Critical sandbox-escape RCE in widely used Firefox and Thunderbird; enables endpoint compromise and is high-impact for many organizations.
CVE-2026-84119
Use-after-free in DOM Navigation allows sandbox escape in Firefox/Thunderbird
Is CVE-2026-84119 being exploited?
Not confirmed. CVE-2026-84119 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.32% probability of exploitation in the next 30 days.
How severe is CVE-2026-84119?
CVE-2026-84119 is rated Critical with a CVSS score of 9.6. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-84119?
Yes. A fix has been recorded for CVE-2026-84119. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-84119 affect?
CVE-2026-84119 affects Mozilla Firefox < 155, Firefox ESR < 115.40, Firefox ESR < 140.15, Firefox ESR < 153.2, and 3 further products or versions. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-84119?
Update to Firefox 155/ESR 115.40,140.15,153.2 and Thunderbird 155/ESR fixed releases immediately
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Crafted web content can trigger sandbox escape and arbitrary code execution, enabling full host compromise from browser/email client.
Immediate action required