CVE-2026-75596

Name	CVE-2026-75596
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.
Source	CVE (at NVD; CERT, ENISA, LWN, oss-sec, fulldisc, Debian ELTS, Red Hat, Ubuntu, Gentoo, SUSE bugzilla/CVE, GitHub advisories/code/issues, web search, more)

Vulnerable and fixed packages

The table below lists information on source packages.

Source Package	Release	Version	Status
netty (PTS)	bullseye	1:4.1.48-4+deb11u2	vulnerable
	bullseye (security)	1:4.1.48-4+deb11u3	vulnerable
	bookworm, bookworm (security)	1:4.1.48-7+deb12u2	vulnerable
	trixie (security), trixie	1:4.1.48-10+deb13u1	vulnerable
	forky, sid	1:4.1.48-16	vulnerable

The information below is based on the following data on fixed versions.

Package	Type	Release	Fixed Version	Urgency	Origin	Debian Bugs
netty	source	(unstable)	(unfixed)

Notes

https://github.com/netty/netty/security/advisories/GHSA-fccg-mwvh-qqg4
https://github.com/netty/netty/pull/17213
Fixed by: https://github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7 (netty-4.2.17.Final)
https://github.com/netty/netty/pull/17217
Fixed by: https://github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961 (netty-4.1.137.Final)