CVE-2026-2777
Privilege escalation in Firefox/Thunderbird Messaging System (pre-148/ESR)
Is CVE-2026-2777 being exploited?
Not confirmed. CVE-2026-2777 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 0.37% probability of exploitation in the next 30 days.
How severe is CVE-2026-2777?
CVE-2026-2777 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-2026-2777?
Yes. A fix has been recorded for CVE-2026-2777. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-2777 affect?
CVE-2026-2777 affects Mozilla Firefox (<148, ESR <115.33/140.8), Mozilla Thunderbird (<148, ESR <140.8). Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-2777?
Patch when available; enable auto-update; limit Messaging System exposure.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Local privilege escalation in Firefox/Thunderbird Messaging System; attacker gains higher privileges in the app.
Immediate action required