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CVE-2026-84119: Sandbox escape due to use-after-free in the DOM: Navigation component

• Severity: critical (CVSS 9.6 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-09-01 · updated 2026-09-02
• Weakness: CWE-416

Description

Sandbox escape due to use-after-free in the DOM: Navigation 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

• Mozilla Firefox: before 115.40.0; from 116.0 before 140.15.0; from 141.0.0 before 153.2.0; from 154.0.0 before 155.0.0 — fixed in 115.40.0, 140.15.0, 153.2.0, 155.0.0
• Mozilla Thunderbird: before 140.15.0; from 141.0 before 153.2.0; from 154.0 before 155.0 — fixed in 140.15.0, 153.2.0, 155.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 115.40.0, 140.15.0, 153.2.0, 155.0.0; Mozilla Thunderbird 140.15.0, 153.2.0, 155.0. Check the vendor advisory for mitigations.

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=2057817
• 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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