CVE-2026-75596: Netty Handler Vulnerability Leading to Quadratic CPU Consumption
Summary
CVE-2026-75596: Netty: Fragmented ClientHello records trigger quadratic pre-handshake reassembly in default SNI parsing. Affects io.netty:netty-handler across 2 version ranges.
< 4.1.137.Final
>= 4.2.0.Final, < 4.2.17.Final
Fixed in 4.1.137.Final or 4.2.17.Final, depending on the release line you run. How to fix ↓
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Background
Netty is a popular asynchronous event-driven network application framework that facilitates the development of network applications such as protocol servers and clients. It provides a rich set of features including support for various protocols, SSL/TLS, and SNI (Server Name Indication) handling. However, a vulnerability has been identified in the default SNI parsing mechanism that can lead to significant CPU resource exhaustion under certain conditions.
Vulnerability Detail
The vulnerability arises from the way Netty's SNI entrypoint reprocesses ClientHello fragments during TLS handshakes. An attacker can exploit this by sending a small initial record that indicates a large ClientHello length, followed by numerous tiny records. This results in a quadratic increase in CPU workload as the handler repeatedly re-copies previously received data. The impact is severe, as it can exhaust CPU resources on the event loop, degrading or stalling TLS connection handling for all clients connected to the affected server.
How To Fix
Latest published version: 5.0.0.Alpha2 as of 23 Oct 2026
Upgrading to it may include breaking changes; the versions below are the minimum fixed release for this vulnerability.
If you can't move to the latest
The minimum fixed version depends on the release line you run.
-
< 4.1.137.Final4.1.137.Final -
>= 4.2.0.Final, < 4.2.17.Final4.2.17.Final
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Workaround
As a temporary measure, users can limit the number of fragments that can be processed or implement rate limiting on incoming TLS handshake requests to mitigate the impact of this vulnerability until an upgrade can be performed.
Conclusion
The CVE-2026-75596 vulnerability in the Netty handler presents a significant risk due to its potential to cause quadratic CPU consumption during TLS handshakes. It is essential for users to update their Netty versions to the latest patched releases to mitigate this risk effectively.
See every vulnerability recorded for io.netty:netty-handler →
FAQ
The vulnerability is related to inefficient algorithmic complexity in the SNI parsing mechanism of Netty, which can lead to excessive CPU consumption.
An unauthenticated remote attacker can exploit this vulnerability by sending specially crafted TLS handshake records.
Versions of Netty prior to 4.2.17.Final and 4.1.137.Final are affected by this vulnerability.
The vulnerability can lead to significant CPU resource exhaustion, degrading or stalling TLS connection handling for all clients on the affected server.
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References
Related vulnerabilities
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