CVE-2026-4692 is a sandbox escape vulnerability in the Responsive Design Mode component of Mozilla Firefox. The flaw allows an attacker to break out of the browser's sandbox environment, which is designed to isolate web content and prevent it from affecting the host system. This vulnerability was addressed and fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9. The vulnerability is rated critical with a CVSS 3.1 score of 9.6, reflecting its potential for remote exploitation without privileges but requiring user interaction. The Mozilla advisories MFSA2026-20 and MFSA2026-21 provide detailed information on this and other related vulnerabilities fixed in these releases.

Successful exploitation of CVE-2026-4692 could allow an attacker to escape the Firefox sandbox, potentially leading to arbitrary code execution on the host system with the privileges of the user running the browser. The vulnerability impacts confidentiality, integrity, and availability of the affected system. Given the critical CVSS score of 9.6, this represents a severe security risk if left unpatched.

This vulnerability is fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9. Users and administrators should upgrade to these versions or later to mitigate the risk. No additional mitigations are required beyond applying the official patches provided by Mozilla.