CVE-2026-59900: Netty: [codec-http2] Lack of Host Header Deduplication in HTTP/2→HTTP/1.x Translation Leads to Request Routing Bypass

Netty’s HTTP/2-to-HTTP/1.x translation layer (Http2StreamFrameToHttpObjectCodec and InboundHttp2ToHttpAdapter) fails to deduplicate or validate Host headers when an HTTP/2 client supplies both the :authority pseudo-header and a literal host header in a single HEADERS frame. The translator maps :authority to Host and separately copies the literal host header, producing an HttpRequest object containing two Host headers with attacker-controlled differing values.