# CVE-2026-56817: Netty XmlDecoder XML external entity reference vulnerability

Source: https://junglewise.ai/threats/cve-2026-56817-netty-xmldecoder-xml-external-entity-reference-vulnerability  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-21. Updated: 2026-07-22  
License: CC BY 4.0 with a link back

CVE-2026-56817 · Severity: high · CVSS 4 · Published 2026-07-21

Netty is a widely used networking framework for Java applications. A vulnerability in its XML processing component could allow an attacker to send specially crafted XML messages that trick the system into accessing unauthorized files or internal network resources. This could lead to the exposure of sensitive data or impact the availability of the service.

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

## Quick answers

**Is CVE-2026-56817 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-56817 being exploited in the wild.

**How severe is CVE-2026-56817?** High severity, with a CVSS score of 4, published 21 July 2026.

**Which products and versions are affected?** io.netty:netty-codec-xml (Maven) ECOSYSTEM:introduced=4.2.0.Final,fixed=4.2.16.Final; Netty Project Netty-Codec-Xml >= 4.1.0.Final, <= 4.1.135.Final.

**Is there a fix for CVE-2026-56817?** Yes. io.netty:netty-codec-xml (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-2026-56817 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-56817?** Check whether you run io.netty:netty-codec-xml (Maven) or Netty Project Netty-Codec-Xml 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-xml (Maven): upgrade to 4.2.16.Final or later

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


## Technical details

An Improper Restriction of XML External Entity Reference (XXE) vulnerability exists in Netty's `netty-codec-xml` module. The `XmlDecoder` component utilizes an `AsyncXMLInputFactory` (often backed by Aalto XML) that is instantiated without disabling DTDs or external entity resolution. A remote, unauthenticated attacker can exploit this by sending XML containing a `DOCTYPE` declaration to a Netty channel pipeline. Successful exploitation could lead to local file disclosure or server-side request forgery (SSRF), though actual exploitability may depend on the specific behavior of the underlying asynchronous parser. The issue is resolved in versions 4.1.136.Final and 4.2.16.Final.

## Affected products

- Netty netty-codec-xml >= 4.1.0.Final, <= 4.1.135.Final
- Netty netty-codec-xml >= 4.2.0.Final, <= 4.2.15.Final

## Identifiers

- CVE-2026-56817
- GHSA-4qhr-g3c6-fcfx

## Timeline

- 2026-07-14: disclosed: Initial disclosure by maintainers
- 2026-07-21: advisory: NVD publication date
- 2026-07-22: patched: GitHub Advisory reviewed and updated with patch information

## References

- [https://github.com/netty/netty/security/advisories/GHSA-4qhr-g3c6-fcfx](https://github.com/netty/netty/security/advisories/GHSA-4qhr-g3c6-fcfx)
- [https://github.com/netty/netty/commit/5b68c61f37aa4a3045cba624cbea239655c9003b](https://github.com/netty/netty/commit/5b68c61f37aa4a3045cba624cbea239655c9003b)
- [https://github.com/netty/netty/commit/bb2ff68a1fb71cb4b0eb9a9e17b66c52aff680c6](https://github.com/netty/netty/commit/bb2ff68a1fb71cb4b0eb9a9e17b66c52aff680c6)
- [https://github.com/netty/netty/releases/tag/netty-4.1.136.Final](https://github.com/netty/netty/releases/tag/netty-4.1.136.Final)
- [https://github.com/netty/netty/releases/tag/netty-4.2.16.Final](https://github.com/netty/netty/releases/tag/netty-4.2.16.Final)

## Related threats

- [Netty: Denial of Service in XmlFrameDecoder via CPU Exhaustion](https://junglewise.ai/threats/cve-2026-73507-netty-denial-of-service-in-xmlframedecoder-via-cpu-exhaustion)