CVE-2021-41335: Windows 10 Kernel Privilege Escalation

CVE-2021-41335 Overview

CVE-2021-41335 is a Windows Kernel Elevation of Privilege Vulnerability that affects a wide range of Microsoft Windows operating systems, including both client and server editions. This vulnerability allows a local attacker with low privileges to escalate their permissions to gain full control over the affected system. The flaw resides in the Windows kernel, the core component responsible for managing system resources and enforcing security boundaries between user-mode applications and critical system operations.

Critical Impact

Successful exploitation enables a local attacker to elevate privileges from a standard user account to SYSTEM level, potentially allowing complete system compromise, unauthorized data access, persistence mechanisms, and lateral movement within enterprise networks.

Affected Products

• Microsoft Windows 10 (multiple versions including 1607, 1809, 1909, 2004, 20H2, 21H1)
• Microsoft Windows 7 SP1
• Microsoft Windows 8.1
• Microsoft Windows RT 8.1
• Microsoft Windows Server 2008 R2 SP1
• Microsoft Windows Server 2012 and Server 2012 R2
• Microsoft Windows Server 2016 (including 2004, 20H2)
• Microsoft Windows Server 2019

Discovery Timeline

• 2021-10-13 - CVE-2021-41335 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-41335

Vulnerability Analysis

This elevation of privilege vulnerability exists due to improper privilege management within the Windows kernel (CWE-269). The Windows kernel is responsible for enforcing security boundaries between processes and managing access to system resources. When this security boundary fails, an attacker operating with limited user privileges can bypass intended restrictions and execute code with elevated SYSTEM privileges.

The vulnerability requires local access to the target system, meaning an attacker must either have physical access or have already compromised the system through another vector such as phishing, malware delivery, or exploitation of a separate vulnerability. Once local access is established, exploitation does not require user interaction and can be executed with low complexity.

Successful exploitation grants the attacker the highest level of privilege on the Windows system. With SYSTEM-level access, an attacker can install programs, view and modify all data, create new accounts with full user rights, disable security software, deploy persistence mechanisms, and potentially use the compromised system as a pivot point for further network intrusion.

Root Cause

The root cause of CVE-2021-41335 is classified as CWE-269 (Improper Privilege Management). This category of vulnerability occurs when a software component does not properly restrict the acquisition or modification of privileges, allowing attackers to gain capabilities beyond their authorized permissions. In this case, the Windows kernel fails to adequately validate or restrict privilege transitions, enabling unauthorized elevation from standard user context to kernel/SYSTEM context.

Attack Vector

The attack vector for CVE-2021-41335 is local, requiring the attacker to have established access to the target system prior to exploitation. The attack scenario typically involves:

1. Initial Access: The attacker gains a foothold on the target system through methods such as social engineering, exploiting another vulnerability, or compromising user credentials.

2. Privilege Escalation: With standard user privileges, the attacker executes a specially crafted program or exploits the kernel vulnerability to escalate privileges.

3. Post-Exploitation: Upon achieving SYSTEM-level access, the attacker can perform malicious activities including credential harvesting, installing backdoors, disabling security controls, or exfiltrating sensitive data.

The vulnerability affects confidentiality, integrity, and availability at a high level, meaning successful exploitation can result in complete system compromise.

Detection Methods for CVE-2021-41335

Indicators of Compromise

• Unusual process execution patterns where standard user processes spawn SYSTEM-level child processes
• Anomalous kernel-mode driver loading or suspicious kernel object manipulation
• Unexpected privilege token modifications or security descriptor changes on critical system objects
• Evidence of exploitation tools or privilege escalation utilities on the system

Detection Strategies

• Monitor for suspicious process creation events where processes running as standard users attempt to spawn processes with elevated privileges
• Implement Windows Event Log monitoring for Event ID 4688 (process creation) with attention to privilege escalation patterns
• Deploy endpoint detection and response (EDR) solutions capable of detecting kernel-level exploitation attempts
• Utilize SentinelOne's behavioral AI engine to identify and block privilege escalation attempts in real-time

Monitoring Recommendations

• Enable advanced audit logging including process tracking and privilege use auditing on all Windows systems
• Configure centralized log collection and analysis using SIEM solutions to correlate privilege escalation indicators across the environment
• Establish baselines for normal kernel activity and alert on deviations that may indicate exploitation attempts
• Regularly review security event logs for patterns consistent with local privilege escalation attacks

How to Mitigate CVE-2021-41335

Immediate Actions Required

• Apply the Microsoft security update addressing CVE-2021-41335 immediately on all affected Windows systems
• Prioritize patching of internet-facing systems and critical infrastructure servers
• Implement application whitelisting to prevent execution of unauthorized privilege escalation tools
• Review and enforce principle of least privilege to minimize the attack surface available to potential attackers

Patch Information

Microsoft has released security updates to address this vulnerability as part of their October 2021 Patch Tuesday release. Administrators should consult the Microsoft Security Advisory for CVE-2021-41335 for detailed patch information specific to each affected operating system version. Updates are available through Windows Update, Windows Server Update Services (WSUS), and the Microsoft Update Catalog.

Organizations should test patches in a staging environment before deploying to production systems, particularly for critical infrastructure. However, given the severity of privilege escalation vulnerabilities, expedited patching is strongly recommended.

Workarounds

• Restrict local access to systems by enforcing strong authentication and access controls
• Implement network segmentation to limit lateral movement if a system is compromised
• Deploy endpoint protection solutions with behavioral detection capabilities to identify and block exploitation attempts
• Monitor for and block known privilege escalation tools and techniques using application control policies

# Configuration example
# Enable Windows Defender Credential Guard to protect against credential theft post-exploitation
# Run in elevated PowerShell on Windows 10/Server 2016 or later
reg add "HKLM\SYSTEM\CurrentControlSet\Control\DeviceGuard" /v EnableVirtualizationBasedSecurity /t REG_DWORD /d 1 /f
reg add "HKLM\SYSTEM\CurrentControlSet\Control\LSA" /v LsaCfgFlags /t REG_DWORD /d 1 /f

# Enable audit logging for privilege use
auditpol /set /subcategory:"Sensitive Privilege Use" /success:enable /failure:enable