A high-severity (CVSS 9.8) flaw in widely used Firefox/Thunderbird can expose user profile data (credentials, cookies), risking account/session takeover and broad user impact.
CVE-2026-84134
Critical profile-backup flaw in Firefox/Thunderbird allowing sensitive profile data disclosure.
Is CVE-2026-84134 being exploited?
Not confirmed. CVE-2026-84134 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 0.31% probability of exploitation in the next 30 days.
How severe is CVE-2026-84134?
CVE-2026-84134 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-2026-84134?
Yes. A fix has been recorded for CVE-2026-84134. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-84134 affect?
CVE-2026-84134 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-2026-84134?
Update to Firefox 155/ESR 153.2 or Thunderbird 155/153.2 immediately.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Remote disclosure of profile data (credentials, cookies, tokens), enabling account takeover, session hijack, and data exfiltration.
Immediate action required