CVE-2026-109736 in Firefox
Summary
by MITRE • 10/13/2026
Frame poisoning in the Layout component. This vulnerability was fixed in Firefox 158 and Firefox ESR 153.5.
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Analysis
by VulDB Data Team • 10/13/2026
The identified security issue represents a frame poisoning vulnerability within the layout rendering engine of Mozilla Firefox, specifically affecting versions prior to 158 and Firefox Extended Support Release (ESR) version 153.5. This class of vulnerability typically arises when user-controlled input or external data sources are improperly validated before being integrated into critical internal structures that govern how web pages are rendered on the screen. In this specific context, the flaw resides in the component responsible for calculating and managing the spatial arrangement of elements within a document object model tree. When an attacker can influence the parameters passed to these layout calculations through crafted HTML or CSS content hosted on a malicious website, they may cause the browser to misinterpret memory boundaries or structural integrity checks associated with frame objects.
From a technical perspective, frame poisoning often involves manipulating properties that control how nested browsing contexts interact with their parent containers. If the layout engine fails to sanitize inputs related to dimensions, positioning, or visibility states of iframes, it can lead to unexpected behavior in how content is displayed and processed. This misalignment between expected state and actual rendered output creates an opportunity for exploitation where malicious scripts might escape sandbox restrictions or access sensitive data from adjacent frames that should remain isolated. The vulnerability exploits the trust relationship between different parts of the browser interface, allowing a compromised page to potentially read or modify information belonging to other origins due to improper isolation mechanisms during layout recalculations.
The operational impact of this vulnerability is significant for users browsing untrusted content on the web. Successful exploitation could allow an attacker to perform cross-site scripting attacks with elevated privileges, steal session cookies, or conduct clickjacking attacks by manipulating the visual presentation of legitimate websites. By poisoning the frame structure, attackers can trick users into interacting with hidden elements or overlaying malicious interfaces over trusted applications. This undermines the fundamental security model of web browsers which relies on strict origin isolation to prevent data leakage between different domains. Users who visit compromised sites may find their sensitive information exposed without any direct interaction beyond loading the page.
Mitigation for this vulnerability is primarily addressed through software updates provided by Mozilla. Administrators and individual users should ensure that Firefox is updated to version 158 or later, or if using the Extended Support Release channel, update to version 153.5 or higher. These releases contain patches that enforce stricter validation rules on layout inputs and improve the robustness of frame isolation mechanisms. Until updates are applied, organizations can implement network-level filtering to block access to known malicious domains and employ content security policies in their own web applications to restrict iframe embedding capabilities where possible. Additionally, enabling browser features such as enhanced tracking protection adds an extra layer of defense against scripts attempting to exploit layout-related flaws for data exfiltration or unauthorized actions within the browsing context.
This vulnerability aligns with Common Weakness Enumeration categories related to improper input validation and security misconfigurations in web applications. Specifically it relates to CWE-20 Improper Input Validation as the root cause lies in insufficient checking of external data before processing by the layout engine. In terms of attack tactics, this falls under MITRE ATT&CK techniques involving client-side code injection or browser exploitation for initial access and credential theft. The lack of proper sanitization allows attackers to manipulate the rendering pipeline effectively bypassing standard security controls designed to keep different web contexts separate from one another.