What is CVE-2026-75596?

CVE-2026-75596 is a medium-severity security vulnerability in io.netty:netty-handler (maven), affecting versions >= 4.2.0.Final, <= 4.2.16.Final. It is fixed in 4.2.17.Final, 4.1.137.Final.

Which versions of io.netty:netty-handler are affected by CVE-2026-75596?

io.netty:netty-handler (maven) versions >= 4.2.0.Final, <= 4.2.16.Final is affected.

Is there a fix for CVE-2026-75596?

Yes. CVE-2026-75596 is fixed in 4.2.17.Final, 4.1.137.Final. Upgrade to this version or later.

Is CVE-2026-75596 exploitable, and should I be worried?

Whether CVE-2026-75596 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk.

What actually determines whether CVE-2026-75596 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

CVE-2026-75596 in io.netty:netty-handler

Q: Which versions of io.netty:netty-handler are affected by CVE-2026-75596?

io.netty:netty-handler (maven) versions >= 4.2.0.Final, <= 4.2.16.Final is affected.

Q: Is there a fix for CVE-2026-75596?

Yes. CVE-2026-75596 is fixed in 4.2.17.Final, 4.1.137.Final. Upgrade to this version or later.

Q: Is CVE-2026-75596 exploitable, and should I be worried?

Whether CVE-2026-75596 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk.

Q: What actually determines whether CVE-2026-75596 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

Q: How do I fix CVE-2026-75596?

Upgrade: io.netty:netty-handler to 4.2.17.Final or later, io.netty:netty-handler to 4.1.137.Final or later.

Summary

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

Netty's default SNI entrypoint reparses and recopies previously received ClientHello fragments on every additional TLS handshake record. A remote peer can send a small first record that advertises a large ClientHello length and then drip the body in many tiny records, causing superlinear (quadratic) CPU work before the handshake completes. With 4095 one-byte fragments, the handler recopies 8,386,560 bytes from only 24,579 bytes on the wire, a 341× amplification ratio.

Affected Entrypoints

io.netty.handler.ssl.SniHandler, default constructors
io.netty.handler.ssl.SslClientHelloHandler, pre-handshake ClientHello aggregation path

Vulnerable Code Locations

handler/src/main/java/io/netty/handler/ssl/SniHandler.java:85
handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java:75 (decode entry)
handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java:165 (handshakeBuffer.clear)
handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java:174 (writeBytes re-copy)
codec-base/src/main/java/io/netty/handler/codec/ByteToMessageDecoder.java:294 (cumulation retention)

Exploit Path

1. TCP connection → SniHandler → SslClientHelloHandler.decode
2. First record: TLS handshake header declaring large ClientHello length (e.g., 4096 bytes)
3. Attacker sends thousands of tiny follow-on handshake records (1 byte each)
4. On each fragment: handshakeBuffer.clear() + writeBytes() re-copies ALL accumulated body bytes
5. Total bytes copied = n*(n+1)/2 where n = number of body bytes → quadratic
6. Event-loop CPU exhausted before SslHandler takes over

Credits

Found by a security research team from the University of Sydney, focusing on detecting open source software vulnerabilities.
Liyi Zhou: https://lzhou1110.github.io/
Ziyue Wang: https://zyy0530.github.io/
Strick: https://str1ckl4nd.github.io/
Maurice: https://maurice.busystar.org/
Chenchen Yu: https://7thparkk.github.io/

Impact

Vulnerability Type: Inefficient Algorithmic Complexity
An unauthenticated network attacker can drive disproportionate CPU consumption on the Netty event loop
Affects all Netty deployments using SniHandler for TLS termination (the default SNI path)
No privileges, user interaction, or special configuration required
Can degrade or stall TLS connection handling for all clients on the affected event loop
The attack requires only modest bandwidth (~25 KB) to trigger significant CPU work

Affected versions

io.netty:netty-handler (>= 4.2.0.Final, <= 4.2.16.Final) io.netty:netty-handler (<= 4.1.136.Final)

Security releases

io.netty:netty-handler → 4.2.17.Final (maven) io.netty:netty-handler → 4.1.137.Final (maven)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

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Remediation advice

Upgrade the following packages to resolve this vulnerability:

io.netty:netty-handler to 4.2.17.Final or later; io.netty:netty-handler to 4.1.137.Final or later

Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.

Frequently Asked Questions

What is CVE-2026-75596? CVE-2026-75596 is a medium-severity security vulnerability in io.netty:netty-handler (maven), affecting versions >= 4.2.0.Final, <= 4.2.16.Final. It is fixed in 4.2.17.Final, 4.1.137.Final.
Which versions of io.netty:netty-handler are affected by CVE-2026-75596? io.netty:netty-handler (maven) versions >= 4.2.0.Final, <= 4.2.16.Final is affected.
Is there a fix for CVE-2026-75596? Yes. CVE-2026-75596 is fixed in 4.2.17.Final, 4.1.137.Final. Upgrade to this version or later.
Is CVE-2026-75596 exploitable, and should I be worried? Whether CVE-2026-75596 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
What actually determines whether CVE-2026-75596 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
How do I fix CVE-2026-75596?
- Upgrade io.netty:netty-handler to 4.2.17.Final or later
- Upgrade io.netty:netty-handler to 4.1.137.Final or later