Traffic redirection via cached Alt-Svc validation
Published Jan 7, 2025 · Updated Apr 13, 2026
Improper ALPN validation in Mozilla Firefox and Thunderbird before 134, including ESR before 128.6, allows man-in-the-middle traffic redirection. The Alt-Svc cache preserves validation from the first negotiated connection, so later connections reuse trusted state without confirming that ALPN matches the advertised protocol. Exploitation requires an origin to advertise an attacker-controlled alternative service and the attacker to forward the first connection to a valid HTTP/2 server before switching later traffic to another TLS endpoint.
Summary
What happened
Improper ALPN validation in Mozilla Firefox and Thunderbird before 134, including ESR before 128.6, allows man-in-the-middle traffic redirection. The Alt-Svc cache preserves validation from the first negotiated connection, so later connections reuse trusted state without confirming that ALPN matches the advertised protocol. Exploitation requires an origin to advertise an attacker-controlled alternative service and the attacker to forward the first connection to a valid HTTP/2 server before switching later traffic to another TLS endpoint.
The record
- CVE
- CVE-2025-0239
- Published
- Jan 7, 2025
- Updated
- Apr 13, 2026
- Vendor
- Unknown vendor
- Product
- Unknown product
- Classifications
- CWE-295, T1557
- Attack vector
- adjacent
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jan 7, 2025CVE publishedPublication date reported by the CVE source.
- Apr 13, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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What is affected?
Affected products and versions are unavailable in this record.
Published CVSS scores
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Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Public exploit references
- Alt-Svc ALPN reproducerproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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