Browser sandbox escape in widely deployed Firefox/Thunderbird enabling remote code execution via web content; affects many endpoints and requires org-wide mitigation.
CVE-2026-84121
Use-after-free in Firefox/Thunderbird DOM allows sandbox escape and potential remote code execution.
Is CVE-2026-84121 being exploited?
Not confirmed. CVE-2026-84121 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-2026-84121?
CVE-2026-84121 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-2026-84121?
Yes. A fix has been recorded for CVE-2026-84121. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-84121 affect?
CVE-2026-84121 affects Firefox < 155, Firefox ESR (patched in 115.40, 140.15, 153.2), Thunderbird (patched in 155, 140.15, 153.2). Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-84121?
Update Firefox/Thunderbird to the patched versions immediately and enforce client updates.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
DOM use-after-free enables sandbox escape, allowing remote arbitrary code execution via crafted web content or email.
Immediate action required