CVE-2025-0239: When using Alt-Svc, ALPN did not properly validate certificates when the original server is
Source GCVE Database
Not known to be exploited · CVSS 4.0 Medium · fixed in 134 : update affected systems when convenient.
Data as of 30 Oct 2026, 16:01 UTC
- Patch
- Available – fixed in 134, 128.6
CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:NExploitation evidence
Disclosed: evidence found.
CVE published 7 Jan 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors
A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 30 Oct 2026.
What to do
AI-suggested
This vulnerability is fixed in Firefox 134 and Firefox ESR 128.6. Users and administrators should update to these versions or later to remediate the issue. No additional mitigation steps are required beyond applying the official update.
Description
CVE-2025-0239 is a vulnerability in Firefox versions prior to 134 and Firefox ESR prior to 128.6 where the Alt-Svc ALPN feature did not properly validate certificates when the original server redirects to an insecure site. This flaw could allow an attacker to bypass certificate validation under these specific conditions.
References
- nvd.nist.gov/vuln/detail/CVE-2025-0239 (opens in a new tab)
- bugzilla.mozilla.org/show_bug.cgi?id=1929156 (opens in a new tab)
- lists.debian.org/debian-lts-announce/2025/01/msg00004.html (opens in a new tab)
- mozilla.org/security/advisories/mfsa2025-01 (opens in a new tab)
- mozilla.org/security/advisories/mfsa2025-02 (opens in a new tab)
- mozilla.org/security/advisories/mfsa2025-04 (opens in a new tab)
- mozilla.org/security/advisories/mfsa2025-05 (opens in a new tab)