Netty 4.1.x/4.2.x SniHandler TLS ClientHello DoS before 4.1.137
CVE-2026-75596 Published on August 19, 2026
Netty: Fragmented ClientHello records trigger quadratic pre-handshake reassembly in default SNI parsing
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.
Vulnerability Analysis
CVE-2026-75596 is exploitable with network access, and does not require authorization privileges or user interaction. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality and integrity and availability.
Weakness Type
Inefficient Algorithmic Complexity
An algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached.
Products Associated with CVE-2026-75596
Want to know whenever a new CVE is published for Netty? stack.watch will email you.
Affected Versions
netty:- Version < 4.1.137.Final is affected.
- Version >= 4.2.0.Final, < 4.2.17.Final is affected.