A flaw in Alt‑Service (Alt‑Svc) handling caused the ALPN mechanism to fail to validate TLS certificates when the original server redirected to an insecure site. The control failure allowed an attacker to trick a client into believing it was communicating securely with a legitimate server while actually connecting to a malicious server, creating a full interception path for traffic modification or eavesdropping.

The vulnerability is relevant to Mozilla Firefox versions older than 134 and the Firefox ESR branch older than 128.6, as well as Mozilla Thunderbird versions older than 134 and its ESR branch older than 128.6. Users of those browsers on any platform that uses Alt‑Svc for protocol negotiation are potentially impacted.

The CVSS score of 4.0 indicates a low overall severity, and the EPSS score of less than 1% suggests a very low probability of exploitation. The vulnerability is not listed in CISA’s KEV catalog. The likely attack vector, inferred from the description, requires an attacker to control a server that a victim’s browser connects to via Alt‑Svc and to orchestrate an insecure redirect; this makes exploitation non-trivial but still possible in environments with exposed Alt‑Svc endpoints.