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Vulnerability · Published Jul 28, 2026 · Assigned by GitHub_M

CVE-2026-59921Netty: CRLF Injection via Multipart Filename in Netty HttpPostRequestEncoder

Medium 5.7

In plain English

A medium-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 0.5% (higher than 40% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)5.7Medium
Exploit odds (30 days)0.5%top 59.5% of all CVEs
Exploited in the wildNo reportsCISA SSVC: poc
Attack toolingNoneno public tooling known

Official description

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. This issue has been fixed in versions 4.1.136.Final and 4.2.16.Final.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Debian:12nettyno fix listedDEBIAN-CVE-2026-59921
Debian:13nettyno fix listedDEBIAN-CVE-2026-59921
Debian:14nettyno fix listedDEBIAN-CVE-2026-59921
Mavenio.netty:netty-codec-http4.2.16.FinalGHSA-gcjf-9mgh-3p7g
Mavenio.netty:netty-codec-http4.1.136.FinalGHSA-gcjf-9mgh-3p7g
Ubuntu:26.10nettyno fix listedUBUNTU-CVE-2026-59921

Affected products (as reported)

VendorProductVersions
netty netty >= 4.2.0.Final, < 4.2.16.Final< 4.1.136.Final

Weakness: CWE-93 · Vector: CVSS:3.1/AV:A/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N

References