CVE-2026-4692: Sandbox escape in the Responsive Design Mode component

Severity: critical (CVSS 10.0 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-03-24 · updated 2026-07-15
Weakness: CWE-653

Description

Sandbox escape in the Responsive Design Mode component. This vulnerability was fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

Affected products

Mozilla Firefox: before 115.34.0; before 149.0; from 128.0 before 140.9.0 — fixed in 115.34.0, 140.9.0, 149.0
Mozilla Thunderbird: before 140.9.0; before 149.0 — fixed in 140.9.0, 149.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 115.34.0, 140.9.0, 149.0; Mozilla Thunderbird 140.9.0, 149.0. Check the vendor advisory for mitigations.

References

https://bugzilla.mozilla.org/show_bug.cgi?id=2017643
https://www.mozilla.org/security/advisories/mfsa2026-20/
https://www.mozilla.org/security/advisories/mfsa2026-21/
https://www.mozilla.org/security/advisories/mfsa2026-22/
https://www.mozilla.org/security/advisories/mfsa2026-23/
https://www.mozilla.org/security/advisories/mfsa2026-24/
https://access.redhat.com/errata/RHSA-2026:5930
https://access.redhat.com/errata/RHSA-2026:5931
https://access.redhat.com/errata/RHSA-2026:5932
https://access.redhat.com/errata/RHSA-2026:6188
https://access.redhat.com/errata/RHSA-2026:6342
https://access.redhat.com/errata/RHSA-2026:6917

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