---
title: "CVE-2026-100666: Netty HttpServerCodec - HTTP Request Smuggling"
description: "Netty HttpServerCodec 4.2.0.Final to 4.2.16.Final and up to 4.1.136.Final contains a response desynchronization vulnerability caused by improper handling of Expect: 100-continue and pipelined requests, letting remote attackers cause response splitting and unsafe connection reuse, exploit requires pipelined HTTP/1.1 requests with specific headers."
url: https://panop.io/threat-heartbeat/CVE-2026-100666/
---

# CVE-2026-100666 Netty HttpServerCodec - HTTP Request Smuggling

Netty HttpServerCodec 4.2.0.Final to 4.2.16.Final and up to 4.1.136.Final contains a response desynchronization vulnerability caused by improper handling of Expect: 100-continue and pipelined requests, letting remote attackers cause response splitting and unsafe connection reuse, exploit requires pipelined HTTP/1.1 requests with specific headers.

CVSS base score 7.3 /10

High

CISA KEV

Not listed

Public exploit

None known

Authentication

Not required

EPSS

0.24% chance of exploitation in 30 days

[View the NVD record ↗](https://nvd.nist.gov/vuln/detail/CVE-2026-100666)

## Details

Type

HTTP Request Smuggling

Impact

Information Disclosure, Denial Of Service

CVSS vector

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

Published

26 September 2026

Updated

15 October 2026

Weakness

[CWE-444](https://cwe.mitre.org/data/definitions/444.html)

## Vendor fix

Upgrade to versions 4.2.17.Final or 4.1.137.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-2026-100666](https://nvd.nist.gov/vuln/detail/CVE-2026-100666)
- [https://www.vulncheck.com/advisories/netty-4.2.0-through-4.2.17-response-desynchronization-via-httpservercodec](https://www.vulncheck.com/advisories/netty-4.2.0-through-4.2.17-response-desynchronization-via-httpservercodec)
- [https://github.com/netty/netty/security/advisories/GHSA-2g6j-r8q9-5hr8](https://github.com/netty/netty/security/advisories/GHSA-2g6j-r8q9-5hr8)

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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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