High-severity (CVSS 9.8) memory-corruption and info-leak in widely used Firefox and Thunderbird could enable RCE across many organizations.
CVE-2026-84143
Critical memory-corruption and info-leak in Firefox/Thunderbird enabling potential remote code execution.
Is CVE-2026-84143 being exploited?
Not confirmed. CVE-2026-84143 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 0.38% probability of exploitation in the next 30 days.
How severe is CVE-2026-84143?
CVE-2026-84143 is rated Critical with a CVSS score of 9.8. 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-2026-84143?
Yes. A fix has been recorded for CVE-2026-84143. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-84143 affect?
CVE-2026-84143 affects Mozilla Firefox <= 154, Firefox ESR <= 153.1, 140.14, Mozilla Thunderbird <= 154, Thunderbird ESR <= 153.1, 140.14. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-84143?
Update Firefox/Thunderbird to the listed patched releases immediately
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Memory corruption may allow remote code execution; information disclosure of sensitive data is also possible.
Immediate action required