Feb 2026: Windows HTTP.sys Elevation of Privilege Vulnerability
CVE-2026-21240 Published on February 10, 2026
Time-of-check time-of-use (toctou) race condition in Windows HTTP.sys allows an authorized attacker to elevate privileges locally.
Weakness Type
What is a TOCTTOU Vulnerability?
The software checks the state of a resource before using that resource, but the resource's state can change between the check and the use in a way that invalidates the results of the check. This can cause the software to perform invalid actions when the resource is in an unexpected state. This weakness can be security-relevant when an attacker can influence the state of the resource between check and use. This can happen with shared resources such as files, memory, or even variables in multithreaded programs.
CVE-2026-21240 has been classified to as a TOCTTOU vulnerability or weakness.
Products Associated with CVE-2026-21240
You can be notified by email with stack.watch whenever vulnerabilities like CVE-2026-21240 are published in these products:
Affected Versions
Microsoft Windows 10 Version 1809:- Version 10.0.17763.0 and below 10.0.17763.8389 is affected.
- Version 10.0.19044.0 and below 10.0.19044.6937 is affected.
- Version 10.0.19045.0 and below 10.0.19045.6937 is affected.
- Version 10.0.22631.0 and below 10.0.22631.6649 is affected.
- Version 10.0.22631.0 and below 10.0.22631.6649 is affected.
- Version 10.0.26100.0 and below 10.0.26100.7840 is affected.
- Version 10.0.26200.0 and below 10.0.26200.7840 is affected.
- Version 10.0.28000.0 and below 10.0.28000.1575 is affected.
- Version 10.0.28000.0 and below 10.0.28000.1575 is affected.
- Version 10.0.17763.0 and below 10.0.17763.8389 is affected.
- Version 10.0.17763.0 and below 10.0.17763.8389 is affected.
- Version 10.0.20348.0 and below 10.0.20348.4773 is affected.
- Version 10.0.25398.0 and below 10.0.25398.2149 is affected.
- Version 10.0.26100.0 and below 10.0.26100.32370 is affected.
- Version 10.0.26100.0 and below 10.0.26100.32370 is affected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.