CVE-2026-100663 Netty HTTP/1 CONNECT authority-form mistranslated to malformed HTTP/3
Attackers can bypass tunnel allow-lists and egress policies in Netty-based HTTP/1-to-HTTP/3 proxies by sending malformed CONNECT requests.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-100663?
CVE-2026-100663 is a high-severity vulnerability in netty, published on 26 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
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.
Weakness: CWE-20
Which versions are affected by CVE-2026-100663?
How do I fix CVE-2026-100663?
No fix has been published for CVE-2026-100663 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-100663 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-100663 exists.
Sources
Also known as GHSA-jgph-cgq3-c627, GHSA-qmrm-mg2w-2mpw, UBUNTU-CVE-2026-100663.