CVE-2025-0239 in Firefox
Summary
by MITRE • 01/07/2025
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.
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Analysis
by VulDB Data Team • 04/15/2026
This vulnerability resides in the HTTP alternative services implementation within Mozilla Firefox and Thunderbird browsers, specifically affecting the Application Layer Protocol Negotiation mechanism. The flaw manifests when the Alt-Svc header is processed during HTTP redirects, creating a certificate validation bypass scenario that undermines the security assurances typically provided by SSL/TLS connections. The vulnerability stems from improper certificate validation logic that fails to adequately verify the authenticity and trustworthiness of certificates when transitioning from secure to insecure connections through redirect mechanisms. This represents a critical breakdown in the browser's security model where the system allows potentially malicious redirects to proceed without proper certificate verification, effectively creating a pathway for man-in-the-middle attacks.
The technical implementation flaw occurs within the ALPN handling code where certificate validation is skipped or weakened when processing redirects that involve insecure sites. This vulnerability directly relates to CWE-295 which addresses improper certificate validation and CWE-310 which covers cryptographic issues. The flaw exploits the trust model where browsers typically maintain certificate validation integrity even during redirects, but in this case the validation process becomes conditional on the redirect target's security status. When an original server responds with an Alt-Svc header directing traffic to a different host, the browser should maintain strict certificate validation regardless of whether the destination is secure or insecure. However, the implementation fails to enforce this requirement, potentially allowing attackers to substitute certificates during the redirect process.
The operational impact of this vulnerability extends beyond simple certificate validation failures to encompass broader security implications for web browsing and email clients. Attackers could leverage this flaw to perform certificate substitution attacks during HTTP redirects, potentially intercepting or modifying traffic between users and web services. The vulnerability affects both HTTP and HTTPS connections, creating potential exposure points where users might be redirected to malicious sites without proper security checks. This particular weakness is especially concerning for email clients like Thunderbird that rely on similar security mechanisms for secure communication protocols. The vulnerability creates a window where users may be unknowingly redirected to compromised sites while maintaining the illusion of secure connections, undermining the fundamental security assurances that users expect from their browsers and email applications.
Mitigation strategies for this vulnerability include immediate deployment of the patched versions mentioned in the advisory, which address the specific certificate validation logic within the ALPN implementation. Organizations should implement comprehensive monitoring for suspicious redirect patterns and certificate changes that might indicate exploitation attempts. Browser administrators should consider additional security measures such as enhanced certificate pinning mechanisms and stricter redirect validation policies. The fix implemented in Firefox 134 and Thunderbird 134 ensures proper certificate validation regardless of redirect destinations, maintaining security integrity throughout the connection lifecycle. Security teams should also review existing security policies to ensure they account for alternative service mechanisms and redirect handling, incorporating these findings into broader security frameworks and compliance requirements. This vulnerability demonstrates the critical importance of maintaining consistent security validation across all connection paths, including those created through HTTP alternative services and redirect mechanisms, aligning with ATT&CK technique T1071.004 for application layer protocol usage and T1566 for credential access through social engineering.