Microsoft has released an urgent security patch for a critical zero-day vulnerability (CVE-2026-21513) affecting the MSHTML Framework, which was actively exploited in the wild before a fix became available.
The flaw allows attackers to bypass Windows security features without requiring elevated privileges, putting millions of systems at risk.
CVE-2026-21513 is a security feature bypass vulnerability in Microsoft’s MSHTML Framework, the core HTML rendering engine used across Windows operating systems and various applications.
FieldValueCVE IDCVE-2026-21513ComponentMSHTML Framework (Windows)Vulnerability typeSecurity Feature Bypass (protection mechanism failure) CVSS base score8.8 CVSS vector (v3.x)AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:HCWE mappingCWE-693 (Protection Mechanism Failure)
The vulnerability stems from a failure in a protection mechanism that enables attackers to circumvent execution prompts when users interact with malicious files.
The MSHTML Framework, also known as Trident, is a proprietary browser engine that renders web pages and HTML content within applications on Windows systems.
This deep integration means the vulnerability can impact a wide range of systems and users across enterprise environments.
Exploitation requires social engineering tactics where attackers convince victims to open specially crafted HTML files or malicious shortcut (.lnk) files.
These files can be delivered through multiple vectors, including email attachments, malicious links, or downloads.
Once opened, the crafted file silently bypasses Windows security prompts and triggers dangerous actions with a single click.
The vulnerability manipulates how Windows Shell and MSHTML handle embedded content, allowing the operating system to process and execute content without proper security validation.
The attacker requires no privileges, and the attack vector is network-based and low-complexity.
Microsoft confirmed that CVE-2026-21513 was both publicly disclosed and actively exploited as a zero-day before patches became available.
The U.S. Cybersecurity and Infrastructure Security Agency (CISA) added this vulnerability to its Known Exploited Vulnerabilities catalog, requiring federal agencies to apply fixes by March 3, 2026.
Security feature bypass vulnerabilities significantly increase the success rate of phishing and malware campaigns.
In enterprise environments, this flaw can lead to unauthorized code execution, malware and ransomware deployment, credential theft, data breaches, and complete system compromise.
The vulnerability affects all supported Windows versions, including Windows 10, Windows 11, and Windows Server editions from 2012 through 2025.
Microsoft released security updates on February 10, 2026, as part of its monthly Patch Tuesday cycle.
Organizations should prioritize patching this vulnerability immediately, especially given its active exploitation in real-world attacks.
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