CVE-2026-84126 in Firefox
Summary
by MITRE • 09/01/2026
Incorrect boundary conditions in the Layout: Grid component. This vulnerability was fixed in Firefox 155.
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Analysis
by VulDB Data Team • 09/01/2026
The identified vulnerability resides within the layout engine of Mozilla Firefox, specifically affecting the Grid component responsible for rendering CSS Grid layouts. The core technical flaw involves incorrect handling of boundary conditions during the calculation and application of grid tracks and items. In complex web pages that utilize intricate grid structures with nested containers or dynamic content adjustments, the browser's layout algorithm fails to correctly validate edge cases where dimensions might exceed expected limits or interact unexpectedly with container constraints. This logical error leads to a state where the computed geometry for certain elements becomes inconsistent with the intended design specifications defined by the website developer.
From an operational perspective, this defect primarily manifests as visual rendering anomalies rather than direct code execution vulnerabilities. Users may observe misaligned content, overlapping text or images, and broken layout structures that disrupt the user experience. In scenarios where precise positioning is critical for functionality, such as interactive dashboards or data-heavy applications relying on strict grid alignment, these errors can lead to usability issues where users cannot effectively interact with interface elements due to obscured buttons or misplaced form fields. While the immediate impact is largely aesthetic and functional rather than security-critical in terms of confidentiality or integrity, it represents a deviation from web standards compliance which can be exploited for phishing attacks if attackers craft malicious pages that mimic legitimate interfaces through layout manipulation.
The vulnerability aligns with CWE-691, which describes use of less restrictive permission level, although in this context it is more accurately mapped to logic errors in boundary condition handling often associated with CWE-20 Improper Input Validation or CWE-835 Loop Involution if the issue stems from iterative layout calculations. In terms of the MITRE ATT&CK framework, while not a direct attack vector for initial access, such rendering inconsistencies can be leveraged in UI redressing attacks where an attacker attempts to deceive users by overlaying deceptive elements over legitimate interface components due to misaligned boundaries. This falls under techniques related to Obfuscated Files or Information and potentially User Interface Redress if the visual distortion is intentionally crafted to trick user interaction.
Mitigation for this issue has been addressed in Firefox version 155 through updates to the layout engine's grid calculation logic. The fix ensures that boundary conditions are rigorously checked before finalizing the position and size of grid items, preventing the erroneous rendering states previously observed. For organizations relying on older versions of Firefox, it is imperative to apply this update promptly to restore correct visual fidelity and prevent potential exploitation in social engineering contexts where layout integrity is crucial for trust verification. Users should also ensure their browsers are configured to auto-update or manually check for updates regularly to maintain compliance with modern web standards and security best practices.