Netty prior 4.2.17.Final/4.1.137.Final HttpServerCodec pollMethod response splitting

Summaryinfo

A vulnerability described as critical has been identified in Netty. This impacts the function pollMethod of the component HttpServerCodec. Executing a manipulation can lead to response splitting. This vulnerability is handled as CVE-2026-100666. The attack can be executed remotely. There is not any exploit available. Upgrading the affected component is recommended.

Detailsinfo

A vulnerability classified as critical was found in Netty. This vulnerability affects the function pollMethod of the component HttpServerCodec. The manipulation with an unknown input leads to a response splitting vulnerability. The CWE definition for the vulnerability is CWE-113. The product receives data from an HTTP agent/component (e.g., web server, proxy, browser, etc.), but it does not neutralize or incorrectly neutralizes CR and LF characters before the data is included in outgoing HTTP headers. As an impact it is known to affect confidentiality, integrity, and availability. CVE summarizes:

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.

The advisory is shared for download at github.com. This vulnerability was named CVE-2026-100666 since 09/26/2026. The exploitation appears to be easy. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. There are known technical details, but no exploit is available.

Upgrading to version 4.2.17.Final or 4.1.137.Final eliminates this vulnerability.

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Name

• Netty

Website

• Product: https://github.com/netty/netty/

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