Medium [CVE-2026-59921] CRLF Injection via Multipart Filename in Netty HttpPostRequestEncoder

Netty is an asynchronous, event-driven network application framework. Prior to versions 4.1.136.Final and 4.2.16.Final, HttpPostRequestEncoder constructs multipart HTTP request bodies by directly concatenating user-supplied filenames and field names into Content-Disposition MIME headers without validating or sanitizing CRLF characters (\r\n).

Since MIME headers are delimited by CRLF, an attacker who controls the filename can inject arbitrary MIME headers into the multipart body part. The root cause is that neither the encoder nor the FileUpload implementations' setFilename() methods, which only check for null, neutralize CRLF characters before the filename is embedded into the header.

A flaw was found in Netty's HttpPostRequestEncoder, a widely used Java networking library component responsible for constructing multipart HTTP request bodies.

This may lead to limited Content-Type spoofing or header manipulation against middleware or storage layers processing the request, though the practical impact is constrained by the context in which Netty is deployed.

Moderate: A CRLF injection flaw in the Netty Java networking library's multipart message encoder allows remote attackers with low privileges to inject arbitrary MIME headers.