---
title: "CVE-2026-75595: Netty: SNI Routing Bypass via Fragmented TLS ClientHello Causing Fallback to Default SslContext"
url: https://erroneo.us/security/cve/CVE-2026-75595/
description: "CVE-2026-75595 (io.netty:netty-handler (Maven)): Netty is an asynchronous, event-driven network application framework. Prior to 4.1.137.Fina and…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-08-19
date_modified: 2026-09-08
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-75595"
kev: false
cvss: 9.1
epss: 0.00277
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-75595: Netty: SNI Routing Bypass via Fragmented TLS ClientHello Causing Fallback to Default SslContext

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
- EPSS 0.3% , percentile 19: FIRST’s model estimates a 0.3% chance of exploitation activity in the next 30 days, higher than 19% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes io.netty:netty-handler (Maven) >= 4.2.0.Final, <= 4.2.16.Final, fixed in 4.2.17.Final ; io.netty:netty-handler (Maven) <= 4.1.136.Final, fixed in 4.1.137.Final . Check your lockfile, including transitive dependencies.

## What it is

Netty is an asynchronous, event-driven network application framework. Prior to 4.1.137.Fina and 4.2.17.Final, io.netty.handler.ssl.SslClientHelloHandler#decode checks the wrong offset before reading the four-byte TLS handshake header, so a ClientHello whose handshake header spans records can cause an IndexOutOfBoundsException and invoke select(ctx, null). This selects the default SslContext instead of the SNI-specific context. In deployments where per-SNI clientAuth=REQUIRE is the sole mutual TLS gate, the default SslContext uses clientAuth=NONE or clientAuth=OPTIONAL, and no application-layer certificate verification exists, an unauthenticated remote attacker can bypass the protected route's mutual TLS requirement. This issue is fixed in versions 4.1.137.Final and 4.2.17.Final.

## Affected packages

- Maven `io.netty:netty-handler` >= 4.2.0.Final, <= 4.2.16.Final → fixed: 4.2.17.Final
- Maven `io.netty:netty-handler` <= 4.1.136.Final → fixed: 4.1.137.Final

## Affected products (CVE record)

- netty netty: < 4.1.137.Final, >= 4.2.0.Final, < 4.2.17.Final

## Severity

- GitHub_M CVSS 4.0: 9.1 `CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N`
- GitHub CVSS 4.0: 9.1 `CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation none , automatable no , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.28% probability of exploitation activity in the next 30 days; percentile 19.

## Sources

- [GitHub Advisory GHSA-c4c3-7fpv-j4q5](https://github.com/advisories/GHSA-c4c3-7fpv-j4q5)
- [CVE.org record CVE-2026-75595](https://www.cve.org/CVERecord?id=CVE-2026-75595)
- [NVD entry CVE-2026-75595](https://nvd.nist.gov/vuln/detail/CVE-2026-75595)
- [github.com](https://github.com/netty/netty/security/advisories/GHSA-c4c3-7fpv-j4q5)
- [github.com](https://github.com/netty/netty/pull/17213)
- [github.com](https://github.com/netty/netty/pull/17217)
- [github.com](https://github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7)
- [github.com](https://github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961)
- [github.com](https://github.com/netty/netty/releases/tag/netty-4.1.137.Final)
- [github.com](https://github.com/netty/netty/releases/tag/netty-4.2.17.Final)

---
Source page: https://erroneo.us/security/cve/CVE-2026-75595/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt