CVE-2026-117915 in Firefox
Summary
by MITRE • 10/27/2026
Incorrect boundary conditions in the Layout: Block and Inline component. This vulnerability was fixed in Firefox 159, Firefox ESR 115.44, and Firefox ESR 153.6.
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Analysis
by VulDB Data Team • 10/27/2026
The identified vulnerability resides within the core rendering engine of Mozilla Firefox, specifically affecting the Layout subsystem where Block and Inline components are processed. This issue stems from incorrect boundary conditions during the calculation of element dimensions and positions. In web layout engines, block-level elements typically establish their own formatting context and occupy a specific rectangular area on the page, while inline elements flow within lines of text or other containers. When these two types interact, particularly in complex nested structures involving margins, padding, borders, or floating elements, precise boundary calculations are critical to ensure correct visual representation. The flaw indicates that under certain edge cases, the engine fails to correctly determine where one element ends and another begins, leading to miscalculations in spatial allocation.
From a technical perspective, this type of error often manifests as incorrect rendering artifacts such as overlapping content, unexpected whitespace gaps, or elements being clipped incorrectly. While primarily a visual correctness issue, improper boundary handling can sometimes be leveraged by attackers if the layout engine interacts with other subsystems like JavaScript execution or memory management in unintended ways. For instance, if an attacker can manipulate the DOM structure to trigger these incorrect boundaries repeatedly or in conjunction with specific CSS properties, it might lead to unstable states within the browser's internal data structures. Although not explicitly classified as a remote code execution vulnerability in this context, such layout bugs are frequently precursors to more severe issues like buffer overflows or use-after-free conditions if memory is allocated based on these flawed boundary calculations.
The operational impact of this vulnerability varies depending on how it is exploited within a malicious webpage. For the average user, the primary consequence is visual degradation where web pages appear broken or misaligned, which can undermine trust in the browser's reliability and potentially disrupt usability for accessibility-focused users who rely on consistent layout structures. However, from a security standpoint, incorrect boundary conditions are significant because they represent a deviation from expected behavior that could be weaponized. Attackers might craft specially designed HTML and CSS payloads to trigger these edge cases, aiming to cause denial of service through resource exhaustion or attempting to bypass content security policies by manipulating the perceived boundaries of iframes or other sandboxed environments.
This vulnerability aligns with Common Weakness Enumeration (CWE) categories such as CWE-682 Incorrect Calculation, which covers errors in arithmetic operations that lead to unexpected behavior. It also relates to CWE-190 Integer Overflow or Wraparound if the boundary calculations involve numeric types that exceed their maximum limits due to incorrect logic. In terms of the MITRE ATT&CK framework for enterprise security, this falls under techniques related to Defense Evasion and potentially Execution, specifically where attackers exploit software vulnerabilities to gain footholds or disrupt services. The specific context of layout engines places it within the broader category of browser-based attacks that target rendering pipelines rather than direct memory corruption in all cases, but the potential for escalation remains a key concern for security analysts.
To mitigate this risk, users must ensure they are running the patched versions of Firefox mentioned, specifically version 159 or later, as well as the Extended Support Release (ESR) channels updated to ESR 115.44 and ESR 153.6. These updates contain the necessary code corrections that enforce strict boundary checks during layout computation. For organizations deploying Firefox at scale, it is imperative to automate patch management processes to ensure all endpoints are upgraded promptly. Additionally, developers should audit their own web applications for complex layouts involving nested block and inline elements, ensuring they do not rely on ambiguous CSS behaviors that might trigger similar issues in other browsers or older versions of Firefox. Regular security assessments and penetration testing focused on rendering engine edge cases can further help identify and remediate such vulnerabilities before they are exploited in the wild.