Critical RCE-level site-isolation bypass in widely used Firefox/Thunderbird browsers; affects many enterprise endpoints and user workstations.
CVE-2026-84140
Site-isolation bypass in Firefox/Thunderbird enabling remote compromise.
Is CVE-2026-84140 being exploited?
Not confirmed. CVE-2026-84140 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-2026-84140?
CVE-2026-84140 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-2026-84140?
Yes. A fix has been recorded for CVE-2026-84140. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-84140 affect?
CVE-2026-84140 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-2026-84140?
Update to Firefox 155/ESR153.2 or Thunderbird 155/153.2 immediately
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Bypass of site isolation via DOM Navigation allowing remote code execution or cross-origin data access from crafted web content.
Immediate action required