CVE-2026-84639
Use of uninitialized memory in Thunderbird MIME parsing allowing RCE or data disclosure.
Is CVE-2026-84639 being exploited?
Not confirmed. CVE-2026-84639 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-2026-84639?
CVE-2026-84639 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-2026-84639?
Yes. A fix has been recorded for CVE-2026-84639. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-84639 affect?
CVE-2026-84639 affects Mozilla Thunderbird (versions prior to 155, 153.2, 140.15). Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-84639?
Upgrade Thunderbird to 155, 153.2, or 140.15 immediately.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Crafted MIME triggers use of uninitialized memory in Thunderbird, enabling remote code execution, memory disclosure, or crashes via a received email.
Immediate action required