Critical sandbox escape in widely used Firefox/Thunderbird; could lead to RCE on end-user systems.
CVE-2026-4689
Sandbox escape in Firefox/Thunderbird via XPCOM; potential RCE.
Is CVE-2026-4689 being exploited?
Not confirmed. CVE-2026-4689 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 0.66% probability of exploitation in the next 30 days.
How severe is CVE-2026-4689?
CVE-2026-4689 is rated Critical with a CVSS score of 9.3. 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-2026-4689?
Yes. A fix has been recorded for CVE-2026-4689. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-4689 affect?
CVE-2026-4689 affects Firefox <149, Firefox ESR <115.34, Firefox ESR <140.9, Thunderbird <149, and 1 further product or version. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-4689?
Update to latest Firefox/Thunderbird versions
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Sandbox escape via boundary/overflow could allow arbitrary code execution in user context.
Patch when possible