CVE-2026-4692: Mozilla Firefox and Thunderbird sandbox escape in Responsive Design Mode

CVE-2026-4692 · Severity: critical · CVSS 10 · Published 2026-03-24

A critical security vulnerability has been identified in Mozilla Firefox and Thunderbird's Responsive Design Mode, a tool used by developers to test how websites look on different screen sizes. This flaw allows a malicious website to break out of the browser's security sandbox, which is designed to keep web content isolated from the rest of your computer. If exploited, an attacker could potentially gain unauthorized access to your local files, sensitive data, or take control of the underlying operating system.

Technologies: Mozilla Firefox, Mozilla Firefox ESR, Mozilla Thunderbird. Vendors: Mozilla.

Quick answers

Is CVE-2026-4692 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-4692 being exploited in the wild.
How severe is CVE-2026-4692?
Critical severity, with a CVSS score of 10, published 24 March 2026.
Which products and versions are affected?
Mozilla Firefox < 149; Mozilla Firefox ESR < 115.34, < 140.9; Mozilla Thunderbird < 149, < 140.9.
Is there a fix for CVE-2026-4692?
A patch was published on 2026-03-24: Fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-4692 attacked?
It is attacked over the network, in a network appliance.
Am I affected by CVE-2026-4692?
Check whether you run Mozilla Firefox, Mozilla Firefox ESR or Mozilla Thunderbird at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

A sandbox escape vulnerability exists in the Responsive Design Mode (RDM) component of Mozilla browsers and mail clients. The flaw is categorized under CWE-653 (Improper Isolation or Compartmentalization) and allows web content to bypass the security boundaries intended to restrict it to the browser process. While specific root cause details are restricted in the associated Bugzilla report (Bug 2017643), the vulnerability is reachable via network vectors and carries a CVSS 3.1 score of 10.0 due to the potential for full system compromise (Scope Change). Patches are available in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, and corresponding Thunderbird releases.

Affected products

  • Mozilla Firefox < 149
  • Mozilla Firefox ESR < 115.34, < 140.9
  • Mozilla Thunderbird < 149, < 140.9
  • Red Hat Enterprise Linux Server (v. 7 ELS) affected

Timeline

  • 2026-03-24: advisory: Mozilla Foundation Security Advisories MFSA2026-20 through MFSA2026-24 published.
  • 2026-03-24: patched: Fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

References

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