CVE-2026-21513
Microsoft MSHTML Framework Protection Mechanism Failure Vulnerability
Microsoft Windows
Protection mechanism failure in MSHTML Framework allows an unauthorized attacker to bypass a security feature over a network.
Exploitation status
CISA Known Exploited Vulnerabilities
Listed by CISA on 10 February 2026. US federal agencies were required to remediate it by 3 March 2026.
EPSS — exploitation probability
FIRST puts the probability of exploitation activity in the next 30 days at 15.9%, which is higher than 96.9% of all scored vulnerabilities. EPSS is a forecast of activity, not a measure of severity, and it is rescored daily.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
Frequently asked questions
Is CVE-2026-21513 being exploited by ransomware groups?
No ransomware group in this database is currently recorded as exploiting CVE-2026-21513.
What does CVE-2026-21513 affect?
CVE-2026-21513 affects Microsoft Windows. Protection mechanism failure in MSHTML Framework allows an unauthorized attacker to bypass a security feature over a network.
How severe is CVE-2026-21513?
CVE-2026-21513 is rated HIGH with a CVSS base score of 8.8. EPSS puts the probability of exploitation in the next 30 days at 15.9%, higher than 96.9% of all scored vulnerabilities. It is listed in the CISA Known Exploited Vulnerabilities catalog, with a federal remediation deadline of 3 March 2026.
How should organisations respond to CVE-2026-21513?
Apply the vendor patch for Microsoft Windows as the first priority, and treat any internet-facing instance as the most urgent. Because this vulnerability is associated with ransomware activity, also review whether the affected systems were reachable before patching, rather than assuming that patching alone closes the incident.