CVE-2026-59921 Netty: CRLF Injection via Multipart Filename in Netty HttpPostRequestEncoder
Attackers can inject arbitrary MIME headers into multipart requests via unsanitized filenames, potentially bypassing security controls or corrupting data.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-59921?
CVE-2026-59921 is a medium-severity vulnerability in Netty Netty, published on 28 Jul 2026. It can only be exploited from the same machine or local network, with no action from the victim.
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.
Weakness: CWE-93
Which versions are affected by CVE-2026-59921?
4 open source packages ship an affected version, including io.netty:netty-codec-http. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
io.netty:netty-codec-http Maven | All versions | 4.1.136.Final |
io.netty:netty-codec-http Maven | All versions | 4.1.136.Final |
io.netty:netty-codec-http Maven | From 4.2.0.Final | 4.2.16.Final |
io.netty:netty-codec-http Maven | From 4.2.0.Final | 4.2.16.Final |
Products include Netty Netty.
How do I fix CVE-2026-59921?
Upgrade io.netty:netty-codec-http to a fixed version for your release line: 4.1.136.Final or 4.2.16.Final.
Is CVE-2026-59921 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-59921 exists.
Other vulnerabilities in Netty Netty
- CVE-2026-75595 Netty: SNI Routing Bypass via Fragmented TLS ClientHello Causing Fallback to Default SslContext
- CVE-2026-75596 Netty: Fragmented ClientHello records trigger quadratic pre-handshake reassembly in default SNI parsing
- CVE-2026-59901 Netty Bzip2Decoder: Infinite Loop in RLE State Machine Leads to Event-Loop Thread Hang
- CVE-2026-55851 Netty codec-haproxy: Signed-Byte Sentinel Collision in HAProxyMessageDecoder Leads to Unbounded Memory Exhaustion
- CVE-2026-56745 Netty SpdyHttpDecoder: ByteBuf Reference Leak on RST_STREAM Leads to Native Memory Exhaustion
Sources
Also known as CLEANSTART-2026-AC10383, CLEANSTART-2026-BL92558, CLEANSTART-2026-CK01312, CLEANSTART-2026-CP85393, CLEANSTART-2026-SM87861, DEBIAN-CVE-2026-59921 and 108 more.