CVE-2026-84129 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 Mozilla Firefox, specifically affecting the Document Object Model navigation components. Site isolation is a fundamental security architecture designed to prevent cross-site scripting attacks by ensuring that data from one origin cannot be accessed by another, even if they share the same process space. In this instance, the flaw lies in how the browser handles navigation events and state transitions within the DOM, allowing an attacker to potentially bypass these isolation boundaries under specific conditions. This breakdown undermines the core assumption of compartmentalization that modern browsers rely on to protect user data from malicious websites or compromised third-party scripts.
From a technical perspective, the defect likely involves improper validation or separation of context during page navigation events. When a user navigates between pages or when dynamic content triggers a state change within the DOM, the browser is supposed to enforce strict origin checks and clear sensitive memory associated with previous contexts. The vulnerability suggests that these safeguards were not consistently applied, potentially allowing information leakage or unauthorized access to cross-origin data. This could manifest as an ability for a malicious script running in one context to infer or extract state from another context through timing attacks, cache side-channels, or direct DOM manipulation if the isolation boundary was incorrectly maintained during the transition phase handled by the navigation component.
The operational impact of this vulnerability is significant because it erodes trust in the browser's ability to contain threats within individual tabs or frames. If an attacker can exploit this flaw, they may be able to perform cross-site request forgery with elevated privileges, steal session tokens that should have been isolated, or conduct sophisticated phishing attacks by manipulating UI elements across origin boundaries. This is particularly dangerous for users accessing sensitive applications such as online banking, email services, or corporate portals where the integrity of site isolation is paramount. The existence of this flaw means that even if a user visits a malicious website, there is a non-negligible risk that data from other open tabs could be compromised depending on how the navigation state was managed during the exploit execution.
This issue aligns with CWE-200, which covers exposure of sensitive information to an unauthorized actor, and potentially CWE-862 regarding missing authorization checks within complex system interactions. In terms of MITRE ATT&CK mapping, this vulnerability facilitates techniques related to Credential Access via Browser Harvesting or Data Exfiltration over Alternative Channels if the isolation breach allows for covert data transfer between origins. It highlights the complexity involved in maintaining robust security boundaries in highly interactive web environments where dynamic content and frequent navigation are common.
Mitigation strategies primarily involve applying the vendor-provided updates immediately, as this issue has been resolved in Firefox version 155 and Firefox Extended Support Release (ESR) version 153.2. Administrators managing enterprise deployments should ensure that all endpoints running these versions of Firefox are patched to prevent exploitation. Additionally, users can employ additional layers of defense such as using strict content security policies on their own websites if they host sensitive applications, although this does not protect against browser-level flaws directly. Regularly updating the browser is the most effective countermeasure for this class of vulnerability, ensuring that the latest site isolation improvements and bug fixes are active to maintain the integrity of the browsing environment.