Bug 2541850 (CVE-2026-100663)

Summary: CVE-2026-100663 io.netty/netty-codec-http3: netty-codec-http3: Security policy bypass via improper HTTP/1 to HTTP/3 CONNECT request translation
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Priority: high    
Version: unspecifiedCC: sdawley
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in Netty's HTTP/3 codec (netty-codec-http3). A remote attacker can bypass security policies and tunnel allowlists when Netty acts as an HTTP/1 to HTTP/3 proxy or gateway. Due to improper translation of HTTP/1 CONNECT requests, the component uses the attacker-controlled Host header to establish the HTTP/3 connection rather than the validated request target. By sending a request with conflicting destination headers, an attacker can route traffic to unauthorized backend services and circumvent egress filtering or audit controls.
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Description OSIDB Bzimport 2026-09-26 13:35:49 UTC 
Netty's HTTP/3 codec (io.netty:netty-codec-http3) from 4.2.2.Final through 4.2.17.Final does not special-case HTTP/1 CONNECT authority-form request-targets when converting HTTP/1 messages to HTTP/3 in HttpConversionUtil.toHttp3Headers. The authority-form target (e.g., "CONNECT trusted.example:443") is parsed as a URI, so its host is emitted as :scheme, :path is set to "/", and the HTTP/1 Host header is used as :authority; if no Host header is present the CONNECT target is dropped. In a Netty-based HTTP/1-to-HTTP/3 proxy or gateway, a remote client can send a CONNECT request whose Host header names a different authority than the request-target, producing a malformed HTTP/3 CONNECT whose tunnel :authority is attacker-controlled. This can bypass tunnel allow-lists, egress policy, backend selection, or audit controls that validate the HTTP/1 CONNECT request-target before forwarding over HTTP/3. The issue is fixed in 4.2.18.Final.