Netty <4.1.136/4.2.16: H2Codec Duplicate Host Header in HTTP/2
CVE-2026-59900 Published on July 29, 2026
Netty codec-http2: Lack of Host Header Deduplication in HTTP/2→HTTP/1.x Translation Leads to Request Routing Bypass
Netty is an asynchronous, event-driven network application framework. Prior to versions 4.1.136.Final and 4.2.16.Final, Netty's HTTP/2-to-HTTP/1.x translation layer (`Http2StreamFrameToHttpObjectCodec` and `InboundHttp2ToHttpAdapter`) fails to deduplicate or validate `Host` headers when an HTTP/2 client supplies both the `:authority` pseudo-header and a literal `host` header in a single HEADERS frame. The translator maps `:authority` to `Host` and separately copies the literal `host` header, producing an `HttpRequest` object containing two `Host` headers with attacker-controlled differing values. This issue has been fixed in versions 4.1.136.Final and 4.2.16.Final.
Weakness Type
What is a HTTP Request Smuggling Vulnerability?
When malformed or abnormal HTTP requests are interpreted by one or more entities in the data flow between the user and the web server, such as a proxy or firewall, they can be interpreted inconsistently, allowing the attacker to "smuggle" a request to one device without the other device being aware of it.
CVE-2026-59900 has been classified to as a HTTP Request Smuggling vulnerability or weakness.
Products Associated with CVE-2026-59900
Want to know whenever a new CVE is published for Netty? stack.watch will email you.
Affected Versions
netty:- Version >= 4.2.0.Final, < 4.2.16.Final is affected.
- Version < 4.1.136.Final is affected.