What’s Vulnerable › Vulnerabilities

CVE-2026-75596: Netty: Fragmented ClientHello records trigger quadratic pre-handshake reassembly in default SNI parsing

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-08-19 · updated 2026-10-26
• Weakness: CWE-407

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 products

• netty: before 4.1.137; from 4.2.0 before 4.2.17 — fixed in 4.1.137, 4.2.17

How to fix

Upgrade to a fixed version: netty 4.1.137, 4.2.17. Check the vendor advisory for mitigations.

References

• https://github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7
• https://github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961
• https://github.com/netty/netty/pull/17213
• https://github.com/netty/netty/pull/17217
• https://github.com/netty/netty/releases/tag/netty-4.1.137.Final
• https://github.com/netty/netty/releases/tag/netty-4.2.17.Final
• https://github.com/netty/netty/security/advisories/GHSA-fccg-mwvh-qqg4

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