HTTP Request Smuggling Affecting io.netty:netty-codec-http3 package, versions [4.2.2.Final,4.2.18.Final)
Threat Intelligence
Snyk has a proof-of-concept or detailed explanation of how to exploit this vulnerability.
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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Test your applications- Snyk IDSNYK-JAVA-IONETTY-20158598
- published27 Sept 2026
- disclosed26 Sept 2026
- creditRex Liu
Introduced: 26 Sep 2026
NewCVE-2026-100663 (opens in a new tab)How to fix?
Upgrade io.netty:netty-codec-http3 to version 4.2.18.Final or higher.
Overview
Affected versions of this package are vulnerable to HTTP Request Smuggling in HttpConversionUtil.toHttp3Headers(HttpMessage, boolean), which has no special case for HTTP/1 CONNECT and instead runs the generic request conversion, drawing :authority from the Host header rather than the authority-form request-target and emitting the forbidden :scheme and :path pseudo-headers. An attacker can establish a tunnel to an upstream authority other than the one requested, bypassing tunnel allowlists, egress policy, backend selection, and audit logic, by sending CONNECT trusted.example:443 HTTP/1.1 with Host: attacker.example:443, which converts to :authority: attacker.example:443. This requires Netty deployed as an HTTP/1 to HTTP/3 proxy or gateway handling CONNECT, and the effect is confined to authority interpretation with no code execution or memory corruption.