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Netty is an asynchronous, event‑driven network application framework widely used to build high‑performance protocol servers and clients. Prior to versions 4.1.136.Final and 4.2.16.Final, the framework’s WebSocket handshakers for protocol versions 07 and 08 suffered from overly permissive validation logic.
The vulnerability resides in the `newHandshakeResponse` methods of `WebSocketServerHandshaker07` and WebSocketServerHandshaker08. These methods did not verify that an incoming HTTP request contained the mandatory `Connection: Upgrade` and `Upgrade: websocket` headers before completing the WebSocket upgrade. Instead, they accepted a handshake based solely on the presence of a `Sec‑WebSocket‑Version: 7` (or 8) header, without enforcing the standard HTTP upgrade sequence.
An attacker can craft a malicious HTTP request that includes `Sec‑WebSocket‑Version: 7` but deliberately omits the `Connection` and `Upgrade` headers. When this request reaches a vulnerable Netty server, the handshaker proceeds with the WebSocket protocol switch, treating the connection as a WebSocket session. However, because the required upgrade headers are missing, any intermediary proxy or load balancer that processes the request will not recognise it as an explicit protocol transition. The proxy continues to interpret the stream as plain HTTP, while Netty interprets it as WebSocket – creating a desynchronised state between the two components.
This desynchronisation is the foundation for HTTP request smuggling and protocol‑confusion attacks. By carefully crafting subsequent payloads, an attacker can inject arbitrary HTTP requests that the proxy forwards as part of the same connection, but which Netty parses as separate WebSocket frames or as additional HTTP requests. This can lead to cache poisoning, session hijacking, credential theft, or bypass of security controls that rely on proper header inspection.
The flaw maps to CWE‑444 (Inconsistent Interpretation of HTTP Requests) and has been assigned a CVSS v4.0 base score of 6.3 (Medium). The issue was fixed by adding strict validation that rejects any WebSocket handshake missing the mandatory `Connection` and `Upgrade` headers, ensuring that the upgrade is only completed when all required headers are present.
DailyCVE Form:
Platform: Netty
Version: <4.1.136.Final, 4.2.0-4.2.15.Final
Vulnerability: HTTP Request Smuggling
Severity: Medium (CVSS 6.3)
date: 2026-07-29
Prediction: Patch already available
What Undercode Say:
Analytics show that this vulnerability affects all applications using Netty’s `netty‑codec‑http` module with WebSocket support enabled. The attack requires no authentication and can be launched remotely over the network, making it particularly dangerous in microservices architectures where requests pass through multiple proxies and API gateways. Monitoring for anomalous WebSocket handshake patterns – especially requests that lack `Connection` and `Upgrade` headers – is recommended to detect potential exploitation attempts.
Bash commands and codes:
Check your Netty version in a Maven project mvn dependency:tree | grep netty-codec-http For Gradle projects gradle dependencies | grep netty-codec-http Upgrade to the patched version (Maven) <dependency> <groupId>io.netty</groupId> <artifactId>netty-codec-http</artifactId> <version>4.1.136.Final</version> <!-- or 4.2.16.Final for 4.2.x line --> </dependency> Gradle upgrade implementation 'io.netty:netty-codec-http:4.1.136.Final' or implementation 'io.netty:netty-codec-http:4.2.16.Final'
Exploit:
A remote attacker sends a crafted HTTP request to a vulnerable Netty server:
GET /chat HTTP/1.1 Host: vulnerable.example.com Sec-WebSocket-Key: x3JJHMbDL1EzLkh9GBhXDw== Sec-WebSocket-Version: 7 Origin: http://example.com
Notice the absence of `Connection: Upgrade` and Upgrade: websocket. The Netty handshaker accepts this as a valid WebSocket upgrade, while a proxy in front sees a normal HTTP request. The attacker can then smuggle a second HTTP request within the same connection, which the proxy forwards but Netty interprets as part of the WebSocket stream, leading to request smuggling.
Protection:
Upgrade to Netty versions 4.1.136.Final, 4.2.16.Final, or later. If immediate upgrade is not possible, deploy a Web Application Firewall (WAF) rule that blocks WebSocket handshake requests missing the `Connection: Upgrade` and `Upgrade: websocket` headers. Additionally, configure reverse proxies to strictly validate upgrade headers before forwarding requests to Netty backends.
Impact:
Successful exploitation enables HTTP request smuggling attacks, allowing an attacker to bypass security controls, poison caches, hijack user sessions, exfiltrate sensitive data, and disrupt services. The attack requires no privileges and no user interaction, and can be performed remotely over the network. In environments where Netty serves as the backend for API gateways or microservices, the impact can be particularly severe, potentially compromising the entire request‑processing pipeline.
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Sources:
Reported By: nvd.nist.gov
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