CVE-2026-21240
Published: 10 February 2026
Summary
CVE-2026-21240 is a high-severity Time-of-check Time-of-use (TOCTOU) Race Condition (CWE-367) vulnerability in Microsoft Windows 10 21H2. Its CVSS base score is 7.8 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploitation for Privilege Escalation (T1068); ranked at the 8.3th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified are NIST 800-53 SI-2 (Flaw Remediation) and AC-25 (Reference Monitor).
Deeper analysis
CVE-2026-21240 is a Time-of-check time-of-use (TOCTOU) race condition vulnerability, classified under CWE-367, affecting the Windows HTTP.sys kernel-mode driver. Published on 2026-02-10, it carries a CVSS v3.1 base score of 7.8 (AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H). This flaw exists in the HTTP.sys component, which handles HTTP requests in Windows operating systems.
A local attacker with low privileges (PR:L) can exploit this race condition due to its low attack complexity (AC:L) and lack of required user interaction (UI:N). Successful exploitation enables privilege escalation, resulting in high impacts to confidentiality, integrity, and availability (C:H/I:H/A:H).
Microsoft's Security Response Center provides an update guide detailing mitigations and patches for CVE-2026-21240 at https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21240.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-7318
Vulnerability details
Time-of-check time-of-use (toctou) race condition in Windows HTTP.sys allows an authorized attacker to elevate privileges locally.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
TOCTOU race condition in kernel driver directly enables local privilege escalation exploit (T1068).
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Directly mitigates the TOCTOU race condition in Windows HTTP.sys by requiring timely identification, testing, and installation of vendor patches.
Implements a tamper-resistant reference monitor to mediate access to system resources, countering race conditions in kernel-mode access enforcement like HTTP.sys.
Limits damage from privilege escalation by enforcing least privilege for local users and processes interacting with vulnerable kernel components.