CVE-2026-21231: Windows 10 1607 Privilege Escalation Flaw

CVE-2026-21231 Overview

CVE-2026-21231 is a race condition vulnerability in the Windows Kernel that allows an authorized attacker to elevate privileges locally. This vulnerability stems from concurrent execution using a shared resource with improper synchronization (CWE-362), enabling low-privileged users to potentially gain SYSTEM-level access on affected Windows systems.

Critical Impact

Local privilege escalation through Windows Kernel race condition affects all modern Windows desktop and server operating systems, potentially allowing complete system compromise.

Affected Products

• Microsoft Windows 10 (versions 1607, 1809, 21H2, 22H2)
• Microsoft Windows 11 (versions 23H2, 24H2, 25H2)
• Microsoft Windows Server 2012, 2012 R2, 2016, 2019, 2022, 2022 23H2, and 2025

Discovery Timeline

• 2026-02-10 - CVE-2026-21231 published to NVD
• 2026-02-11 - Last updated in NVD database

Technical Details for CVE-2026-21231

Vulnerability Analysis

This vulnerability exists within the Windows Kernel due to improper synchronization when accessing shared resources during concurrent execution. Race conditions occur when multiple threads or processes attempt to access and modify shared data simultaneously without proper locking mechanisms. In this case, the Windows Kernel fails to adequately synchronize access to a critical resource, creating a time-of-check to time-of-use (TOCTOU) window that attackers can exploit.

The attack requires local access and low privileges, but successful exploitation can result in a scope change, affecting resources beyond the vulnerable component. An attacker who successfully exploits this vulnerability could execute arbitrary code with elevated privileges, potentially gaining complete control over the affected system with SYSTEM-level permissions.

Root Cause

The root cause is improper synchronization of concurrent execution using shared resources in the Windows Kernel (CWE-362). The kernel code path handling the affected resource lacks adequate locking primitives or uses incorrect synchronization, allowing a race condition to occur between security checks and the use of the checked resource.

Attack Vector

The vulnerability requires local access to the target system with low-privilege user credentials. An attacker must craft a malicious application or script that triggers the race condition by rapidly performing operations that access the vulnerable kernel resource. By winning the race condition, the attacker can manipulate the kernel's state to escalate privileges.

The attack involves:

1. Identifying the specific kernel code path with improper synchronization
2. Creating concurrent threads that race to access the shared resource
3. Timing the attack to exploit the window between the security check and resource use
4. Leveraging the resulting privilege escalation to execute code as SYSTEM

Due to the nature of race conditions requiring precise timing, exploitation may require multiple attempts, though successful exploitation grants significant system access.

Detection Methods for CVE-2026-21231

Indicators of Compromise

• Unusual process creation events from low-privileged user accounts spawning SYSTEM-level processes
• Abnormal kernel memory access patterns or unexpected system calls to vulnerable kernel functions
• Evidence of repeated rapid thread creation targeting specific kernel resources
• Suspicious privilege escalation activity in Windows Security Event Logs (Event ID 4688, 4672)

Detection Strategies

• Monitor for processes that rapidly spawn multiple threads performing similar kernel operations
• Implement behavioral detection for privilege escalation patterns where low-privileged processes suddenly gain elevated token privileges
• Deploy kernel-level monitoring to detect suspicious synchronization patterns and TOCTOU exploitation attempts
• Enable and monitor Windows Security Event Logs for privilege escalation indicators

Monitoring Recommendations

• Enable advanced audit policies for process creation and privilege use events
• Configure SentinelOne agents to detect and block kernel exploitation techniques targeting race conditions
• Implement endpoint detection and response (EDR) rules for privilege escalation behaviors
• Monitor for unusual system call sequences associated with kernel race condition exploitation

How to Mitigate CVE-2026-21231

Immediate Actions Required

• Apply the latest Windows security updates from Microsoft immediately on all affected systems
• Prioritize patching domain controllers, critical servers, and systems accessible by untrusted users
• Implement application whitelisting to prevent unauthorized code execution
• Review and restrict local user privileges following the principle of least privilege

Patch Information

Microsoft has released security updates to address this vulnerability. Organizations should apply patches through Windows Update, Windows Server Update Services (WSUS), or Microsoft Update Catalog. Refer to the Microsoft Security Advisory for specific patch details and affected version information.

Workarounds

• Restrict local logon rights to trusted users only using Group Policy
• Implement network segmentation to limit lateral movement if a system is compromised
• Enable Credential Guard and other virtualization-based security features where supported
• Monitor systems closely for exploitation attempts until patches can be applied

# Example: Restrict local logon using Group Policy (PowerShell)
# Review current local logon rights
secedit /export /cfg c:\temp\secpol.cfg
# Edit the exported policy to restrict SeDenyInteractiveLogonRight
# Apply the modified policy
secedit /configure /db c:\windows\security\local.sdb /cfg c:\temp\secpol.cfg