Time-of-check Time-of-use (TOCTOU) Race Condition in Microsoft Windows and Windows Server - CVE-2026-21240

Vulnerability identifier: #VU122553

CSH Severity: Low

CVSS v4: 8.5 [CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N]

CVE-ID: CVE-2026-21240

CWE-ID: CWE-367

Exploitation vector: Local access

Exploit availability: No public exploit available

The vulnerability allows a local user to escalate privileges on the system.

The vulnerability exists due to a time-of-check, time-of-use (TOCTOU) race condition in Windows HTTP.sys. A local user can gain elevated privileges on the target system.

Microsoft Windows
Windows Server

Install updates from vendor's website.

Microsoft Windows - addressed in versions 10 21H2 10.0.19044.6937, 10 22H2 10.0.19045.6937, 10 1809 10.0.17763.8389, 11 23H2 10.0.22631.6649, 11 24H2 10.0.26100.7781, 11 24H2 10.0.26100.7840, 11 26H1 10.0.28000.1575
Windows Server - addressed in versions 2012 R2 6.3.9600.23022, 2019 10.0.17763.8389, 2022 23H2 10.0.25398.2149, 2022 10.0.20348.4711, 2022 10.0.20348.4773, 2025 10.0.26100.32313, 2025 10.0.26100.32370

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

• Multiple vulnerabilities in Microsoft Windows HTTP.sys