CVE-2025-0239

Name: CVE-2025-0239
Creator: NVD / NIST
Published: 2025-01-07T16:15:38.563+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 4/10EPSS 0.23%

Last modified Jun 17, 2026

CVE-2025-0239 is a medium-severity vulnerability rated 4/10 on the CVSS scale. 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.. EPSS estimates a 0.23% chance of exploitation in the next 30 days.

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.

Metrics

CVSS 3.1

4/10

CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

EPSS Probability

0.23%

13.5th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-295

Affected Software

Vendor	Product	Versions
Mozilla	Firefox	< 128.6.0
Mozilla	Firefox	< 134.0
Mozilla	Thunderbird	< 128.6.0
Mozilla	Thunderbird	>= 129.0, < 134.0

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1929156Issue Tracking, Permissions Required
https://www.mozilla.org/security/advisories/mfsa2025-01/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2025-02/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2025-04/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2025-05/Vendor Advisory
https://lists.debian.org/debian-lts-announce/2025/01/msg00004.html

Timeline

Published: Jan 7, 2025
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2025-0239?

How severe is CVE-2025-0239?

CVE-2025-0239 has a CVSS score of 4/10 (MEDIUM severity). The EPSS model estimates a 0.23% probability of exploitation in the next 30 days.

How do I fix CVE-2025-0239?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2025-0239?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST