CVE-2026-21513 in Windows
Summary
by MITRE • 02/10/2026
Protection mechanism failure in MSHTML Framework allows an unauthorized attacker to bypass a security feature over a network.
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Analysis
by VulDB Data Team • 03/27/2026
The vulnerability identified as CVE-2026-21513 represents a critical protection mechanism failure within the Microsoft HTML Viewer Framework commonly known as MSHTML. This flaw exists in the core rendering engine responsible for processing and displaying web content across numerous Microsoft applications and services. The vulnerability manifests as a failure in the security controls that are designed to prevent unauthorized access to system resources or data. When exploited, this weakness enables attackers to circumvent established security boundaries that should normally prevent unauthorized network-based access to protected system components.
The technical implementation of this vulnerability resides in the MSHTML framework's handling of specific HTML parsing and rendering operations. The flaw occurs during the processing of malformed or specially crafted HTML content that triggers an unexpected behavior in the security module responsible for validating network requests and access controls. This mechanism failure allows an attacker to manipulate the normal execution flow and bypass authentication or authorization checks that would typically prevent network-based exploitation. The vulnerability is particularly concerning because it operates at the framework level, meaning it affects multiple applications that rely on MSHTML for web content rendering.
From an operational impact perspective, this vulnerability creates significant risks for organizations that depend on Microsoft applications and services that utilize the MSHTML framework. Attackers can leverage this weakness to gain unauthorized access to systems that should be protected by network-level security controls, potentially leading to data breaches, system compromise, or lateral movement within network environments. The network-based nature of the attack means that exploitation can occur without requiring local system access, making it particularly dangerous in environments where network segmentation is relied upon for security. The vulnerability's impact extends across various Microsoft products including Internet Explorer, Office applications, and other software that integrates the MSHTML rendering engine.
Security professionals should consider this vulnerability in the context of the CWE-119 weakness category, which addresses improper restriction of operations within a limited scope, and the ATT&CK technique T1071.004 for application layer protocol: web protocols, as it involves bypassing security controls related to web content processing. The mitigation approach should include immediate deployment of Microsoft security patches and updates to address the underlying framework vulnerability. Organizations should also implement network-based security controls such as web application firewalls and content filtering systems to detect and block suspicious HTML content. Additionally, security teams should monitor for exploitation attempts and consider implementing network segmentation strategies to limit the potential impact of successful exploitation attempts. The vulnerability highlights the importance of maintaining up-to-date security controls and the need for continuous monitoring of framework-level security mechanisms that underpin enterprise applications and services.