CVE-2025-0239: Vulnerability in Mozilla Firefox
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.
AI Analysis
Technical Summary
The vulnerability CVE-2025-0239 involves a failure in the Alt-Svc ALPN certificate validation process in Mozilla Firefox. Specifically, when the original server redirects to an insecure site, the ALPN validation did not properly verify certificates, potentially allowing security bypass. This issue was addressed and fixed in Firefox 134 and Firefox ESR 128.6 releases, as documented in Mozilla's security advisories.
Potential Impact
The vulnerability impacts the integrity of certificate validation during Alt-Svc ALPN negotiation when redirects to insecure sites occur. While it does not affect confidentiality or availability, it could allow an attacker to interfere with the validation process, potentially leading to security bypass scenarios. The CVSS score is 4.0 (medium severity), indicating limited impact without user interaction or elevated privileges. There are no known exploits in the wild.
Mitigation Recommendations
This vulnerability has been 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 actions are required beyond applying the official vendor update.
CVE-2025-0239: Vulnerability in Mozilla Firefox
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.
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The vulnerability CVE-2025-0239 involves a failure in the Alt-Svc ALPN certificate validation process in Mozilla Firefox. Specifically, when the original server redirects to an insecure site, the ALPN validation did not properly verify certificates, potentially allowing security bypass. This issue was addressed and fixed in Firefox 134 and Firefox ESR 128.6 releases, as documented in Mozilla's security advisories.
Potential Impact
The vulnerability impacts the integrity of certificate validation during Alt-Svc ALPN negotiation when redirects to insecure sites occur. While it does not affect confidentiality or availability, it could allow an attacker to interfere with the validation process, potentially leading to security bypass scenarios. The CVSS score is 4.0 (medium severity), indicating limited impact without user interaction or elevated privileges. There are no known exploits in the wild.
Mitigation Recommendations
This vulnerability has been 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 actions are required beyond applying the official vendor update.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- mozilla
- Date Reserved
- 2025-01-06T14:49:04.597Z
- Cvss Version
- 3.1
- State
- PUBLISHED
Threat ID: 6909325c35043901e83099ff
Added to database: 11/3/2025, 10:53:16 PM
Last enriched: 4/14/2026, 11:32:30 AM
Last updated: 5/10/2026, 3:06:00 AM
Views: 222
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