CVE-2026-109753 in Firefox
Summary
by MITRE • 10/13/2026
Incorrect boundary conditions in the Layout component. This vulnerability was fixed in Firefox 158, Firefox ESR 115.43, and Firefox ESR 153.5.
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Analysis
by VulDB Data Team • 10/13/2026
The identified security issue resides within the layout engine of Mozilla Firefox, specifically affecting how boundary conditions are calculated during rendering operations. This vulnerability stems from an incorrect handling of edge cases in the CSS box model or geometric calculations used to position elements on a web page. When the browser processes complex layouts involving nested containers, absolute positioning, or specific overflow properties, it fails to correctly validate the boundaries of these elements against their parent contexts. This logical error allows for situations where internal state variables representing coordinates, dimensions, or offsets become inconsistent with the actual rendered output, creating a discrepancy between what is displayed and how the browser internally tracks element positions.
From a technical perspective, this flaw represents a classic instance of improper input validation within layout algorithms. The vulnerability does not necessarily involve memory corruption in the traditional sense but rather logic errors that can lead to unexpected behavior when interacting with web content. Such boundary condition failures are often categorized under CWE-20 Improper Input Validation or CWE-697 Incorrect Comparison, as the software fails to correctly evaluate whether a calculated position falls within acceptable limits before proceeding with rendering actions. The impact of this flaw is primarily related to visual integrity and potential exploitation through crafted web pages that trigger specific layout states. An attacker could potentially use maliciously designed HTML and CSS to manipulate how elements are rendered, possibly leading to UI redressing attacks where legitimate interface components are obscured or replaced by deceptive overlays.
The operational impact of this vulnerability extends beyond mere cosmetic issues. In certain scenarios, incorrect boundary calculations can lead to information disclosure if the rendering engine exposes internal memory states through visual artifacts or timing side-channels during layout recalculation. Furthermore, these logic errors can serve as a stepping stone for more severe exploits by destabilizing the browser's internal state management. For instance, an attacker might leverage the inconsistent boundary data to confuse subsequent security checks that rely on accurate element positioning, potentially bypassing content security policies or other mitigations designed to restrict cross-origin interactions. The vulnerability highlights the complexity of modern web rendering engines and the difficulty in maintaining robust validation across all possible combinations of CSS properties and DOM structures.
Mozilla addressed this issue by refining the boundary condition logic within the layout component, ensuring that edge cases are properly validated before being processed for rendering. This fix was incorporated into Firefox version 158 as well as the extended support releases ESR 115.43 and ESR 153.5 to ensure broad coverage across different user bases including enterprise environments relying on stable release channels. Users of these browsers are advised to update immediately to mitigate any potential risks associated with malicious web content designed to exploit layout inconsistencies. Defense in depth strategies should also be employed, such as keeping browser plugins and extensions updated, since they may interact with the rendering engine and amplify the effects of such vulnerabilities. Regular security audits and adherence to secure coding practices for web developers can further reduce the attack surface by minimizing reliance on complex or ambiguous CSS behaviors that might trigger these boundary errors.