CVE-2026-21240: Windows 10 1809 Privilege Escalation Flaw

CVE-2026-21240 Overview

A Time-of-check Time-of-use (TOCTOU) race condition vulnerability exists in the Windows HTTP.sys driver that allows an authorized attacker to elevate privileges locally. This vulnerability arises from improper synchronization between the verification of resource state and its subsequent use, creating a window of opportunity for exploitation.

Critical Impact

Successful exploitation of this vulnerability allows a local attacker with low privileges to gain elevated system privileges, potentially compromising the confidentiality, integrity, and availability of the affected system.

Affected Products

• Microsoft Windows 10 1809 (x64, x86)
• Microsoft Windows 10 21H2 (x64, x86, ARM64)
• Microsoft Windows 10 22H2 (x64, x86, ARM64)
• Microsoft Windows 11 23H2 (x64, ARM64)
• Microsoft Windows 11 24H2 (x64, ARM64)
• Microsoft Windows 11 25H2 (x64, ARM64)
• Microsoft Windows Server 2019
• Microsoft Windows Server 2022
• Microsoft Windows Server 2022 23H2
• Microsoft Windows Server 2025

Discovery Timeline

• 2026-02-10 - CVE-2026-21240 published to NVD
• 2026-02-11 - Last updated in NVD database

Technical Details for CVE-2026-21240

Vulnerability Analysis

This vulnerability is classified under CWE-367 (Time-of-check Time-of-use Race Condition). The flaw exists within the Windows HTTP.sys kernel-mode driver, which is responsible for handling HTTP requests in Windows. The race condition occurs when the driver checks a security-relevant condition and then uses the result of that check in a way that allows an attacker to change the state between the check and the use.

The vulnerability requires local access and an authorized user account to exploit, though the attack complexity is high due to the timing-dependent nature of race conditions. A successful exploit could allow the attacker to execute arbitrary code with elevated privileges, potentially gaining SYSTEM-level access on the affected machine.

Root Cause

The root cause is a synchronization failure in the HTTP.sys driver where a security validation check occurs asynchronously from the operation that relies on that validation. This creates a race window where an attacker can manipulate the state of a resource after it has been validated but before it is used. The lack of atomic operations or proper locking mechanisms in the validation-to-use path allows this exploitation window to exist.

Attack Vector

The attack vector is local, requiring the attacker to have authenticated access to the target system. The attacker must be able to execute code on the local machine and precisely time their exploitation attempts to win the race condition. The attack works as follows:

1. An attacker initiates a request that triggers the vulnerable code path in HTTP.sys
2. The driver performs a security check on a resource or permission
3. Before the driver can use the result of that check, the attacker manipulates the underlying state
4. The driver proceeds with the operation using stale validation data, allowing unauthorized privilege escalation

Due to the high attack complexity inherent in race conditions, successful exploitation may require multiple attempts to achieve the precise timing needed.

Detection Methods for CVE-2026-21240

Indicators of Compromise

• Unusual HTTP.sys driver activity or errors in Windows Event logs
• Unexpected privilege escalation events for low-privileged user accounts
• Anomalous kernel-mode driver behavior detected by endpoint security solutions
• Suspicious process creation chains originating from HTTP-related services

Detection Strategies

• Monitor Windows Security Event Log for Event ID 4672 (Special Privileges Assigned) with unexpected user accounts
• Deploy kernel-mode monitoring to detect anomalous HTTP.sys behavior patterns
• Implement behavioral analysis to identify processes attempting rapid repeated operations consistent with race condition exploitation
• Use SentinelOne's Singularity XDR platform to detect privilege escalation attempts and kernel-level anomalies

Monitoring Recommendations

• Enable advanced auditing for process creation and privilege use events
• Configure alerts for repeated failures followed by successful privilege escalation attempts
• Monitor for unusual timing patterns in HTTP.sys-related system calls
• Implement real-time kernel integrity monitoring through SentinelOne's endpoint protection

How to Mitigate CVE-2026-21240

Immediate Actions Required

• Apply the Microsoft security update as soon as it becomes available
• Audit systems for any unauthorized privilege escalation that may indicate prior exploitation
• Restrict local user access to essential personnel only
• Enable enhanced logging and monitoring on affected systems pending patch deployment

Patch Information

Microsoft has released a security update to address this vulnerability. Refer to the Microsoft Security Update Guide for CVE-2026-21240 for detailed patch information and deployment guidance. Organizations should prioritize patching systems where local user access is less tightly controlled.

Workarounds

• Limit local user account access to reduce the attack surface
• Implement application control policies to restrict unauthorized code execution
• Deploy SentinelOne endpoint protection with behavioral AI to detect and prevent exploitation attempts
• Consider disabling non-essential HTTP.sys features if operationally feasible until patching is complete

# Check current HTTP.sys driver status
sc query http

# Review HTTP.sys related services
netsh http show servicestate

# Enable enhanced logging for privilege escalation monitoring
auditpol /set /subcategory:"Sensitive Privilege Use" /success:enable /failure:enable