerability exists in the Windows AppContainer implementation and can allow an attacker to escalate privileges on a vulnerable system. The flaw is related to insufficiently protected credentials (CWE-522) and requires local access with user interaction to exploit.

Critical Impact

Successful exploitation of this vulnerability could allow an attacker running within an AppContainer sandbox to escape the container restrictions and gain elevated privileges on the affected system, potentially compromising the confidentiality, integrity, and availability of the target environment.

Affected Products

• Microsoft Windows 10 (multiple versions including 1607, 1809, 1909, 2004, 20H2, 21H1)
• Microsoft Windows 11 (ARM64 and x64)
• Microsoft Windows 8.1 and Windows RT 8.1
• Microsoft Windows Server 2012, 2012 R2, 2016, 2019, and 2022

Discovery Timeline

• 2021-10-13 - CVE-2021-40476 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-40476

Vulnerability Analysis

This vulnerability affects the Windows AppContainer sandboxing mechanism, specifically within the WSAQuerySocketSecurity function. AppContainers are designed to provide application isolation by restricting the capabilities and access rights of applications running within them. The vulnerability allows a malicious application running inside an AppContainer to bypass these security restrictions and gain elevated privileges.

The flaw is categorized under CWE-522 (Insufficiently Protected Credentials), indicating that the vulnerability involves improper handling of credential information that can be leveraged by an attacker to escalate privileges. An attacker must have local access to the target system and requires user interaction to successfully exploit this vulnerability.

Root Cause

The root cause of this vulnerability lies in the improper validation and protection of security-related information within the Windows socket security query functionality. When an application within an AppContainer makes certain socket security queries via WSAQuerySocketSecurity, the system fails to properly enforce the sandbox restrictions, allowing access to privileged information or operations that should be restricted within the AppContainer environment.

Attack Vector

The attack requires local access to the vulnerable system and user interaction to execute. An attacker could craft a malicious application designed to run within an AppContainer context. Once executed with user interaction, the application exploits the vulnerability in the socket security query mechanism to break out of the AppContainer sandbox and gain elevated privileges.

The exploitation flow involves:

1. An attacker delivers a malicious application to the target system
2. The user executes the application, which runs within an AppContainer sandbox
3. The application makes specific calls to WSAQuerySocketSecurity that exploit the vulnerability
4. The vulnerability allows the application to escape the sandbox restrictions and gain elevated privileges

Technical details regarding this vulnerability can be found in the Packet Storm Security advisory and the Microsoft Security Advisory.

Detection Methods for CVE-2021-40476

Indicators of Compromise

• Unusual process behavior from applications running within AppContainers attempting to access privileged resources
• Anomalous calls to WSAQuerySocketSecurity from sandboxed applications
• Processes unexpectedly elevating from low-integrity AppContainer contexts to higher privilege levels

Detection Strategies

• Monitor for suspicious activity from processes running within AppContainer sandboxes, particularly those making socket security-related API calls
• Implement behavioral analysis to detect privilege escalation attempts from low-integrity processes
• Deploy endpoint detection and response (EDR) solutions like SentinelOne Singularity to identify exploitation attempts in real-time

Monitoring Recommendations

• Enable Windows Security event logging for process creation and privilege changes
• Monitor for unusual parent-child process relationships involving AppContainer applications
• Track API calls to Windows socket security functions from sandboxed contexts
• Implement SentinelOne's Behavioral AI to detect and block privilege escalation attempts

How to Mitigate CVE-2021-40476

Immediate Actions Required

• Apply the Microsoft security update released as part of the October 2021 Patch Tuesday immediately
• Review and audit applications running within AppContainers for suspicious behavior
• Ensure endpoint protection solutions are updated with the latest detection signatures
• Limit user execution of untrusted applications that run in AppContainer contexts

Patch Information

Microsoft has released security updates to address this vulnerability as part of their October 2021 security release. Administrators should apply the appropriate patches for their affected Windows versions. Detailed patch information is available in the Microsoft Security Advisory for CVE-2021-40476.

Workarounds

• Restrict execution of untrusted applications, particularly those utilizing AppContainer sandboxes
• Implement application whitelisting to prevent unauthorized applications from running
• Enable Windows Defender Application Control (WDAC) policies to limit application execution
• Deploy SentinelOne Singularity Platform for real-time threat detection and automated response to exploitation attempts

# Verify patch installation status via PowerShell
Get-HotFix | Where-Object {$_.Description -like "*Security Update*"} | Sort-Object InstalledOn -Descending
# Review installed Windows updates for October 2021 patches
wmic qfe list brief | findstr /i "KB5006"