Arbitrary file upload in Mozilla Thunderbird
CVE-2026-84637
Malicious calendar invitations could use file URI attachments to launch local or network-hosted executables on Windows, bypassing Thunderbird's normal executable attachment protections. With the new invitation display enabled, the attachment could also appear under a misleading filename. This vulnerability was fixed in Thunderbird 154 and Thunderbird 153.2.
Vulnerability class: Unrestricted File Upload
Published · last modified .
CVSS v3 metric
CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.
EPSS exploit prediction
EPSS: 0.006 (48.9th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 49th percentile — 48.9% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.006 · change: +0.003
Affected products
Weakness classification (CWE)
References
- bugzilla.mozilla.org/show_bug.cgi (Permissions Required)
- www.mozilla.org/security/advisories/mfsa2026-78/ (Vendor Advisory)
- www.mozilla.org/security/advisories/mfsa2026-88/ (Vendor Advisory)
Frequently asked questions
- What is CVE-2026-84637?
- CVE-2026-84637 is a critical-severity vulnerability in Mozilla Thunderbird, classified under Unrestricted Upload of File with Dangerous Type. CVSS score: 9.8/10. Published 2026-09-01.
- How severe is CVE-2026-84637?
- Critical severity. CVSS v3 base score is 9.8 out of 10.