CVE-2026-100808 in Firefox
Summary
by MITRE • 09/29/2026
Mitigation bypass in the DOM: Service Workers component. This vulnerability was fixed in Firefox ESR 153.4 and Firefox 157.
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Analysis
by VulDB Data Team • 09/29/2026
The disclosed vulnerability represents a critical failure within the service worker implementation of Mozilla Firefox, specifically affecting versions prior to ESR 153.4 and version 157. Service workers act as network proxies sitting between web applications and the browser, enabling features such as offline caching, push notifications, and background synchronization. The core issue lies in a mitigation bypass mechanism that allows an attacker to circumvent security controls designed to restrict how service workers interact with sensitive browsing contexts or data stores. This flaw undermines the isolation boundaries intended by the browser's security model, potentially allowing malicious scripts executed via service workers to access resources or perform actions that should be prohibited under normal operational constraints.
From a technical perspective, this vulnerability likely stems from an improper validation of scope or permissions when registering or executing service worker scripts. In many modern web architectures, service workers are bound by specific URL scopes and must adhere to strict same-origin policies unless explicitly configured otherwise via headers like Cross-Origin-Opener-Policy or Cross-Origin-Embedder-Policy. The bypass suggests that the browser failed to correctly enforce these restrictions during the registration phase or subsequent message passing events between the main thread and the service worker context. This could allow a malicious site to register a service worker with an overly broad scope, thereby gaining access to data from other origins or manipulating network requests in ways that violate standard security expectations. Such behavior aligns closely with CWE-20 Improper Input Validation, as the browser did not adequately sanitize or verify the constraints applied to the service worker registration process.
The operational impact of this vulnerability is significant for users relying on Firefox for secure web interactions. An attacker could exploit this flaw to perform cross-site request forgery attacks by manipulating network requests made through the compromised service worker. Additionally, it may facilitate data exfiltration if the service worker gains unauthorized access to cached resources or local storage belonging to other domains. In more severe scenarios, this bypass could be leveraged in conjunction with other vulnerabilities to achieve persistent malicious activity on a user's device, as service workers can persist even after the browser is closed. This persistence capability makes the exploitation of such flaws particularly dangerous for long-term security and privacy.
This incident highlights the complexity of maintaining robust isolation boundaries in complex web platforms like Firefox. As browsers continue to adopt more powerful APIs that blur the lines between native applications and web pages, ensuring strict adherence to security policies becomes increasingly challenging. The fix included in Firefox ESR 153.4 and version 157 addresses these logic errors by tightening the validation checks during service worker registration and execution phases. Users are strongly advised to update their browsers immediately to mitigate this risk. Furthermore, developers should ensure that their applications implement appropriate Content Security Policy directives and cross-origin resource sharing configurations to add additional layers of defense against potential exploitation vectors related to service workers.
In terms of threat intelligence classification, this type of vulnerability is often associated with techniques found in the MITRE ATT&CK framework under Web Application Server components or potentially browser-based attacks aimed at bypassing security restrictions. The ability to manipulate network requests and access sensitive data through a privileged context like a service worker places it within the realm of high-severity web vulnerabilities that require immediate attention from both end-users and application developers. By updating to the patched versions, users ensure that these specific mitigation mechanisms are correctly enforced, thereby restoring the intended security posture of their browsing environment.