Widespread browser SOP bypass enabling cross-origin cookie access; could lead to session hijack across organizations.
CVE-2026-12304
Same-origin policy bypass in browser cookies (Firefox/Thunderbird).
Is CVE-2026-12304 being exploited?
Not confirmed. CVE-2026-12304 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 0.19% probability of exploitation in the next 30 days.
How severe is CVE-2026-12304?
CVE-2026-12304 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-2026-12304?
Yes. A fix has been recorded for CVE-2026-12304. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-12304 affect?
CVE-2026-12304 affects Firefox, Firefox ESR, Thunderbird, Thunderbird ESR. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-12304?
Update to patched versions (Firefox 152+/ESR 140.12+).
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Cross-origin cookie access could permit data theft or session hijack via malicious sites.
Patch when possible