Is CVE-2026-100659 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-100659 being exploited in the wild.

How severe is CVE-2026-100659?

Medium severity, with a CVSS score of 6.5, published 26 September 2026.

Which products and versions are affected?

Netty Project Netty-Codec-Http3 4.2.0.Final through 4.2.17.Final.

Is there a fix for CVE-2026-100659?

A patch was published on 2026-09-10: Fix available in 4.2.18.Final Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-100659 attacked?

It is attacked over the network, in a software library.

Am I affected by CVE-2026-100659?

Check whether you run Netty Project Netty-Codec-Http3 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.

CVE-2026-100659: Netty HTTP/3 codec host header validation bypass

CVE-2026-100659 · Severity: medium · CVSS 6.5 · Published 2026-09-26

Netty's HTTP/3 codec is a library that processes incoming web requests over the HTTP/3 protocol. An attacker can send a specially crafted request with conflicting host identifiers that different security systems interpret differently, allowing them to bypass routing rules and access controls that should protect sensitive resources.

Technologies: Netty Project Netty-Codec-Http3. Vendors: Netty Project.

Quick answers

Is CVE-2026-100659 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-100659 being exploited in the wild.
How severe is CVE-2026-100659?: Medium severity, with a CVSS score of 6.5, published 26 September 2026.
Which products and versions are affected?: Netty Project Netty-Codec-Http3 4.2.0.Final through 4.2.17.Final.
Is there a fix for CVE-2026-100659?: A patch was published on 2026-09-10: Fix available in 4.2.18.Final Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-100659 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-100659?: Check whether you run Netty Project Netty-Codec-Http3 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.

Technical details

The HTTP/3 codec fails to validate RFC 9114 requirements that the :authority pseudo-header and host header field must match when both are present in a single HEADERS frame. An unauthenticated remote attacker can send a HEADERS frame with mismatched authority values, which the codec accepts and delivers to the application, enabling HTTP request smuggling and authorization bypass when downstream components consult different authority fields.

Affected products

Netty Project netty-codec-http3 4.2.0.Final through 4.2.17.Final

Timeline

2026-09-26: disclosed
2026-09-10: patched: Fix available in 4.2.18.Final

References

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