Netty WebSocket Upgrade Smuggling (CVE-202659898) before 4.1.136/4.2.16
CVE-2026-59898 Published on July 29, 2026
Netty: WebSockets V07/V08 handshaker missing Connection/Upgrade validation
Netty is an asynchronous, event-driven network application framework. Prior to versions 4.1.136.Final and 4.2.16.Final, ab attacker can force WebSocket upgrade via the lax V07 (or V08) handshaker by sending `Sec-WebSocket-Version: 7` and omitting `Connection: Upgrade` / `Upgrade: websocket` headers, completing a protocol switch that a proxy would not recognize as an Upgrade request and enabling HTTP request smuggling / protocol-confusion attacks. 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-2026-59898 has been classified to as a HTTP Request Smuggling vulnerability or weakness.
Products Associated with CVE-2026-59898
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Affected Versions
netty:- Version >=4.2.0.Final, < 4.2.16.Final is affected.
- Version < 4.1.136.Final is affected.