High-severity (CVSS 9.8) vulnerability in widely used Mozilla products (Firefox/Thunderbird); likely to attract broad attention and requires org-wide remediation.
CVE-2026-16366
Privilege escalation in Firefox/Thunderbird DOM navigation (fixed in 153).
Is CVE-2026-16366 being exploited?
Not confirmed. CVE-2026-16366 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 0.27% probability of exploitation in the next 30 days.
How severe is CVE-2026-16366?
CVE-2026-16366 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-2026-16366?
Yes. A fix has been recorded for CVE-2026-16366. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-16366 affect?
CVE-2026-16366 affects Mozilla Firefox < 153, Mozilla Thunderbird < 153. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-16366?
Update Firefox and Thunderbird to version 153+ immediately
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Allows privilege escalation in the browser/mail client DOM navigation, potentially enabling sandbox escape, arbitrary code execution, or profile compromise via crafted content.
Immediate action required