High-severity flaw in widely used browsers (Firefox/Thunderbird) with CVSS 9.8 that can enable broad compromise if exploited.
CVE-2026-16396
Privilege escalation in Firefox/Thunderbird WebExtensions allows elevated code or data access.
Is CVE-2026-16396 being exploited?
Not confirmed. CVE-2026-16396 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 0.24% probability of exploitation in the next 30 days.
How severe is CVE-2026-16396?
CVE-2026-16396 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-2026-16396?
Yes. A fix has been recorded for CVE-2026-16396. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-16396 affect?
CVE-2026-16396 affects Firefox < 153, Firefox ESR < 140.13, Thunderbird < 153, Thunderbird ESR < 140.13. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-16396?
Update Firefox/Thunderbird to 153 or ESR 140.13 immediately; restrict untrusted extensions.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Local or extension-based privilege escalation enabling elevated code execution or access to sensitive extension/browser data.
Immediate action required