CVE-2026-117961 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 security issue 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 documents contain intricate typographic elements such as variable fonts, multi-line text boxes with specific alignment properties, or nested layout containers, the browser's engine must accurately calculate spatial boundaries to prevent visual artifacts and memory corruption. The flaw indicates that under certain edge cases, likely triggered by malformed CSS rules or specially crafted HTML structures, the internal calculations for these boundary conditions fail to validate input data correctly against allocated memory limits or logical constraints. This type of vulnerability is characteristic of improper limit enforcement where the software does not adequately check whether a requested operation exceeds predefined safe thresholds before execution.
From a technical perspective, this defect aligns with CWE-190 Integer Overflow or Wraparound and potentially CWE-787 Out-of-bounds Write if the miscalculation leads to memory being written beyond its intended allocation. When the layout engine processes text metrics such as width, height, or offset positions, it relies on precise arithmetic operations. If these calculations overflow due to large values or negative numbers resulting from complex CSS computations, they may wrap around to small positive integers or invalid addresses. This can result in the browser attempting to access memory regions that are either unmapped or belong to other processes, leading to a crash of the renderer process or potentially allowing an attacker to execute arbitrary code if the vulnerability is combined with other exploitation techniques such as heap spraying or use-after-free conditions within the same rendering pipeline.
The operational impact of this vulnerability is primarily centered on browser stability and potential remote code execution risks for users visiting maliciously crafted web pages. An attacker could host a webpage containing specific CSS stylesheets designed to trigger these incorrect boundary calculations during layout reflow events. Upon loading such a page, Firefox would attempt to process the malformed instructions, leading to immediate application termination in less severe cases or memory corruption that compromises browser security isolation mechanisms in more critical scenarios. This undermines the principle of least privilege by potentially allowing content from an untrusted origin to gain elevated privileges within the sandboxed environment, thereby threatening user data confidentiality and integrity.
Mitigation strategies for this vulnerability involve both technical updates and defensive browsing practices. The primary remediation is upgrading Mozilla Firefox to version 159 or later, including the Extended Support Release (ESR) version 153.6, where these boundary checks have been corrected by the development team. Organizations should ensure that all endpoints running Firefox are patched promptly through automated update mechanisms to minimize exposure windows. Additionally, users and administrators can employ browser hardening techniques such as disabling JavaScript on untrusted sites or using extensions that restrict complex layout rendering features if feasible within their operational context. Monitoring for unusual memory usage patterns in the renderer process may also serve as an early detection indicator for exploitation attempts targeting this class of vulnerabilities before patches are applied.