High-severity same-origin bypass in widely used browser and email client enabling cross-origin data theft across many users.
CVE-2026-16349
Same-origin policy bypass in Firefox/Thunderbird enabling cross-origin data access.
Is CVE-2026-16349 being exploited?
Not confirmed. CVE-2026-16349 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 0.21% probability of exploitation in the next 30 days.
How severe is CVE-2026-16349?
CVE-2026-16349 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-2026-16349?
Yes. A fix has been recorded for CVE-2026-16349. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-16349 affect?
CVE-2026-16349 affects Mozilla Firefox < 153, Firefox ESR < 115.38, Firefox ESR < 140.13, Mozilla Thunderbird < 153, 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-2026-16349?
Immediately update Firefox and Thunderbird to the fixed versions.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Bypasses same-origin policy to read/modify cross-origin DOM, enabling data exfiltration, session theft, and account takeover via web content.
Immediate action required