High-severity site-isolation bypass in widely used Mozilla products (Firefox/Thunderbird) enabling cross-origin data theft; requires org-wide patching.
CVE-2026-84129
Critical site-isolation bypass in Firefox/Thunderbird enabling cross-origin data disclosure.
Is CVE-2026-84129 being exploited?
Not confirmed. CVE-2026-84129 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 0.18% probability of exploitation in the next 30 days.
How severe is CVE-2026-84129?
CVE-2026-84129 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-2026-84129?
Yes. A fix has been recorded for CVE-2026-84129. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-84129 affect?
CVE-2026-84129 affects Firefox < 155, Firefox ESR < 153.2, Thunderbird < 155, Thunderbird ESR < 153.2. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-84129?
Upgrade to Firefox 155 / ESR 153.2 and Thunderbird 155 / ESR 153.2 immediately.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Same-origin policy bypass allowing cross-origin data disclosure; may enable remote code execution via crafted web content.
Immediate action required