Vulnerability in Mozilla Thunderbird
CVE-2026-84642
The values of the mail.allowed_attachment_hostnames advanced config setting were used in a regular expression without escaping. For some possible valid hostnames, this could allow certain unintended hostnames to also match and serve remote attachments. This vulnerability was fixed in Thunderbird 155 and Thunderbird 153.2.
Vulnerability class: ReDoS (Regular Expression Denial of Service)
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N.
EPSS exploit prediction
EPSS: 0.004 (34.6th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 35th percentile — 34.6% 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: 0.004 · change: +0.002
Affected products
Weakness classification (CWE)
References
- bugzilla.mozilla.org/show_bug.cgi (Permissions Required)
- www.mozilla.org/security/advisories/mfsa2026-86/ (Vendor Advisory)
- www.mozilla.org/security/advisories/mfsa2026-88/ (Vendor Advisory)
Frequently asked questions
- What is CVE-2026-84642?
- CVE-2026-84642 is a high-severity vulnerability in Mozilla Thunderbird, classified under Inefficient Regular Expression Complexity. CVSS score: 7.5/10. Published 2026-09-01.
- How severe is CVE-2026-84642?
- High severity. CVSS v3 base score is 7.5 out of 10.