CVE-2026-21513
Published: 10 February 2026
Summary
CVE-2026-21513 is a high-severity Protection Mechanism Failure (CWE-693) vulnerability in Microsoft Windows 10 21H2. Its CVSS base score is 8.8 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploitation for Client Execution (T1203); ranked in the top 3.2% of CVEs by exploit likelihood; CISA has added it to the Known Exploited Vulnerabilities catalog.
The strongest mitigations our analysis identified are NIST 800-53 SI-2 (Flaw Remediation) and SI-5 (Security Alerts, Advisories, and Directives).
Threat & Defense at a Glance
Threat & Defense Details
Mitigating Controls (NIST 800-53 r5)AI
Directly requires timely identification, reporting, and correction of flaws like the MSHTML protection mechanism failure in CVE-2026-21513 via patching.
Mandates receiving, disseminating, and acting on security alerts and directives such as CISA's Known Exploited Vulnerabilities listing for CVE-2026-21513 to ensure rapid remediation.
Provides vulnerability scanning to identify systems affected by CVE-2026-21513, enabling targeted patching of the MSHTML security feature bypass.
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
The vulnerability is a security feature bypass in the MSHTML rendering engine exploited via crafted webpages or documents requiring user interaction, directly enabling arbitrary code execution through client-side software exploitation.
NVD Description
Protection mechanism failure in MSHTML Framework allows an unauthorized attacker to bypass a security feature over a network.
Deeper analysisAI
CVE-2026-21513 is a protection mechanism failure vulnerability in the MSHTML Framework, classified under CWE-693. This security feature bypass affects the MSHTML rendering engine, a component historically used in Microsoft Internet Explorer and legacy Edge browsers. Published on February 10, 2026, it carries a CVSS v3.1 base score of 8.8 (AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H), indicating high severity due to its potential for significant impact on confidentiality, integrity, and availability.
An unauthorized attacker can exploit this vulnerability over a network with low complexity and no required privileges, though user interaction is necessary. By tricking a user into interacting with malicious content, such as via a crafted webpage or document, the attacker can bypass a security feature in MSHTML, potentially leading to arbitrary code execution or other high-impact compromises on the affected system.
Microsoft's update guide at msrc.microsoft.com provides details on patches and remediation. Vicarius offers specific detection and mitigation scripts tailored to this MSHTML security feature bypass. The vulnerability is listed in CISA's Known Exploited Vulnerabilities catalog, signaling active exploitation in the wild and urging immediate patching by federal agencies.
As a high-severity issue in a legacy but still-deployed component, CVE-2026-21513 underscores the risks of unpatched Microsoft browser engines, with confirmed real-world exploitation per CISA. Security practitioners should prioritize inventorying and updating affected systems.
Details
- CWE(s)
- KEV Date Added
- 10 February 2026