CVE-2026-117933 in Firefox
Summary
by MITRE • 10/27/2026
Frame poisoning in the CSS Parsing and Computation component. This vulnerability was fixed in Firefox 159 and Firefox ESR 153.6.
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Analysis
by VulDB Data Team • 10/27/2026
The identified security issue represents a frame poisoning vulnerability located within the Cascading Style Sheets parsing and computation subsystem of Mozilla Firefox browsers, specifically affecting versions prior to 159 and Firefox Extended Support Release version 153.6. This class of defect typically arises when the browser fails to adequately sanitize or validate input data during the processing of CSS rules that influence frame properties such as width, height, position, or visibility. In web development contexts, frames are used to embed one HTML document within another, and their behavior is heavily dictated by associated style sheets. When an attacker can inject maliciously crafted CSS content into a webpage, they may exploit parsing inconsistencies to manipulate the dimensions or positioning of these embedded frames in unintended ways.
From a technical perspective, frame poisoning often involves manipulating computed styles to cause layout shifts that exceed expected boundaries or interfere with adjacent DOM elements. This manipulation can lead to visual overlap where critical interface components are obscured by maliciously sized iframes, potentially covering legitimate user interaction targets such as login buttons or confirmation dialogs. Such behavior is frequently associated with Clickjacking attacks, where the attacker tricks users into clicking on hidden or disguised elements that perform actions under a different context than what the user perceives they are interacting with. The vulnerability stems from insufficient boundary checks during the CSS computation phase, allowing malformed style properties to propagate and alter the rendering engine's internal state regarding frame geometry.
The operational impact of this vulnerability is primarily centered around social engineering and interface manipulation rather than direct code execution or data exfiltration through memory corruption. By distorting the visual layout of a webpage, an attacker can create deceptive user interfaces that mimic trusted applications or websites. This capability enables sophisticated phishing campaigns where victims are led to believe they are interacting with secure forms while their inputs are actually being captured by malicious overlays controlled via poisoned frames. Additionally, persistent layout distortions can degrade accessibility and usability for legitimate users, causing navigation elements to become inaccessible or misaligned, which constitutes a denial of service against the user experience even if system stability remains intact.
Mitigation strategies primarily involve applying the vendor-provided patches released in Firefox 159 and ESR version 153.6 immediately across all client endpoints. For organizations unable to patch instantly due to compatibility concerns, implementing strict Content Security Policy directives can help mitigate risks by restricting frame embedding sources and preventing unauthorized style injection through external stylesheets. Developers should also ensure that any dynamic CSS manipulation is validated against a whitelist of allowed properties and values before being applied to the DOM. Regular security audits focusing on cross-origin resource sharing policies and iframe sandboxing attributes further reduce the attack surface associated with frame-based vulnerabilities by limiting the privileges granted to embedded content.
This vulnerability aligns with Common Weakness Enumeration categories such as CWE-830 which pertains to inclusion of functionality from an incorrect source, or more broadly CWE-20 regarding improper input validation within parsing logic. In terms of adversary tactics and techniques reference framework mapping, this defect facilitates the initial access and execution phases by enabling clickjacking scenarios that fall under ATT&CK technique T1534 which involves internal spearphishing through interface manipulation. The exploitation relies heavily on user interaction to trigger the malicious layout changes, making it a critical concern for web applications handling sensitive transactions or authentication flows where visual integrity is paramount to security assurance.