CVE-2026-119331 in Ladybird
Summary
by MITRE • 10/30/2026
Ladybird is a truly independent web browser. Prior to the c0d68bddf3e3616e5173835f0f4c155ee9700d91 commit, CSSStyleSheet.cssRules, insertRule(), and deleteRule() in Libraries/LibWeb/CSS/CSSStyleSheet.cpp did not enforce the stylesheet origin-clean flag with the required SecurityError, even though cross-origin imported stylesheets were correctly marked non-origin-clean. A malicious page could read parsed rules from a stylesheet loaded without CORS and could insert or delete rules in the victim stylesheet, bypassing the Same-Origin Policy. Reading could expose selectors, generated content, feature markers, or other sensitive rule text, while mutation could alter page visibility or layout. The issue does not provide memory corruption, code execution, or persistence after the attacker page closes. This vulnerability is fixed by commit c0d68bddf3e3616e5173835f0f4c155ee9700d91.
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Analysis
by VulDB Data Team • 10/30/2026
The Ladybird web browser, an independent project aiming to provide a modern browsing experience, contained a significant security flaw related to the enforcement of cross-origin restrictions on CSS stylesheets. This vulnerability centered around the handling of the origin-clean flag within the CSSStyleSheet implementation located in Libraries/LibWeb/CSS/CSSStyleSheet.cpp. Specifically, prior to commit c0d68bddf3e3616e5173835f0f4c155ee9700d91, the methods cssRules, insertRule(), and deleteRule() failed to properly check whether a stylesheet was origin-clean before allowing access or modification. While the browser correctly identified cross-origin imported stylesheets as non-origin-clean during their loading phase, it neglected to enforce this status when JavaScript code attempted to interact with these sheets via standard DOM APIs. This discrepancy created a gap in the Same-Origin Policy enforcement mechanisms that are critical for isolating content from different origins and preventing unauthorized data access or manipulation.
The technical flaw allowed malicious web pages to bypass same-origin restrictions by reading parsed rules from stylesheets loaded without Cross-Origin Resource Sharing (CORS) permissions. By invoking cssRules on such a stylesheet, an attacker could extract sensitive information embedded within the CSS, including selectors, generated content definitions, and feature markers that might reveal internal application logic or design structures not intended for public consumption. Furthermore, the ability to call insertRule() and deleteRule() enabled attackers to mutate the victim's stylesheet dynamically. This mutation capability meant that a malicious page could alter the visual presentation of a target website by changing visibility properties, layout configurations, or other stylistic elements. Such manipulation could be used to facilitate phishing attacks by disguising legitimate interfaces or causing denial-of-service conditions through disruptive style changes, thereby compromising both data confidentiality and integrity within the context of the victim's browsing session.
From an industry standard perspective, this vulnerability aligns with CWE-200, which covers Information Exposure, as it allowed unauthorized reading of internal stylesheet details that should have been restricted by origin boundaries. Additionally, the ability to modify DOM elements without proper authorization corresponds closely to CWE-749, where a system exposes a dangerous command or function to an actor not authorized to control that mechanism. In terms of attack tactics, this behavior reflects techniques associated with client-side data exfiltration and UI redressing found in the MITRE ATT&CK framework for browser-based attacks. The failure to enforce origin checks on CSS manipulation APIs represents a classic instance where security controls are implemented inconsistently across different code paths, leading to privilege escalation within the sandboxed environment of the web page context.
The operational impact of this vulnerability is limited by its scope and persistence characteristics. It does not result in memory corruption or arbitrary code execution, which mitigates risks associated with remote code execution vulnerabilities that often lead to full system compromise. Furthermore, the effects are transient; once the attacker's malicious page is closed or navigated away from, any modifications made to the victim stylesheet via insertRule() or deleteRule() do not persist beyond that session. This means the vulnerability primarily affects active browsing sessions and does not allow for long-term persistence on the user's device. However, even without persistent access, the ability to exfiltrate CSS data or alter page layout in real-time poses a tangible risk to users relying on Ladybird for secure web interactions, particularly when visiting sites that mix content from multiple origins without proper CORS headers.
To mitigate this vulnerability and prevent similar issues in future development, it is essential to ensure strict enforcement of the origin-clean flag across all API endpoints that interact with stylesheet objects. Developers must verify the security context before allowing read or write operations on CSSStyleSheet instances, ensuring that cross-origin resources are treated as opaque unless explicitly permitted by CORS policies. Regular code audits focusing on consistency in security checks across related functions can help identify such gaps early in the development lifecycle. The issue was resolved in commit c0d68bddf3e3616e5173835f0f4c155ee9700d91, which implemented the necessary checks to throw SecurityError exceptions when unauthorized access is attempted. Users of Ladybird should ensure they are running a version that includes this fix to maintain robust protection against same-origin policy bypasses involving CSS manipulation and data exfiltration through stylesheet APIs.