CVE-2026-84133 in Firefox
Summary
by MITRE • 09/01/2026
Site isolation issue in the DOM: Push Subscriptions 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 site isolation mechanisms within the Document Object Model, specifically affecting the Push Notifications subsystem of Mozilla Firefox. Site isolation is a fundamental security architecture designed to prevent cross-site data leakage by ensuring that resources from different origins are processed in separate processes with isolated memory spaces. When this boundary is compromised, particularly through complex components like push subscriptions which often interact across multiple domains and service workers, it creates an avenue for attackers to bypass these protections. The flaw likely stems from improper handling of context boundaries or state management during the lifecycle of a push subscription, allowing malicious scripts running in one origin to access sensitive data belonging to another origin that should remain strictly isolated.
From a technical perspective, this issue falls under the category of cross-origin information leakage and violates the principle of least privilege enforced by modern browser security models. The vulnerability allows an attacker who controls or influences a webpage with push notification capabilities to potentially read state or credentials associated with a different site's push subscription context. This could lead to unauthorized access to user-specific identifiers, session tokens, or other sensitive metadata that is typically protected by the same-origin policy and enhanced isolation features like Site Isolation in Firefox. The complexity of the Push API, which involves asynchronous messaging between service workers, background tasks, and main thread scripts, increases the attack surface for such logic errors where context boundaries are not strictly enforced during state transitions or data retrieval operations.
The operational impact of this vulnerability is significant as it undermines user privacy and security by enabling potential cross-site tracking or session hijacking scenarios. An attacker could exploit this flaw to correlate user activities across different websites, build detailed profiles without consent, or in more severe cases, escalate privileges within the browser environment if combined with other vulnerabilities. Since push notifications are often used for critical alerts and updates, compromising their isolation could also facilitate phishing attacks by allowing malicious sites to mimic legitimate notification behaviors while stealing underlying context data. Users relying on Firefox for sensitive activities such as online banking, email access, or corporate portal usage would be exposed to these risks until the browser is updated.
Mitigation strategies primarily involve applying the vendor-provided patches immediately upon availability. Mozilla has addressed this issue in Firefox version 155 and Firefox Extended Support Release (ESR) version 153.2. Organizations should enforce automated update policies to ensure all endpoints are running these patched versions or later. For environments where immediate patching is not feasible, network-level controls such as strict Content Security Policy headers can help limit the execution of untrusted scripts that might attempt to exploit this flaw. Additionally, disabling push notifications for non-essential sites reduces the attack surface by removing the vulnerable component from active use in high-security contexts.
This vulnerability aligns with CWE-200, which covers exposure of sensitive information to an unauthorized actor, and relates to ATT&CK technique T1537, involving communication through cloud accounts or services that may be abused for data exfiltration if isolation is broken. It also reflects broader concerns regarding browser engine security as defined in industry standards like OWASP Web Security Testing Guide sections on client-side protection mechanisms. Continuous monitoring of vulnerability databases and adherence to rapid patching cycles are essential defenses against such architectural flaws in web browsers.