Is CVE-2026-100663 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-100663 being exploited in the wild.

How severe is CVE-2026-100663?

High severity, with a CVSS score of 7.5, published 26 September 2026.

Which products and versions are affected?

Netty-Codec-Http3 4.2.2.Final through 4.2.17.Final.

Is there a fix for CVE-2026-100663?

A patch was published on 2026-09-26: Fixed in version 4.2.18.Final Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-100663 attacked?

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

Am I affected by CVE-2026-100663?

Check whether you run 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-2026-100663: Netty HTTP/3 codec CONNECT authority-form parsing bypass

CVE-2026-100663 · Severity: high · CVSS 7.5 · Published 2026-09-26

Netty's HTTP/3 codec library mishandles conversion of HTTP/1 CONNECT requests, allowing an attacker to specify a different destination server than what was authorized. In HTTP/1-to-HTTP/3 proxy or gateway deployments, an attacker can craft a request that bypasses security controls such as tunnel allow-lists or egress policies by using a conflicting Host header. This enables unauthorized tunneling to attacker-controlled destinations.

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

Quick answers

Is CVE-2026-100663 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-100663 being exploited in the wild.
How severe is CVE-2026-100663?: High severity, with a CVSS score of 7.5, published 26 September 2026.
Which products and versions are affected?: Netty-Codec-Http3 4.2.2.Final through 4.2.17.Final.
Is there a fix for CVE-2026-100663?: A patch was published on 2026-09-26: Fixed in version 4.2.18.Final Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-100663 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-100663?: Check whether you run 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 vulnerability exists in HttpConversionUtil.toHttp3Headers(), which fails to special-case HTTP/1 CONNECT authority-form request-targets. When a CONNECT request like "CONNECT trusted.example:443" is supplied with a conflicting Host header (e.g., "Host: attacker.example:443"), Netty parses the target as a URI and incorrectly emits the Host header value as the HTTP/3 :authority pseudo-header, bypassing validation logic that checks the original request-target. The fix, available in 4.2.18.Final, properly extracts the authority from the CONNECT request-target and rejects conflicting Host headers.

Affected products

Netty netty-codec-http3 4.2.2.Final through 4.2.17.Final

Timeline

2026-09-26: disclosed
2026-09-26: patched: Fixed in version 4.2.18.Final

References

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