CVE-2026-87550 in Chrome
Summary
by MITRE • 09/09/2026
Improper encoding or escaping of output in CSS in Google Chrome prior to 153.0.8010.36 allowed a remote attacker to potentially bypass web origin policy via a crafted HTML page. (Chromium security severity: Medium)
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Analysis
by VulDB Data Team • 09/09/2026
The vulnerability identified as an improper encoding or escaping of output within the Cascading Style Sheets engine of Google Chrome prior to version 153.0.8010.36 represents a significant deviation from secure web rendering practices. This flaw resides in how the browser processes and renders CSS properties when they are dynamically generated or injected into HTML documents via crafted scripts. The core technical issue stems from insufficient validation and sanitization of user-supplied data before it is interpreted by the style engine, allowing special characters to be misinterpreted as structural elements rather than literal string content. This failure in input handling enables an attacker to manipulate the rendering context beyond its intended boundaries, specifically targeting the isolation mechanisms that separate different web origins.
From a technical perspective, this vulnerability falls under the category of Improper Output Handling or Encoding issues, which is formally classified under CWE-79: Cross-site Scripting and more specifically related to CSS injection vectors where style properties are not properly escaped against HTML context boundaries. The attack vector involves an adversary hosting a malicious webpage containing crafted HTML and CSS code designed to exploit this parsing inconsistency. When a victim visits the page, the browser's rendering engine processes the malformed styles without correctly escaping critical characters such as quotes or semicolons that delimit property values. This allows the attacker to inject arbitrary commands into the style context, effectively breaking out of the sandboxed environment provided by CSS and interacting with DOM elements in ways that were previously restricted.
The operational impact of this vulnerability is primarily centered on the bypassing of web origin policies, which are fundamental security mechanisms designed to prevent malicious scripts from accessing data across different domains. By exploiting this flaw, a remote attacker can potentially read sensitive information stored within cookies, local storage, or session tokens belonging to other origins that share the same browser instance. This capability undermines the principle of least privilege and confidentiality guarantees provided by modern web browsers. Although classified with a medium severity rating in Chromium's internal security framework, the potential for data exfiltration makes this a serious concern for users who interact with untrusted websites or click on malicious links embedded in emails or social media platforms.
This type of exploitation aligns closely with techniques documented in the MITRE ATT&CK framework, particularly those involving client-side code injection and browser security policy bypasses. Attackers often leverage such vulnerabilities as part of a broader campaign to establish persistence or move laterally within a compromised user environment by stealing authentication credentials. The ability to read cross-origin data effectively neutralizes the Same-Origin Policy, which is the cornerstone of web application security for decades. Consequently, any interaction with content that triggers this rendering bug can lead to unauthorized access to private user information without requiring additional privileges on the host system.
Mitigation strategies focus primarily on timely software updates and defensive coding practices. Users must ensure their Google Chrome installation is updated to version 153.0.8010.36 or later, as this release contains patches that enforce stricter escaping rules for CSS output generated from dynamic sources. For developers integrating third-party content or handling user input in web applications, it is critical to implement robust sanitization libraries that encode special characters before they are rendered by the browser engine. Additionally, deploying Content Security Policy headers can provide an additional layer of defense by restricting the sources from which styles and scripts can be loaded, thereby reducing the attack surface available for such injection attempts. Regular security audits and static code analysis tools should also be employed to detect similar encoding flaws during the development lifecycle before deployment.