CVE-2026-84121: mozilla firefox, mozilla thunderbird

CVSS 9.6 (critical) · published 2026-09-01

Sandbox escape due to use-after-free in the DOM: Security component. This vulnerability was fixed in Firefox 155, Firefox ESR 115.40, Firefox ESR 140.15, Firefox ESR 153.2, Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.

Affected products (NVD): mozilla firefox, mozilla thunderbird

Fixed in (NVD): 115.40.0, 140.15.0, 153.2.0, 155.0.0, 155.0

Weakness: CWE-416

Source: NVD CVE-2026-84121

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=2059018
• https://www.mozilla.org/security/advisories/mfsa2026-82/
• https://www.mozilla.org/security/advisories/mfsa2026-83/
• https://www.mozilla.org/security/advisories/mfsa2026-84/
• https://www.mozilla.org/security/advisories/mfsa2026-85/
• https://www.mozilla.org/security/advisories/mfsa2026-86/
• https://www.mozilla.org/security/advisories/mfsa2026-87/
• https://www.mozilla.org/security/advisories/mfsa2026-88/

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