CVE-2026-21240: Windows HTTP.sys Elevation of Privilege Vulnerability

Severity: high (CVSS 7.0 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-02-10 · updated 2026-08-19
Weakness: CWE-367

Description

Time-of-check time-of-use (toctou) race condition in Windows HTTP.sys allows an authorized attacker to elevate privileges locally.

Affected products

Microsoft Windows 10: before 10.0.17763.8389 — fixed in 10.0.17763.8389
Microsoft Windows 10: before 10.0.19044.6937 — fixed in 10.0.19044.6937
Microsoft Windows 10: before 10.0.19045.6937 — fixed in 10.0.19045.6937
Microsoft Windows 11: before 10.0.22631.6649 — fixed in 10.0.22631.6649
Microsoft Windows 11: before 10.0.26100.7781 — fixed in 10.0.26100.7781
Microsoft Windows 11 25H2: before 10.0.26200.7781 — fixed in 10.0.26200.7781
Microsoft Windows Server 2019: before 10.0.17763.8389 — fixed in 10.0.17763.8389
Microsoft Windows Server 2022: before 10.0.20348.4711 — fixed in 10.0.20348.4711
Microsoft Windows Server 2022, 23H2 Edition (Server Core installation): before 10.0.25398.2149 — fixed in 10.0.25398.2149
Microsoft Windows Server 2025: before 10.0.26100.32313 — fixed in 10.0.26100.32313
Microsoft Windows 10: from 10.0.17763.0 before 10.0.17763.8389 — fixed in 10.0.17763.8389
Microsoft Windows 10: from 10.0.19044.0 before 10.0.19044.6937 — fixed in 10.0.19044.6937
Microsoft Windows 10: from 10.0.19045.0 before 10.0.19045.6937 — fixed in 10.0.19045.6937
Microsoft Windows 11 version 22H3: from 10.0.22631.0 before 10.0.22631.6649 — fixed in 10.0.22631.6649
Microsoft Windows 11: from 10.0.22631.0 before 10.0.22631.6649 — fixed in 10.0.22631.6649
Microsoft Windows 11: from 10.0.26100.0 before 10.0.26100.7840 — fixed in 10.0.26100.7840
Microsoft Windows 11 25H2: from 10.0.26200.0 before 10.0.26200.7840 — fixed in 10.0.26200.7840
Microsoft Windows 11 version 26H1: from 10.0.28000.0 before 10.0.28000.1575 — fixed in 10.0.28000.1575
Microsoft Windows Server 2019: from 10.0.17763.0 before 10.0.17763.8389 — fixed in 10.0.17763.8389
Microsoft Windows Server 2022: from 10.0.20348.0 before 10.0.20348.4773 — fixed in 10.0.20348.4773
Microsoft Windows Server 2022, 23H2 Edition (Server Core installation): from 10.0.25398.0 before 10.0.25398.2149 — fixed in 10.0.25398.2149
Microsoft Windows Server 2025: from 10.0.26100.0 before 10.0.26100.32370 — fixed in 10.0.26100.32370

How to fix

Upgrade to a fixed version: Microsoft Windows 10 10.0.17763.8389; Microsoft Windows 10 10.0.19044.6937; Microsoft Windows 10 10.0.19045.6937; Microsoft Windows 11 10.0.22631.6649; Microsoft Windows 11 10.0.26100.7781; Microsoft Windows 11 25H2 10.0.26200.7781; Microsoft Windows Server 2019 10.0.17763.8389; Microsoft Windows Server 2022 10.0.20348.4711; Microsoft Windows Server 2022, 23H2 Edition (Server Core installation) 10.0.25398.2149; Microsoft Windows Server 2025 10.0.26100.32313; Microsoft Windows 10 10.0.17763.8389; Microsoft Windows 10 10.0.19044.6937. Check the vendor advisory for mitigations.

References

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21240

Get alerted when CVE-2026-21240 or the next vulnerability affects your stack