io.netty:netty-handler@4.1.136.Final

io.netty:netty-handler is a library that provides an asynchronous event-driven network application framework and tools for rapid development of maintainable high performance and high scalability protocol servers and clients. In other words, Netty is a NIO client server framework which enables quick and easy development of network applications such as protocol servers and clients. It greatly simplifies and streamlines network programming such as TCP and UDP socket server.

Affected versions of this package are vulnerable to Improper Check for Unusual or Exceptional Conditions through the SslClientHelloHandler.decode ClientHello parser in handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java. An attacker can bypass per-SNI mTLS selection by sending a fragmented TLS ClientHello whose 4-byte handshake header spans multiple records, causing parsing to fail and the connection to fall back to the default SslContext. This breaks deployments that rely on SNI-driven SslContext selection as the only client-certificate check: the request reaches the default TLS configuration instead of the intended SNI-specific context, allowing an unauthenticated client to complete a connection where mTLS was expected.

Upgrade io.netty:netty-handler to version 4.2.17.Final, 4.1.137.Final or higher.