GHSA-fccg-mwvh-qqg4

### 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/