CVE-2026-75596
Netty: Fragmented ClientHello records trigger quadratic pre-handshake reassembly in default SNI parsing
Impact
Netty is an asynchronous, event-driven network application framework. Prior to 4.1.137.Final and 4.2.17.Final, the default io.netty.handler.ssl.SniHandler constructors use the pre-handshake ClientHello aggregation path in handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java at io.netty.handler.ssl.SslClientHelloHandler#decode, where handshakeBuffer.clear() and writeBytes() recopy all previously received body bytes for every additional TLS record. An unauthenticated remote peer can advertise a large ClientHello and deliver its body in thousands of tiny records, causing quadratic CPU work on the event loop before the TLS handshake completes and degrading TLS handling for other clients. This issue is fixed in versions 4.1.137.Final and 4.2.17.Final.
Affected packages
| Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|
| maven | io.netty:netty-handler | >= 4.2.0.Final, <= 4.2.16.Final | 4.2.17.Final |
Affected products
| Vendor | Product | Affected versions |
|---|---|---|
| Netty | Netty | before 4.1.137 from 4.2.0 before 4.2.17 |
Weakness classification
References
- NVD record
- CVE.org record
- GitHub advisory
- https://github.com/advisories/GHSA-fccg-mwvh-qqg4
- https://github.com/netty/netty/security/advisories/GHSA-fccg-mwvh-qqg4
- https://nvd.nist.gov/vuln/detail/CVE-2026-75596
- https://github.com/netty/netty/pull/17213
- https://github.com/netty/netty/pull/17217
- https://github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7
- https://github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961
- https://github.com/netty/netty/releases/tag/netty-4.1.137.Final
- https://github.com/netty/netty/releases/tag/netty-4.2.17.Final