# CVE-2026-75595: Netty SNI routing bypass via fragmented TLS ClientHello

Source: https://junglewise.ai/threats/cve-2026-75595-netty-sni-routing-bypass-via-fragmented-tls-clienthello  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-19. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-75595 · Severity: critical · CVSS 9.1 · Published 2026-08-19

Netty is a widely-used Java networking library that handles TLS encryption for secure communications. A flaw in its TLS ClientHello parsing allows an attacker to bypass mutual TLS (mTLS) authentication by sending a specially fragmented TLS handshake. When mTLS is the sole security gate for a service route and the fallback configuration is permissive, an unauthenticated attacker can access protected resources.

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

## Quick answers

**Is CVE-2026-75595 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-75595 being exploited in the wild.

**How severe is CVE-2026-75595?** Critical severity, with a CVSS score of 9.1, published 19 August 2026.

**Which products and versions are affected?** io.netty:netty-handler (Maven) ECOSYSTEM:introduced=4.2.0.Final,fixed=4.2.17.Final.

**Is there a fix for CVE-2026-75595?** Yes. io.netty:netty-handler (Maven): upgrade to 4.2.17.Final or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-75595 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-75595?** Check whether you run io.netty:netty-handler (Maven) or Netty 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-handler (Maven): upgrade to 4.2.17.Final or later

  ```sh
  # set io.netty:netty-handler to 4.2.17.Final in your pom.xml or build.gradle
  ```


## Technical details

The vulnerability exists in io.netty.handler.ssl.SslClientHelloHandler#decode, where the handshake header length guard checks the wrong offset and fails to account for the 5-byte TLS record header. When a record payload is smaller than 4 bytes, an IndexOutOfBoundsException is thrown and caught by a generic exception handler that invokes select(ctx, null), falling back to the default permissive SslContext. This permits bypass of per-SNI mTLS authentication only when: (1) mTLS enforcement relies solely on per-SNI SslContext with clientAuth=REQUIRE, (2) the default/fallback SslContext has clientAuth=NONE or OPTIONAL, and (3) no secondary peer-certificate verification exists at the application layer. The attack requires network access and no authentication. Patches are available in versions 4.1.137.Final and 4.2.17.Final.

## Affected products

- Netty netty-handler 4.1.0.Final through 4.1.136.Final, 4.2.0.Final through 4.2.16.Final

## Identifiers

- CVE-2026-75595
- GHSA-c4c3-7fpv-j4q5

## Timeline

- 2026-08-07: disclosed: Published by Netty maintainers
- 2026-08-19: patched: NVD publication date; patches released in versions 4.1.137.Final and 4.2.17.Final
- 2026-09-08: advisory: GitHub Advisory Database publication

## References

- [https://api.github.com/users/violetagg](https://api.github.com/users/violetagg)
- [https://github.com/violetagg](https://github.com/violetagg)
- [https://api.github.com/users/violetagg/gists%7B/gist_id%7D](https://api.github.com/users/violetagg/gists%7B/gist_id%7D)
- [https://api.github.com/users/violetagg/repos](https://api.github.com/users/violetagg/repos)
- [https://avatars.githubusercontent.com/u/696661?v=4](https://avatars.githubusercontent.com/u/696661?v=4)
- [https://api.github.com/users/violetagg/events%7B/privacy%7D](https://api.github.com/users/violetagg/events%7B/privacy%7D)

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