Vulnerability · Published Aug 19, 2026 · Assigned by GitHub_M
CVE-2026-75596Netty: Fragmented ClientHello records trigger quadratic pre-handshake reassembly in default SNI parsing
High 8.7
In plain English
A high-severity security flaw in netty from netty. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 32% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
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 & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | netty | no fix listed | DEBIAN-CVE-2026-75596 |
| Debian:13 | netty | no fix listed | DEBIAN-CVE-2026-75596 |
| Debian:14 | netty | no fix listed | DEBIAN-CVE-2026-75596 |
| Maven | io.netty:netty-handler | 4.2.17.Final | GHSA-fccg-mwvh-qqg4 |
| Maven | io.netty:netty-handler | 4.1.137.Final | GHSA-fccg-mwvh-qqg4 |
| Ubuntu:26.10 | netty | no fix listed | UBUNTU-CVE-2026-75596 |
Affected products (as reported)
Weakness: CWE-407 · Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
References
- https://github.com/netty/netty/security/advisories/GHSA-fccg-mwvh-qqg4 x_refsource_CONFIRM
- https://github.com/netty/netty/pull/17213 x_refsource_MISC
- https://github.com/netty/netty/pull/17217 x_refsource_MISC
- https://github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7 x_refsource_MISC
- https://github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961 x_refsource_MISC
- https://github.com/netty/netty/releases/tag/netty-4.1.137.Final x_refsource_MISC
- https://github.com/netty/netty/releases/tag/netty-4.2.17.Final x_refsource_MISC