CVE-2026-117959 in Firefox
Summary
by MITRE • 10/27/2026
Site isolation issue in the DOM: Service Workers 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 vulnerability represents a critical failure in the site isolation mechanisms within Mozilla's Gecko rendering engine, specifically affecting the service workers subsystem. 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 memory spaces and contexts. When this boundary is compromised, particularly through complex interactions involving service workers, it creates an avenue for attackers to bypass same-origin policies that are intended to protect user privacy and application integrity. The flaw indicates a lapse in the strict enforcement of origin boundaries during the lifecycle management or message passing phases handled by service workers, which operate as background scripts capable of intercepting network requests and caching resources independently of the main page execution context.
From a technical perspective, this issue likely stems from an improper validation of the originating document's security attributes when communicating with or being controlled by a service worker. Service workers have elevated privileges compared to standard JavaScript contexts because they can persist data across sessions and manipulate HTTP traffic. If the browser fails to correctly isolate these operations based on their associated origin, it may allow one site to access sensitive state information, cookies, or local storage belonging to another site that shares the same top-level frame but is not a direct parent-child relationship in the DOM hierarchy. This type of flaw undermines the core assumption of modern web security models where each tab and its associated service workers are treated as distinct trust boundaries. The vulnerability was addressed in Firefox version 159 and Firefox Extended Support Release (ESR) version 153.6, indicating that Mozilla prioritized this fix to maintain the integrity of their multi-process architecture against sophisticated cross-site attacks.
The operational impact of such a site isolation breach is severe, as it effectively neutralizes one of the primary defenses against Cross-Site Scripting and data exfiltration attacks in modern browsers. An attacker who can exploit this flaw could potentially read sensitive information from other tabs or windows that are not directly related to the malicious page but share certain browser-level contexts. This capability allows for persistent tracking, session hijacking, or theft of authentication tokens stored within service worker caches. Unlike traditional cross-site request forgery which relies on user actions, an isolation bypass can be triggered programmatically without direct user interaction once a vulnerable site is loaded and its associated service workers are active. The persistence of service workers means that even after the initial malicious page is closed, the compromised state might remain accessible to subsequent visits by other sites if they share the same scope or registration path, leading to long-term privacy violations.
This vulnerability aligns with Common Weakness Enumeration (CWE) categories such as CWE-200: Exposure of Sensitive Information to an Unauthorized Actor and CWE-156: Unspecified Wrongly Stored Value in Cache, given that service workers heavily rely on caching mechanisms for performance optimization. In terms of the MITRE ATT&CK framework for browsers, this flaw facilitates techniques related to Collection via Browser Extension or Scripting, specifically allowing attackers to gather data from other origins without proper authorization checks. To mitigate risks associated with similar architectural weaknesses in web applications and browser environments, developers should ensure that service worker scopes are tightly constrained to only the necessary paths and avoid registering workers at root levels unless absolutely required. Furthermore, users must keep their browsers updated to the latest stable or ESR versions where such isolation fixes have been applied. Security teams monitoring for exploitation of this specific flaw in Firefox-based ecosystems should look for anomalous network activity originating from service worker contexts that attempt to access resources outside their registered scope, although detection at runtime remains challenging due to the background nature of these processes.