CVE-2026-84140 in Firefox
Summary
by MITRE • 09/01/2026
Site isolation issue in the DOM: Navigation component. This vulnerability was fixed in Firefox 155 and Firefox ESR 153.2.
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Analysis
by VulDB Data Team • 09/01/2026
The identified vulnerability represents a critical failure in the site isolation mechanisms within the Document Object Model navigation components of Mozilla Firefox, specifically affecting versions prior to 155 and Firefox Extended Support Release 153.2. Site isolation is a fundamental browser security architecture designed to prevent cross-site scripting attacks from accessing sensitive data across different origins by ensuring that each website runs in its own isolated process with distinct memory spaces. When this mechanism fails during navigation events, it creates a window of opportunity for attackers to bypass these boundaries and access resources or state information belonging to other sites, thereby undermining the core trust model of modern web browsers.
The technical flaw likely stems from an improper handling of context transitions when navigating between different origins within the DOM structure. In typical site isolation implementations, the browser must ensure that all data associated with a specific origin is strictly segregated and not accessible by processes running code from other origins. The vulnerability suggests that during certain navigation sequences, such as redirects or frame loads, the browser may have inadvertently retained references to sensitive objects or allowed cross-origin access to internal state before the isolation boundaries were fully enforced. This lapse allows malicious scripts loaded on one origin to potentially read cookies, local storage data, or DOM elements belonging to another origin if they can manipulate the navigation flow in a specific way that triggers this race condition or logic error.
From an operational perspective, this vulnerability significantly increases the risk of cross-site scripting and data exfiltration attacks. Attackers could exploit this flaw to steal session tokens, personal identifiable information, or other sensitive user data by tricking victims into navigating through maliciously crafted links or embedded frames that trigger the flawed navigation logic. The impact is particularly severe because it targets a foundational security feature rather than an application-level bug, meaning that even well-secured websites with strict content security policies could be compromised if their users are directed to exploit this browser-level weakness. This type of vulnerability erodes user trust and can lead to widespread data breaches across the web ecosystem.
The classification of this issue aligns closely with CWE-200, which covers exposure of sensitive information to an unauthorized actor, as well as CWE-862, involving missing authorization checks in critical security features. In terms of the MITRE ATT&CK framework for browsers and extensions, this vulnerability facilitates techniques related to client-side protocol manipulation and potentially cross-site scripting by weakening the isolation guarantees that prevent lateral movement or data theft between tabs and frames. It highlights the complexity of maintaining robust process boundaries in complex rendering engines where navigation events must be handled atomically to prevent state leakage.
To mitigate this risk, users are strongly advised to update their Firefox installation immediately to version 155 or later, including the ESR branch at version 153.2, as these releases contain the necessary patches to correct the site isolation logic in the DOM navigation components. For organizations deploying Firefox across enterprise environments, it is crucial to ensure that automatic updates are enabled or to manually deploy the patched versions through centralized management tools to prevent exposure during any window of vulnerability. Additionally, developers should continue to adhere to strict content security policies and avoid relying solely on browser-level isolation for application security, ensuring defense-in-depth strategies remain robust even if underlying engine vulnerabilities exist.