GHSA-fccg-mwvh-qqg4 Netty: Fragmented ClientHello records trigger quadratic pre-handshake reassembly in default SNI parsing

• Maven io.netty:netty-handler: >= 4.2.0.Final, < 4.2.17.Final → fixed in 4.2.17.Final • Maven io.netty:netty-handler: < 4.1.137.Final → fixed in 4.1.137.Final

Summary Netty's default SNI entrypoint reparses and recopies previously received ClientHello fragments on every additional TLS handshake record. A remote peer can send a small first record that advertises a large ClientHello length and then drip the body in many tiny records, causing superlinear (quadratic) CPU work before the handshake completes. With 4095 one-byte fragments, the handler recopies 8,386,560 bytes from only 24,579 bytes on the wire — a 341× amplification ratio. Affected Entrypoints • `io.netty.handler.ssl.SniHandler` — default constructors • `io.netty.handler.ssl.SslClientHelloHandler` — pre-handshake ClientHello aggregation path Vulnerable Code Locations • `handler/src/main/java/io/netty/handler/ssl/SniHandler.java:85` • `handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java:75` (decode entry) • `handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java:165` (handshakeBuffer.clear) • `handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java:174` (writeBytes re-copy) • `codec-base/src/main/java/io/netty/handler/codec/ByteToMessageDecoder.java:294` (cumulation retention) Exploit Path ``` 1. TCP connection → SniHandler → SslClientHelloHandler.decode 2. First record: TLS handshake header declaring large ClientHello length (e.g., 4096 bytes) 3. Attacker sends thousands of tiny follow-on handshake records (1 byte each) 4. On each fragment: handshakeBuffer.clear() + writeBytes() re-copies ALL accumulated body bytes 5. Total bytes copied = n*(n+1)/2 where n = number of body bytes → quadratic 6. Event-loop CPU exhausted before SslHandler takes over ``` Impact • Vulnerability Type: Inefficient Algorithmic Complexity • An unauthenticated network attacker can drive disproportionate CPU consumption on the Netty event loop • Affects all Netty deployments using `SniHandler` for TLS termination (the default SNI path) • No privileges, user interaction, or special configuration required • Can degrade or stall TLS connection handling for all clients on the affected event loop • The attack requires only modest bandwidth (~25 KB) to trigger significant CPU work --- Credits Found by a security research team from the University of Sydney, focusing on detecting open source software vulnerabilities. Liyi Zhou: https://lzhou1110.github.io/ Ziyue Wang: https://zyy0530.github.io/ Strick: https://str1ckl4nd.github.io/ Maurice: https://maurice.busystar.org/ Chenchen Yu: https://7thparkk.github.io/