CVE-2026-117991 in Firefox
Summary
by MITRE • 10/27/2026
Incorrect boundary conditions in the Layout: Text and Fonts 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, Text, and Fonts subsystem of Mozilla Firefox, specifically involving incorrect boundary condition handling during text rendering operations. In complex web environments where dynamic content injection or rapid style updates occur, the engine may fail to properly validate memory offsets or array indices associated with font metrics and layout boxes. This logical error typically manifests when processing edge cases in Unicode string lengths, variable-width fonts, or nested flexbox/grid layouts that push internal data structures beyond their expected limits. The flaw allows for out-of-bounds reads or writes within the rendering pipeline's memory space, which can lead to unpredictable behavior ranging from visual corruption of web pages to more severe security implications depending on how the browser handles the corrupted state.
From a technical perspective, this issue aligns with CWE-125 (Out-of-Bounds Read) or CWE-787 (Out-of-Bounds Write), as it involves accessing memory locations outside the intended buffer boundaries due to insufficient validation of input parameters related to text layout calculations. The vulnerability is particularly relevant in scenarios involving high-density typography, international character sets with complex shaping requirements, or CSS properties that trigger reflow events under constrained viewport sizes. Attackers could potentially craft malicious web pages containing specially designed HTML and CSS structures to exploit these boundary errors. While the immediate impact might be limited to denial of service through browser crashes due to memory access violations, such vulnerabilities can sometimes be chained with other flaws to achieve arbitrary code execution or information disclosure if adjacent memory regions contain sensitive data like cookies, session tokens, or credentials stored in the process space.
The operational impact of this vulnerability is primarily centered on stability and reliability for end-users encountering specific web content. Users may experience sudden tab crashes, browser freezes, or rendering artifacts that disrupt normal browsing activities. For enterprise environments relying on Firefox Extended Support Release (ESR), such instability can affect productivity if critical internal applications depend on consistent layout behavior across different devices and screen resolutions. Furthermore, the existence of this flaw indicates a potential attack surface for sophisticated adversaries aiming to fingerprint user configurations or trigger side-channel attacks by analyzing timing differences in rendering failures, although direct exploitation requires precise control over memory state which is mitigated by modern browser sandboxing mechanisms like Content Security Policy and process isolation.
To mitigate these risks, it is imperative that all instances of Firefox are updated to version 159 for standard releases or version 153.6 for Extended Support Release channels immediately upon availability. These versions contain the necessary patches that enforce strict boundary checks within the text layout engine before processing font metrics and box dimensions. Administrators should ensure automated update policies are in place to prevent exposure windows where unpatched browsers remain connected to the internet. Additionally, users can reduce their risk profile by disabling JavaScript on untrusted sites or using browser extensions that restrict dynamic content loading until security patches are verified as deployed across the organization's endpoint fleet. Regular auditing of third-party add-ons is also recommended since they may interact with layout engines in ways that exacerbate such boundary condition flaws if not properly sandboxed themselves.