Websocket vulnerabilities

The websocket protocol is a protocol in the application layer of the OSI model, enabling full-duplex communication between a client (browser) and a web server. This makes it possible to create dynamic, real-time web applications such as instant messaging.

WebSockets are used for all kinds of purposes, including performing user actions and transmitting sensitive information. Virtually any HTTP web security vulnerability can also exist in WebSockets communications.


The two main tools for testing WebSockets are Burp Suite and OWASP ZAP. These tools allow for intercepting and modifying WebSockets frames on the fly.

  1. Check if there is indeed an authentication system and how it is implemented (access to functionalities without authentication).

  2. Test access control in detail to try to obtain a privilege escalation.

Cross-Site WebSocket Hijacking (CSWH)

A WebSocket communication is initiated through HTTP communication via the WebSocket handshake. The client tells the server that it wants to initiate a WebSocket connection:

GET /chat HTTP/1.1
Host: www.websocket.com:8000
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Firefox/68.0
Accept: */*
Accept-Language: fr,fr-FR;q=0.8,en-US;q=0.5,en;q=0.3
Accept-Encoding: gzip, deflate
Sec-WebSocket-Version: 13
Origin: http://www.websocket.com
Sec-WebSocket-Key: xxxxxxxxxxxxxxxxxxxxxxx
DNT: 1
Connection: keep-alive, Upgrade
Cookie: X-Authorization=yyyyyyyyyyyyyyyyyyyyyyyyyyyy
Pragma: no-cache
Cache-Control: no-cache
Upgrade: websocket

The server accepts the connection with:

HTTP/1.1 101 Switching Protocols
Upgrade: WebSocket
Connection: Upgrade
Sec-WebSocket-Accept: zzzzzzzzzzzzzzzzzzzzzzzzzzzzz

The WebSocket protocol does not let a server authenticate the client during the handshake process. Only the normal mechanisms for HTTP connections are available, including HTTP and TLS authentication and cookies. The upgraded handshake still happens from HTTP to WebSocket. The HTTP sends the authentication information directly to WS. And this can be exploited in Cross-Site WebSocket Hijacking (CSWH).

The Cross-Site WebSocket Hijacking attack is possible when the WebSocket handshake is vulnerable to CSRF. The communication channel between the two parties (client/server) is created according to the origin of the opening request. If the protocol change request is only based on cookies, it is possible to lure a victim into initiating a request with its session on the attacker’s server.


Once the Websocket communication is initiated, the client and the server communicate asynchronously. The format of the exchanged data can be of any form. In practice, the most used WebSocket libraries use JSON format.

If a CSWH attack is successful, it becomes possible to communicate with the server via WebSockets without a victim’s knowledge, to act in the place of a user, and also read the server messages sent via WebSockets.

Portswigger lab writeups


  • The WebSocket protocol does not have a native mechanism for authentication, so during development, a clean solution must be implemented.

  • As with authentication, there is no system in the WebSocket protocol for managing authorisations, guaranteeing users only have access to the data and services they should have access to. This gives an attacker the ability to raise privileges vertically or to access a user’s data with the same level of rights the attacker has.

  • The data entered by users, also via WebSockets, is the major cause of attacks: XSS, SQL injections, code injections, etc. All inputs must be sanitised with the most appropriate method according to the context, before being used.

  • Data transmission via the WebSocket protocol is done in clear text just like HTTP. This data can be read and stolen in on-path attacks. To avoid information leakage, the WebSocket Secure (wss) protocol must be implemented. Using wss does not mean that the web application is secure, it is only the encrypted transport of data via TLS.

  • To protect from Cross-Site WebSocket Hijacking, add a unique token per session that cannot be guessed as a parameter of the handshake request.