Improper Input Validation in io.netty:netty-codec-http3

Summary

Netty's HTTP/3 codec versions from 4.2.2.Final through 4.2.17.Final does not special-case "HTTP/1 CONNECT" authority-form request-targets when converting HTTP/1 messages to HTTP/3 in "HttpConversionUtil.toHttp3Headers". The authority-form target (e.g., "CONNECT trusted.example:443") is parsed as a URI, so its host is emitted as :scheme, :path is set to "/", and the HTTP/1 Host header is used as :authority; if no Host header is present the "CONNECT" target is dropped. In a Netty-based HTTP/1-to-HTTP/3 proxy or gateway, a remote client can send a "CONNECT" request whose Host header names a different authority than the request-target, producing a malformed "HTTP/3 CONNECT" whose tunnel ":authority" is attacker-controlled. This can bypass tunnel allow-lists, egress policy, backend selection, or audit controls that validate the "HTTP/1 CONNECT" request-target before forwarding over HTTP/3.