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

> CVE-2026-75596: Netty: Fragmented ClientHello records trigger quadratic pre-handshake reassembly in default SNI parsing. CVSS 7.5 (CVSS 3.1).

Canonical: https://rewindex.com/cve/CVE-2026-75596
Last updated: 2026-09-22T19:28:32Z

- Published: 2026-08-19 20:56 UTC · Updated: 2026-09-22 19:28 UTC · Assigner: GitHub\_M
- CVSS: 7.5 (CVSS 3.1) · Severity: High · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- EPSS: 0.37%, 31th percentile (scored 2026-10-25)
- CISA KEV: no
- CWE: 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 vendors and products

- netty: netty

## Tracked companies

- [Jira](https://rewindex.com/company/jira.md)

## References

- [CVE record on cve.org](https://www.cve.org/CVERecord?id=CVE-2026-75596)
- [NVD entry](https://nvd.nist.gov/vuln/detail/CVE-2026-75596)
- [https://github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7](https://github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7)
- [https://github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961](https://github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961)
- [https://github.com/netty/netty/pull/17213](https://github.com/netty/netty/pull/17213)
- [https://github.com/netty/netty/pull/17217](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-4.1.137.Final)
- [https://github.com/netty/netty/releases/tag/netty-4.2.17.Final](https://github.com/netty/netty/releases/tag/netty-4.2.17.Final)
- [https://github.com/netty/netty/security/advisories/GHSA-fccg-mwvh-qqg4](https://github.com/netty/netty/security/advisories/GHSA-fccg-mwvh-qqg4)

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_Sources: CVE records from the CVE Program (cve.org). CVE® is a registered trademark of The MITRE Corporation. (https://www.cve.org/) This product uses data from the NVD API but is not endorsed or certified by the NVD. (https://nvd.nist.gov/) Known Exploited Vulnerabilities catalog from CISA. (https://www.cisa.gov/known-exploited-vulnerabilities-catalog) EPSS scores from FIRST (Exploit Prediction Scoring System). (https://www.first.org/epss/)_


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Canonical: https://rewindex.com/cve/CVE-2026-75596 · Markdown: https://rewindex.com/cve/CVE-2026-75596.md · JSON: https://rewindex.com/api/v1/cves/CVE-2026-75596 · Data licensed CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) · Generated 2026-10-26T02:46:49.514Z