Vulnerability · Published Sep 26, 2026 · Assigned by VulnCheck
CVE-2026-100666Netty 4.2.0 through 4.2.16 Response Desynchronization via HttpServerCodec
High 7.3
In plain English
A high-severity security flaw in netty from netty. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 22% of all known vulnerabilities).
Official description
Netty's HttpServerCodec (io.netty:netty-codec-http) in versions 4.2.0.Final through 4.2.16.Final and in versions up to and including 4.1.136.Final pairs each outbound response with an inbound request by calling pollMethod() once per response, including for 1xx informational responses. If a client pipelines an HTTP/1.1 GET carrying an Expect: 100-continue header followed by a HEAD request, the 100 Continue response consumes the queued GET method, so the subsequent 200 OK for the GET is paired with HEAD and its body is dropped, while the following 200 OK for the HEAD request is written with a body. This desynchronizes HTTP parsing on the connection: the GET entity is never delivered and the HEAD response body is interpreted as the GET body, resulting in response splitting and unsafe connection reuse. Fixed in 4.2.17.Final and 4.1.137.Final.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | netty | no fix listed | DEBIAN-CVE-2026-100666 |
| Debian:13 | netty | no fix listed | DEBIAN-CVE-2026-100666 |
| Debian:14 | netty | no fix listed | DEBIAN-CVE-2026-100666 |
| Ubuntu:26.10 | netty | no fix listed | UBUNTU-CVE-2026-100666 |
Affected products (as reported)
Weakness: CWE-444 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L
References
- GitHub Security Advisory (GHSA-2g6j-r8q9-5hr8) vendor-advisory
- VulnCheck Advisory: Netty 4.2.0 through 4.2.17 Response Desynchronization via HttpServerCodec third-party-advisory