Affects widely used Firefox/Thunderbird; sandbox escape could enable remote code execution.
CVE-2026-2778
DOM sandbox escape in Firefox/Thunderbird could enable remote code execution.
Is CVE-2026-2778 being exploited?
Not confirmed. CVE-2026-2778 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 0.48% probability of exploitation in the next 30 days.
How severe is CVE-2026-2778?
CVE-2026-2778 is rated Critical with a CVSS score of 10.0. 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-2026-2778?
Yes. A fix has been recorded for CVE-2026-2778. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-2778 affect?
CVE-2026-2778 affects Firefox <148, Firefox ESR <115.33, Firefox ESR <140.8, Thunderbird <148, and 1 further product or version. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-2778?
Update to latest when patch is released; enable auto-update.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Remote code execution via crafted web content exploiting DOM sandbox escape in Firefox/Thunderbird.
Immediate action required