Affects widely used Mozilla Firefox and Thunderbird; critical browser/email client flaw that can expose cross-origin data across organizations.
CVE-2026-74986
CSS parsing bug bypasses site isolation in Firefox/Thunderbird, exposing cross-origin data.
Is CVE-2026-74986 being exploited?
Not confirmed. CVE-2026-74986 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 0.32% probability of exploitation in the next 30 days.
How severe is CVE-2026-74986?
CVE-2026-74986 is rated Critical with a CVSS score of 9.1. 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-2026-74986?
Yes. A fix has been recorded for CVE-2026-74986. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-74986 affect?
CVE-2026-74986 affects Mozilla Firefox < 154, Firefox ESR < 153.1, Mozilla Thunderbird < 154, Thunderbird ESR < 153.1. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-74986?
Update to Firefox 154 / ESR 153.1 and Thunderbird 154 / 153.1 immediately.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Remote CSS parsing bug can bypass site isolation to read cross-origin sensitive data, enabling data exfiltration from browser/email processes.
Immediate action required