---
title: "CVE-2026-100663: Netty - HTTP Request Smuggling"
description: "Netty HTTP/3 codec 4.2.2.Final through 4.2.17.Final contains an HTTP request smuggling caused by improper handling of HTTP/1 CONNECT authority-form request-targets in HttpConversionUtil.toHttp3Headers, letting remote attackers bypass tunnel allow-lists and egress policies by sending malformed HTTP/3 CONNECT requests, exploit requires attacker to send crafted CONNECT requests with mismatched Host headers."
url: https://panop.io/threat-heartbeat/CVE-2026-100663/
---

# CVE-2026-100663 Netty - HTTP Request Smuggling

Netty HTTP/3 codec 4.2.2.Final through 4.2.17.Final contains an HTTP request smuggling caused by improper handling of HTTP/1 CONNECT authority-form request-targets in HttpConversionUtil.toHttp3Headers, letting remote attackers bypass tunnel allow-lists and egress policies by sending malformed HTTP/3 CONNECT requests, exploit requires attacker to send crafted CONNECT requests with mismatched Host headers.

CVSS base score 7.5 /10

High

CISA KEV

Not listed

Public exploit

None known

Authentication

Not required

EPSS

0.30% chance of exploitation in 30 days

[View the NVD record ↗](https://nvd.nist.gov/vuln/detail/CVE-2026-100663)

## Details

Type

HTTP Request Smuggling

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Published

26 September 2026

Updated

15 October 2026

Weakness

[CWE-20](https://cwe.mitre.org/data/definitions/20.html)

## Vendor fix

Upgrade to version 4.2.18.Final or later.

Patching closes this CVE. It does not tell you which of your systems run the affected version, or what else is exposed alongside them.

## References

- [https://nvd.nist.gov/vuln/detail/CVE-2026-100663](https://nvd.nist.gov/vuln/detail/CVE-2026-100663)
- [https://github.com/netty/netty/security/advisories/GHSA-jgph-cgq3-c627](https://github.com/netty/netty/security/advisories/GHSA-jgph-cgq3-c627)
- [https://www.vulncheck.com/advisories/netty-http-1-connect-authority-form-mistranslated-to-malformed-http-3](https://www.vulncheck.com/advisories/netty-http-1-connect-authority-form-mistranslated-to-malformed-http-3)

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## What this record can't tell you

- Whether you run the affected software, and where
- Whether an attacker can actually reach and exploit it
- The misconfigurations that never get a CVE number
- What it would cost the business, ranked into what to fix first

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