Affects widely used Mozilla products (Firefox/Thunderbird); site‑isolation bypass can expose cross‑origin data across browsers, requiring org‑wide mitigation.
CVE-2026-16387
Site‑isolation bypass in Firefox/Thunderbird enabling cross‑origin data exposure.
Is CVE-2026-16387 being exploited?
Not confirmed. CVE-2026-16387 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 0.20% probability of exploitation in the next 30 days.
How severe is CVE-2026-16387?
CVE-2026-16387 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-2026-16387?
Yes. A fix has been recorded for CVE-2026-16387. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-16387 affect?
CVE-2026-16387 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-2026-16387?
Update to Firefox 153 / ESR 140.13 and Thunderbird 153 / ESR 140.13 immediately
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Remote bypass of site isolation to read cross‑origin content and browser data, enabling data exfiltration and session compromise.
Immediate action required