Netty's HTTP/3 codec versions 4.2.0.Final through 4.2.17.Final do not enforce the RFC 9114 requirement that the :authority pseudo-header and the literal host header, when both are present, must have the same value. This flaw allows a remote unauthenticated attacker to send a HEADERS frame with conflicting authority values, potentially bypassing routing, virtual-host, and access-control decisions. The issue is fixed in version 4.2.18.Final.