CVE-2026-74960 in Firefox
Summary
by MITRE • 08/18/2026
Site isolation issue in the WebExtensions 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 • 08/18/2026
The identified site isolation flaw within the Mozilla WebExtensions architecture represents a critical breach of browser security boundaries designed to prevent cross-origin data leakage. Site isolation is a fundamental defense mechanism that ensures processes handling content from one origin are strictly separated from those processing content from other origins, thereby mitigating attacks such as Spectre and preventing unauthorized access to sensitive user data across different websites. In this specific instance, the vulnerability stems from an implementation error in how WebExtensions interact with or share context between isolated frames, allowing a malicious extension or compromised webpage to potentially bypass these isolation boundaries. This failure undermines the core assumption that extensions operate within a sandboxed environment distinct from the main browsing context and other tabs, creating a pathway for privilege escalation where code intended to be restricted by origin can access resources it should not see.
From a technical perspective, this vulnerability likely involves improper handling of cross-frame communication or shared memory segments between extension contexts and web content frames. When site isolation is compromised, an attacker could exploit the lack of strict process separation to read cookies, local storage data, or DOM elements belonging to other origins that are loaded in separate tabs but processed within a context where boundaries have been blurred due to the WebExtensions bug. This type of flaw falls squarely under CWE-200: Information Exposure and CWE-156: Unspecified Wrong Key Value, as it allows for the unauthorized disclosure of sensitive information through improper key management or boundary checks in the browser's internal routing logic. The impact is severe because extensions often possess elevated privileges compared to standard web pages; if an extension can be tricked into leaking data across site boundaries, that data becomes accessible to any script running within the compromised context, effectively neutralizing the protection offered by same-origin policies and content security policies for users relying on those specific features.
The operational impact of this vulnerability extends beyond simple data theft to include potential session hijacking and credential harvesting if attackers can correlate user activity across different sites through shared extension contexts. For enterprise environments using Firefox ESR, such vulnerabilities pose a significant risk as they may be leveraged in targeted attacks against employees who use specific extensions for work-related tasks. The ability to bypass site isolation means that even if users are cautious about which websites they visit, the browser's own security model becomes the vector of compromise. This is particularly dangerous because it exploits trust relationships between different parts of the user agent itself rather than relying on external social engineering or network-level attacks alone.
Mitigation for this issue relies primarily on applying the vendor-provided patches released in Firefox version 154, as well as the extended support release versions 140.14 and 153.1. Organizations should prioritize updating their browser fleets to these patched versions immediately to close the gap in site isolation enforcement. In addition to patching, administrators should audit installed WebExtensions for any that request excessive permissions or exhibit suspicious behavior, as malicious extensions could potentially exploit this vulnerability before patches are applied. Enforcing strict extension policies through group policy or configuration profiles can help limit which extensions are allowed to run, reducing the attack surface. Furthermore, users and security teams should remain vigilant against phishing attempts that might attempt to trigger specific extension behaviors known to interact with vulnerable components until all systems are fully updated. This incident highlights the importance of maintaining up-to-date browser software and regularly reviewing extension permissions to ensure alignment with least-privilege principles in web browsing environments.