Firefox/Thunderbird are widely deployed; a critical site-isolation bypass in DOM Push Subscriptions can expose cross-origin data and affect many users/orgs.
CVE-2026-84133
Site-isolation bypass in Firefox/Thunderbird Push Subscriptions
Is CVE-2026-84133 being exploited?
Not confirmed. CVE-2026-84133 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-2026-84133?
CVE-2026-84133 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-2026-84133?
Yes. A fix has been recorded for CVE-2026-84133. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-84133 affect?
CVE-2026-84133 affects Mozilla Firefox < 155, Firefox ESR < 153.2, Mozilla 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-2026-84133?
Upgrade to Firefox/Thunderbird fixed releases (Firefox 155 / ESR 153.2+).
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Origin isolation bypass via Push Subscriptions allowing cross-origin data exposure and unauthorized script access in browser/email contexts.
Immediate action required