Vulnerability · Published Sep 26, 2026 · Assigned by VulnCheck

CVE-2026-100663Netty HTTP/1 CONNECT authority-form mistranslated to malformed HTTP/3

High 7.5

In plain English

A high-severity improper input validation in netty from netty. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 23% of all known vulnerabilities).

Severity (CVSS 3.1)7.5High

Exploit odds (30 days)0.3%top 77.0% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official description

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.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Ubuntu:26.10	netty	no fix listed	UBUNTU-CVE-2026-100663

Affected products (as reported)

Vendor	Product	Versions
netty	netty	4.2.2.Final – before 4.2.18.Final

Weakness: CWE-20 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

References

GitHub Security Advisory (GHSA-jgph-cgq3-c627) vendor-advisory
VulnCheck Advisory: Netty HTTP/1 CONNECT authority-form mistranslated to malformed HTTP/3 third-party-advisory