# Netty: SNI Routing Bypass via Fragmented TLS ClientHello Causing Fallback to Default SslContext

Advisory record · `GHSA-c4c3-7fpv-j4q5`

- **Identifiers:** `GHSA-c4c3-7fpv-j4q5` · `CVE-2026-75595`
- **State:** Confirmed
- **Severity:** CVSS 9.1 Critical
- **Critical:** Yes: CVSS 9+
- **Weakness (CWE):** [CWE-475](https://cwe.mitre.org/data/definitions/475.html) · [CWE-754](https://cwe.mitre.org/data/definitions/754.html)
- **Tags:** [CWE-475](https://agentprime.io/advisories/tag/cwe-475.md) · [CWE-754](https://agentprime.io/advisories/tag/cwe-754.md)
- **Published:** 8 Sept 2026
- **Updated:** 8 Sept 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 8 Sept 2026, 18:16 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Critical

Critical, because:

- CVSS score 9.1: 9.0 or more.

How we decide what’s critical: <https://agentprime.io/advisories/critical.md>

## Affected packages

- `io.netty:netty-handler` (Maven): affected `>=4.2.0.Final, <4.2.17.Final || <4.1.137.Final`; fixed in `4.2.17.Final`, `4.1.137.Final`

## Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

> Netty's io.netty:netty-handler is affected in versions from 4.2.0.Final before 4.2.17.Final and from 4.1.x before 4.1.137.Final. A remote, unauthenticated attacker can send a TLS ClientHello split across records so that Netty falls back to the default SslContext instead of the per-SNI one. This bypasses mTLS only when the default context does not require client authentication and no other peer-certificate check exists. Upgrade to 4.2.17.Final or 4.1.137.Final.

Written 2 Nov 2026.

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 9.1 Critical, CVSS 4.0: `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`, from CNA (GitHub_M)

- **CISA KEV:** Not listed.
- **EPSS:** 0.0032, the 24th percentile, on 5 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 31 Oct 2026, 14:18 UTC: First seen by agentprime
- 31 Oct 2026, 14:18 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-c4c3-7fpv-j4q5)

## Sources

- [GitHub Advisory Database: GHSA-c4c3-7fpv-j4q5](https://github.com/advisories/GHSA-c4c3-7fpv-j4q5)
- [OSV: GHSA-c4c3-7fpv-j4q5](https://osv.dev/vulnerability/GHSA-c4c3-7fpv-j4q5)
- <https://github.com/netty/netty/security/advisories/GHSA-c4c3-7fpv-j4q5>
- <https://nvd.nist.gov/vuln/detail/CVE-2026-75595>
- <https://github.com/netty/netty/pull/17213>
- <https://github.com/netty/netty/pull/17217>
- <https://github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7>
- <https://github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961>
- <https://github.com/netty/netty>
- <https://github.com/netty/netty/releases/tag/netty-4.1.137.Final>
- <https://github.com/netty/netty/releases/tag/netty-4.2.17.Final>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-c4c3-7fpv-j4q5>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-c4c3-7fpv-j4q5/>. Everything for agents: <https://agentprime.io/llms.txt>.