CVE-2026-4700
HTTP mitigation bypass in Firefox/Thunderbird (<149/140.9).
Is CVE-2026-4700 being exploited?
Not confirmed. CVE-2026-4700 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 0.46% probability of exploitation in the next 30 days.
How severe is CVE-2026-4700?
CVE-2026-4700 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-2026-4700?
Yes. A fix has been recorded for CVE-2026-4700. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-4700 affect?
CVE-2026-4700 affects Firefox <149, Firefox ESR <140.9, Thunderbird <149, Thunderbird <140.9. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-4700?
Update to Firefox 149+ / Thunderbird 149 or ESR 140.9+
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Bypasses HTTP mitigations; could enable exploit delivery via web content and data exposure.
Patch when possible