Vulnerability in Mozilla Firefox
CVE-2025-0239
When using Alt-Svc, ALPN did not properly validate certificates when the original server is redirecting to an insecure site. This vulnerability was fixed in Firefox 134, Firefox ESR 128.6, Thunderbird 134, and Thunderbird 128.6.
Vulnerability class: Improper Certificate Validation
Published · last modified .
CVSS v3 metric
CVSS v3 base score 4.0 (Medium). Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N.
EPSS exploit prediction
EPSS: 0.002 (12.2th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 12th percentile — 12.2% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.002 · newest: 0.002 · change: 0.000
Affected products
- Mozilla Firefox — versions 128.6, 134
- Mozilla Thunderbird — versions 128.6, 134
Weakness classification (CWE)
References
- bugzilla.mozilla.org/show_bug.cgi (Permissions Required, Issue Tracking)
- www.mozilla.org/security/advisories/mfsa2025-01/ (Vendor Advisory)
- www.mozilla.org/security/advisories/mfsa2025-02/ (Vendor Advisory)
- www.mozilla.org/security/advisories/mfsa2025-04/ (Vendor Advisory)
- www.mozilla.org/security/advisories/mfsa2025-05/ (Vendor Advisory)
- af854a3a-2127-422b-91ae-364da2661108
Frequently asked questions
- What is CVE-2025-0239?
- CVE-2025-0239 is a medium-severity vulnerability in Mozilla Firefox, classified under Improper Certificate Validation. CVSS score: 4.0/10. Published 2025-01-07.
- How severe is CVE-2025-0239?
- Medium severity. CVSS v3 base score is 4.0 out of 10.