CVE-2026-84639: Uninitialized memory in MIME parsing
Telemetry drift only. No threshold crossed and no authority ruling changed.
CVE record, CISA KEV and EPSS as of 2026-10-18.
CVEs like this one computed, by meaning
What to do now held facts and links
- Upgrade to the fixed version for your release line, or later: thunderbird 140.15.0; thunderbird 153.2.0; thunderbird 155.0 upper bounds of the CVE record's affected ranges
- Record your decision, dated and frozen with today's evidence: patch by a date · mitigated by a compensating control · deferred · accepted the risk · not affected
- Check your version against the ranges this record publishes.
What it is stated by the CNA
Triggering an error condition in certain MIME bodies would cause uninitialized memory to be used. This vulnerability was fixed in Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.
- weakness
- Not named by the CNA. NVD maps it to CWE-457.
- affected
- Mozilla Thunderbird
- published ranges
- mozilla thunderbird < 140.15.0; mozilla thunderbird >= 141.0 < 153.2.0; mozilla thunderbird >= 154.0 < 155.0
- first unaffected version, by branch
- thunderbird 140.15.0; thunderbird 153.2.0; thunderbird 155.0
- assigned by
- mozilla
- published
- 2026-09-01
check us bugzilla.mozilla.org mozilla.org mozilla.org mozilla.org
Exploit code held, dated
- exploit code
- None held in the PoC-in-GitHub feed. A public repository changes this brief.
What moved held, dated
| date | EPSS | event |
|---|---|---|
| 2 Sep 2026 | 0.0015 | |
| 3 Sep 2026 | 0.00344 | |
| 4 Sep 2026 | 0.00323 |
3 observations held. A step marked with a scoring model moved because the measurement was replaced, not because the vulnerability changed.
Outside the record researched, cited
This section reads the record's references and adds cited findings from outside it. It needs an account. Everything above is held facts and is complete without it.
What would change this brief held, dated
- CISA KEV
- Not listed in the catalogue we hold. A listing changes this brief.
- exploit probability
- 0.00 on 2026-09-04 (FIRST's EPSS, a probability and not evidence of exploitation). Next band line: 10% (0.10). Crossing a line changes this brief.
- fix
- Held: thunderbird 140.15.0; thunderbird 153.2.0; thunderbird 155.0. A newer fix from the CNA or OSV changes this brief.
How to check this
Every value above is held with two dates: when it was true upstream (as_of) and when we recorded it (tx_from). Re-derive the exploit probability as it stood on any date:
SELECT value, source, source_version, as_of, tx_from
FROM volatile_valid_as_of('CVE-2026-84639', '2026-10-29T23:59:59.999999Z', now());