CVE-2026-74959 in Firefox
Summary
by MITRE • 08/18/2026
Mitigation bypass in the Storage: Cache API component. This vulnerability was fixed in Firefox 154, Firefox ESR 140.14, and Firefox ESR 153.1.
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Analysis
by VulDB Data Team • 08/19/2026
The identified vulnerability represents a critical failure within the storage cache application programming interface of Mozilla Firefox browsers, specifically affecting versions prior to Firefox 154, Firefox Extended Support Release 140.14, and Firefox Extended Support Release 153.1. This flaw is categorized as a mitigation bypass, which indicates that existing security controls designed to restrict access or enforce isolation policies were successfully circumvented by an attacker. In the context of web browsers, cache APIs are fundamental for performance optimization but also serve as potential attack surfaces if their sandboxing and permission models are not rigorously enforced. The vulnerability likely stems from a logic error or improper validation within the code that manages how data is stored in the browser's local storage mechanisms, allowing malicious scripts to access resources beyond their intended scope.
From a technical perspective, this type of flaw often involves a violation of the same-origin policy or an elevation of privileges where content running under one origin gains unauthorized read or write access to cache entries belonging to another origin. Such bypasses can occur due to race conditions in asynchronous operations, improper handling of service worker contexts, or flaws in how the browser distinguishes between trusted and untrusted storage requests. When these mitigations fail, an attacker who has managed to inject malicious code into a webpage through cross-site scripting or other injection vectors can exploit this weakness to exfiltrate sensitive data stored locally by legitimate applications. This includes session tokens, authentication cookies, user preferences, or any other persistent state information that the cache API is designed to hold efficiently.
The operational impact of this vulnerability is significant for both individual users and enterprise environments relying on Firefox Extended Support Release versions. For end-users, successful exploitation could lead to account takeover if session identifiers are cached and subsequently stolen by a malicious script running in a different context. It also compromises user privacy by allowing unauthorized tracking or profiling based on stored browsing data that was assumed to be isolated from other web content. In enterprise settings where Firefox ESR is deployed for stability and long-term support, this vulnerability poses a risk of lateral movement if cached credentials are accessed across multiple internal applications hosted under different subdomains but sharing the same browser instance storage scope. The lack of immediate isolation means that a compromise in one less-secure site could potentially impact more critical sites within the same browsing session.
To address this issue, users must upgrade to Firefox 154 or later, as well as the specified Extended Support Release versions which include the necessary patches for this mitigation bypass. Organizations should ensure their patch management processes are updated to prioritize these specific version numbers given the severity of privilege escalation and data exposure risks associated with cache API vulnerabilities. Additionally, implementing strict Content Security Policy directives can help mitigate the impact by restricting the execution context of scripts that might attempt to interact with storage APIs in unauthorized ways. Regular auditing of browser configurations and ensuring that third-party extensions do not override default security settings are also recommended practices to maintain a robust defense-in-depth posture against such client-side vulnerabilities.