How severe is CVE-2026-21240?

CVE-2026-21240 has a CVSS 3.x base score of 7.8, rated high severity. It is exploitable over local access with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2026-21240 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (18th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-21240?

CVE-2026-21240 primarily affects Microsoft Windows 10 1809. In total, 18 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2026-21240?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

Does CVE-2026-21240 have an EU (EUVD) identifier?

Yes. CVE-2026-21240 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-7318.

When was CVE-2026-21240 published?

CVE-2026-21240 was published on 2026-02-10 and last updated on 2026-06-17.

CVE-2026-21240

CVE-2026-21240 is a high-severity vulnerability in Microsoft Windows 10 1809 with a CVSS 3.x base score of 7.8. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-367.

Key facts

Severity: High (CVSS 3.x base score 7.8)
EPSS exploit prediction: 0% (18th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-7318
Weakness: CWE-367
Affected product: Microsoft Windows 10 1809
Published: 2026-02-10
Last modified: 2026-06-17

Description

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

Frequently asked questions

What is CVE-2026-21240?: Time-of-check time-of-use (toctou) race condition in Windows HTTP.sys allows an authorized attacker to elevate privileges locally.
How severe is CVE-2026-21240?: CVE-2026-21240 has a CVSS 3.x base score of 7.8, rated high severity. It is exploitable over local access with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2026-21240 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (18th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-21240?: CVE-2026-21240 primarily affects Microsoft Windows 10 1809. In total, 18 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2026-21240?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
Does CVE-2026-21240 have an EU (EUVD) identifier?: Yes. CVE-2026-21240 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-7318.
When was CVE-2026-21240 published?: CVE-2026-21240 was published on 2026-02-10 and last updated on 2026-06-17.

References

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

Affected products (18)

cpe:2.3:o:microsoft:windows_10_1809:*:*:*:*:*:*:x64:*
cpe:2.3:o:microsoft:windows_10_1809:*:*:*:*:*:*:x86:*
cpe:2.3:o:microsoft:windows_10_21h2:*:*:*:*:*:*:arm64:*
cpe:2.3:o:microsoft:windows_10_21h2:*:*:*:*:*:*:x64:*
cpe:2.3:o:microsoft:windows_10_21h2:*:*:*:*:*:*:x86:*
cpe:2.3:o:microsoft:windows_10_22h2:*:*:*:*:*:*:arm64:*
cpe:2.3:o:microsoft:windows_10_22h2:*:*:*:*:*:*:x64:*
cpe:2.3:o:microsoft:windows_10_22h2:*:*:*:*:*:*:x86:*
cpe:2.3:o:microsoft:windows_11_23h2:*:*:*:*:*:*:arm64:*
cpe:2.3:o:microsoft:windows_11_23h2:*:*:*:*:*:*:x64:*
cpe:2.3:o:microsoft:windows_11_24h2:*:*:*:*:*:*:arm64:*
cpe:2.3:o:microsoft:windows_11_24h2:*:*:*:*:*:*:x64:*
cpe:2.3:o:microsoft:windows_11_25h2:*:*:*:*:*:*:arm64:*
cpe:2.3:o:microsoft:windows_11_25h2:*:*:*:*:*:*:x64:*
cpe:2.3:o:microsoft:windows_server_2019:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_server_2022:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_server_2022_23h2:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_server_2025:*:*:*:*:*:*:*:*

More vulnerabilities in Microsoft Windows 10 1809

CVE-2025-49708 — Critical (CVSS 9.9): Use after free in Microsoft Graphics Component allows an authorized attacker to elevate privileges over a network.
CVE-2026-47291 — Critical (CVSS 9.8): Integer overflow or wraparound in Windows HTTP.sys allows an unauthorized attacker to execute code over a network.
CVE-2026-44815 — Critical (CVSS 9.8): Stack-based buffer overflow in Windows DHCP Client allows an unauthorized attacker to execute code over a network.
CVE-2026-33824 — Critical (CVSS 9.8): Double free in Windows IKE Extension allows an unauthorized attacker to execute code over a network.
CVE-2025-60724 — Critical (CVSS 9.8): Heap-based buffer overflow in Microsoft Graphics Component allows an unauthorized attacker to execute code over a…
CVE-2025-53766 — Critical (CVSS 9.8): Heap-based buffer overflow in Windows GDI+ allows an unauthorized attacker to execute code over a network.

All CVEs affecting Microsoft Windows 10 1809 →

Other CWE-367 (Time-of-check Time-of-use (TOCTOU) Race Condition) vulnerabilities

CVE-2026-25641 — Critical (CVSS 10.0): SandboxJS is a JavaScript sandboxing library. Prior to 0.8.29, there is a sandbox escape vulnerability due to a…
CVE-2025-64180 — Critical (CVSS 10.0): Manager-io/Manager is accounting software. In Manager Desktop and Server versions 25.11.1.3085 and below, a critical…
CVE-2025-34027 — Critical (CVSS 10.0): The Versa Concerto SD-WAN orchestration platform is vulnerable to an authentication bypass in the Traefik reverse proxy…
CVE-2026-25052 — Critical (CVSS 9.9): n8n is an open source workflow automation platform. Prior to versions 1.123.18 and 2.5.0, a vulnerability in the file…
CVE-2025-13032 — Critical (CVSS 9.9): Double fetch in sandbox kernel driver in Avast/AVG Antivirus <25.3 on windows allows local attacker to escalate…
CVE-2026-53838 — Critical (CVSS 9.8): OpenClaw before 2026.5.27 contains a state mutation vulnerability in node pairing reconnection that allows paired nodes…

Browse all CWE-367 (Time-of-check Time-of-use (TOCTOU) Race Condition) vulnerabilities →