Affects widely used Firefox and Thunderbird clients; mitigation bypass can enable remote compromise via web/email content and warrants org-wide action.
CVE-2026-16383
Mitigation bypass in Firefox/Thunderbird DOM networking enabling remote compromise.
Is CVE-2026-16383 being exploited?
Not confirmed. CVE-2026-16383 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 0.39% probability of exploitation in the next 30 days.
How severe is CVE-2026-16383?
CVE-2026-16383 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-2026-16383?
Yes. A fix has been recorded for CVE-2026-16383. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-16383 affect?
CVE-2026-16383 affects Firefox 153 (fixed), Firefox ESR 140.13 (fixed), Thunderbird 153 (fixed), Thunderbird ESR 140.13 (fixed). Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-16383?
Update Firefox/Thunderbird to the fixed versions (153 / ESR 140.13) immediately.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Remote web or email content can bypass browser mitigations to access cross-origin data, exfiltrate information, or enable arbitrary code execution on client systems.
Immediate action required