# CVE-2026-59921: Netty CRLF injection in HttpPostRequestEncoder multipart filenames

Source: https://junglewise.ai/threats/cve-2026-59921-netty-crlf-injection-in-httppostrequestencoder-multipart  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-28. Updated: 2026-07-28  
License: CC BY 4.0 with a link back

CVE-2026-59921 · Severity: medium · CVSS 5.7 · Published 2026-07-28

Netty is a widely used networking framework that helps developers build high-performance web applications and services. A security flaw in its HTTP request handling allows attackers to inject malicious data into file upload requests by using specially crafted filenames containing hidden line-break characters. This could allow an attacker to bypass security filters, perform unauthorized actions on downstream systems, or deliver malicious scripts (XSS) that compromise user sessions.

Technologies: [io.netty:netty-codec-http (Maven)](https://junglewise.ai/threats/technologies/io-netty-netty-codec-http), [Netty Project Netty codec-http](https://junglewise.ai/threats/technologies/codec-http). Vendors: [Maven](https://junglewise.ai/threats/vendors/maven), [Netty Project](https://junglewise.ai/threats/vendors/netty-project).

## Quick answers

**Is CVE-2026-59921 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-59921 being exploited in the wild.

**How severe is CVE-2026-59921?** Medium severity, with a CVSS score of 5.7, published 28 July 2026.

**Which products and versions are affected?** io.netty:netty-codec-http (Maven) ECOSYSTEM:introduced=4.2.0.Final,fixed=4.2.16.Final.

**Is there a fix for CVE-2026-59921?** Yes. io.netty:netty-codec-http (Maven): upgrade to 4.2.16.Final or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-59921 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-59921?** Check whether you run io.netty:netty-codec-http (Maven) or Netty Project Netty codec-http at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- io.netty:netty-codec-http (Maven): upgrade to 4.2.16.Final or later

  ```sh
  # set io.netty:netty-codec-http to 4.2.16.Final in your pom.xml or build.gradle
  ```


## Technical details

A CRLF injection vulnerability exists in Netty's `HttpPostRequestEncoder` and `FileUpload` implementations (including `DiskFileUpload`, `MemoryFileUpload`, and `MixedFileUpload`). The root cause is the direct concatenation of user-supplied filenames and field names into `Content-Disposition` MIME headers without sanitizing carriage return (\r) or line feed (\n) characters. An attacker who can control the filename of an uploaded file can inject arbitrary MIME headers, override Content-Type values, or manipulate multipart boundaries. This can lead to Cross-Site Scripting (XSS), security bypasses in downstream middleware, or request smuggling within the multipart body. The issue is fixed in versions 4.1.136.Final and 4.2.16.Final by adding validation to the `setFilename()` and encoder methods.

## Affected products

- Netty netty-codec-http < 4.1.136.Final, >= 4.2.0.Final, < 4.2.16.Final

## Identifiers

- CVE-2026-59921
- GHSA-gcjf-9mgh-3p7g

## Timeline

- 2026-07-14: advisory: GitHub Security Advisory published
- 2026-07-28: disclosed: NVD publication date
- 2026-07-28: patched: Fixes released in 4.1.136.Final and 4.2.16.Final

## References

- [https://github.com/netty/netty/security/advisories/GHSA-gcjf-9mgh-3p7g](https://github.com/netty/netty/security/advisories/GHSA-gcjf-9mgh-3p7g)

## Related threats

- [Netty HttpServerCodec response desynchronization](https://junglewise.ai/threats/cve-2026-100666-netty-s-httpservercodec-io-netty-netty-codec-http-in-versions-4)
- [Netty CorsHandler cache poisoning via Vary header overwrite](https://junglewise.ai/threats/cve-2026-59903-netty-corshandler-cache-poisoning-via-vary-header-overwrite)
- [Netty HTTP request smuggling via lax WebSocket handshake validation](https://junglewise.ai/threats/cve-2026-59898-netty-http-request-smuggling-via-lax-websocket-handshake)
- [Netty resource exhaustion in HttpContentEncoder via HTTP pipelining](https://junglewise.ai/threats/cve-2026-59899-netty-resource-exhaustion-in-httpcontentencoder-via-http)
- [Netty security bypass in CorsHandler origin evaluation](https://junglewise.ai/threats/cve-2026-56746-netty-security-bypass-in-corshandler-origin-evaluation)