# CVE-2026-59898: Netty HTTP request smuggling via lax WebSocket handshake validation

Source: https://junglewise.ai/threats/cve-2026-59898-netty-http-request-smuggling-via-lax-websocket-handshake  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-29. Updated: 2026-07-29  
License: CC BY 4.0 with a link back

CVE-2026-59898 · Severity: medium · CVSS 4 · Published 2026-07-29

Netty is a widely used networking framework for Java applications. A vulnerability in its handling of WebSocket connections allows an attacker to trick the system into switching communication protocols without a security proxy noticing. This can lead to 'request smuggling,' where an attacker can bypass security filters to access internal data or interfere with other users' web traffic.

Technologies: [io.netty:netty-codec-http (Maven)](https://junglewise.ai/threats/technologies/io-netty-netty-codec-http). Vendors: [Maven](https://junglewise.ai/threats/vendors/maven).

## Quick answers

**Is CVE-2026-59898 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-59898 being exploited in the wild.

**How severe is CVE-2026-59898?** Medium severity, with a CVSS score of 4, published 29 July 2026.

**Which products and versions are affected?** io.netty:netty-codec-http (Maven) ECOSYSTEM:introduced=4.2.0.Final,fixed=4.2.16.Final.

**Is there a fix for CVE-2026-59898?** Yes. io.netty:netty-codec-http (Maven): upgrade to 4.2.16.Final or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-59898 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-59898?** Check whether you run io.netty:netty-codec-http (Maven) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- io.netty:netty-codec-http (Maven): upgrade to 4.2.16.Final or later

  ```sh
  # set io.netty:netty-codec-http to 4.2.16.Final in your pom.xml or build.gradle
  ```


## Technical details

A vulnerability exists in Netty's `netty-codec-http` component due to insufficient validation in the WebSocket V07 and V08 handshakers. An attacker can initiate a protocol switch by sending a `Sec-WebSocket-Version: 7` header while omitting the mandatory `Connection: Upgrade` and `Upgrade: websocket` headers. Because these standard upgrade headers are missing, intermediary proxies may fail to recognize the protocol switch, while the Netty backend completes it. This inconsistency enables HTTP request smuggling and protocol-confusion attacks. The issue is resolved in versions 4.1.136.Final and 4.2.16.Final.

## Affected products

- Netty netty-codec-http < 4.1.136.Final, >= 4.2.0.Final, < 4.2.16.Final

## Identifiers

- CVE-2026-59898
- GHSA-4mp9-239f-g9hg

## Timeline

- 2026-07-07: patched: Version 4.2.16.Final released
- 2026-07-08: patched: Version 4.1.136.Final released
- 2026-07-14: advisory: GitHub Security Advisory GHSA-4mp9-239f-g9hg published
- 2026-07-29: disclosed: CVE-2026-59898 published to NVD

## References

- [https://github.com/netty/netty/releases/tag/netty-4.1.136.Final](https://github.com/netty/netty/releases/tag/netty-4.1.136.Final)
- [https://github.com/netty/netty/releases/tag/netty-4.2.16.Final](https://github.com/netty/netty/releases/tag/netty-4.2.16.Final)
- [https://github.com/netty/netty/security/advisories/GHSA-4mp9-239f-g9hg](https://github.com/netty/netty/security/advisories/GHSA-4mp9-239f-g9hg)

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