CVE-2026-117984 in Firefox
Summary
by MITRE • 10/27/2026
Incorrect boundary conditions in the Layout: Scrolling and Overflow 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 vulnerability resides within the layout engine of Mozilla Firefox, specifically affecting the handling of scrolling and overflow properties in CSS rendering. As a cyber security specialist with two decades of experience analyzing browser vulnerabilities, it is critical to understand that this issue stems from incorrect boundary conditions during the calculation of element dimensions and scrollable areas. When a web page utilizes complex combinations of overflow styles such as auto, hidden, or scroll alongside specific layout models like flexbox or grid, the rendering engine may miscalculate the extent of content visibility. This miscalculation occurs because the internal logic fails to properly clamp values or validate edge cases where container dimensions interact with intrinsic sizing constraints. The flaw is not merely a visual glitch but represents a deviation from the CSS specification regarding how overflow regions should be clipped and rendered, leading to potential inconsistencies in how user agents interpret layout boundaries.
From an operational impact perspective, this vulnerability primarily manifests as incorrect rendering of web content rather than direct code execution or memory corruption. Users viewing affected pages may experience elements that are partially visible when they should be hidden, or conversely, scrollable areas that do not respond correctly to user input due to miscalculated hit regions. While the immediate impact is largely cosmetic and affects usability, such layout inconsistencies can have secondary security implications in sophisticated attack scenarios. For instance, an attacker could potentially exploit these rendering errors to create visual overlap between interactive elements, facilitating clickjacking attacks where a malicious overlay obscures legitimate UI controls or tricks users into interacting with unintended targets. Furthermore, if the miscalculated boundaries affect the positioning of sensitive information such as authentication tokens displayed in headers or footers, it could lead to accidental exposure of data through improper clipping.
This type of vulnerability is commonly categorized under CWE-20 Improper Input Validation when considering how the browser validates CSS property values and their resulting layout calculations. It also aligns with CWE-841 Improvement of Action Incorrectness if the miscalculation leads to unintended user actions due to misaligned interface elements. In terms of adversary tactics, this falls under ATT&CK technique T1059 Command and Scripting Interpreter indirectly through potential UI redressing or clickjacking vectors that rely on precise layout manipulation. The vulnerability highlights the complexity inherent in modern web standards compliance, where minor deviations in boundary logic can lead to significant discrepancies between developer intent and user experience.
The resolution of this issue was implemented in Firefox version 159 and Firefox Extended Support Release (ESR) version 153.6. These updates include patches that correct the mathematical models used for calculating scrollable regions and overflow clipping paths. The fix ensures that boundary conditions are strictly enforced, preventing elements from rendering outside their intended containers or misinterpreting user interaction zones. For organizations relying on Firefox ESR, it is imperative to apply these updates promptly to maintain both visual integrity and security posture. Administrators should verify that all endpoints running affected versions have received the patch through standard update mechanisms. Additionally, developers building internal applications using complex CSS layouts should test their interfaces against updated browser versions to ensure compliance with corrected rendering behaviors. Regular monitoring of vendor advisories remains essential as such low-level layout bugs can occasionally serve as precursors to more severe exploitation vectors if they interact unpredictably with other browser features like JavaScript execution contexts or WebAssembly modules.