CVE-2025-0239 Alt-Svc ALPN validation failure when redirected
Attackers can bypass certificate validation during Alt-Svc redirects, potentially enabling man-in-the-middle attacks against Firefox and Thunderbird users.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-0239?
CVE-2025-0239 is a medium-severity vulnerability in Mozilla Firefox, published on 7 Jan 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
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.
Weakness: CWE-295
Which versions are affected by CVE-2025-0239?
It affects 2 products.
Products include Mozilla Firefox, Mozilla Thunderbird.
How do I fix CVE-2025-0239?
Upgrade to a fixed version.
Is CVE-2025-0239 being exploited?
There is no known exploitation of CVE-2025-0239 in the wild and no public exploit code.
Other vulnerabilities in Mozilla Firefox
Sources
Also known as DEBIAN-CVE-2025-0239, GHSA-p4q7-g7ff-823j, UBUNTU-CVE-2025-0239.