Exploiting clickjacking vulnerability to trigger DOM-based XSS | Jan 2, 2023

Introduction

Welcome to my another writeup! In this Portswigger Labs lab, you’ll learn: Exploiting clickjacking vulnerability to trigger DOM-based XSS! Without further ado, let’s dive in.

• Overall difficulty for me (From 1-10 stars): ★☆☆☆☆☆☆☆☆☆

Background

This lab contains an XSS vulnerability that is triggered by a click. Construct a clickjacking attack that fools the user into clicking the “Click me” button to call the print() function.

Exploitation

Home page:

Feedback page:

In here, we can submit a feedback to the back-end.

Let’s try to submit a test feedback:

When we clicked the “Submit feedback” button, it’ll display our name.

View source page:

<form id="feedbackForm" action="/feedback/submit" method="POST" enctype="application/x-www-form-urlencoded" personal="true">
    <input required type="hidden" name="csrf" value="BX4Ak51cXyol4PcY7g5vtLn4JKtYucEo">
    <label>Name:</label>
    <input required type="text" name="name">
    <label>Email:</label>
    <input required type="email" name="email">
    <label>Subject:</label>
    <input required type="text" name="subject">
    <label>Message:</label>
    <textarea required rows="12" cols="300" name="message"></textarea>
    <button class="button" type="submit">
        Submit feedback
    </button>
    <span id="feedbackResult"></span>
</form>
<script src="/resources/js/submitFeedback.js"></script>

/resources/js/submitFeedback.js:

document.getElementById("feedbackForm").addEventListener("submit", function(e) {
    submitFeedback(this.getAttribute("method"), this.getAttribute("action"), this.getAttribute("enctype"), this.getAttribute("personal"), new FormData(this));
    e.preventDefault();
});

function submitFeedback(method, path, encoding, personal, data) {
    var XHR = new XMLHttpRequest();
    XHR.open(method, path);
    if (personal) {
        XHR.addEventListener("load", displayFeedbackMessage(data.get('name')));
    } else {
        XHR.addEventListener("load", displayFeedbackMessage());
    }
    if (encoding === "multipart/form-data") {
        XHR.send(data)
    } else {
        var params = new URLSearchParams(data);
        XHR.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
        XHR.send(params.toString())
    }
}

function displayFeedbackMessage(name) {
    return function() {
        var feedbackResult = document.getElementById("feedbackResult");
        if (this.status === 200) {
            feedbackResult.innerHTML = "Thank you for submitting feedback" + (name ? ", " + name : "") + "!";
            feedbackForm.reset();
        } else {
            feedbackResult.innerHTML =  "Failed to submit feedback: " + this.responseText
        }
    }
}

In here, function displayFeedbackMessage(name) looks interesting:

feedbackResult.innerHTML = "Thank you for submitting feedback" + (name ? ", " + name : "") + "!";

It’s using innerHTML sink (Dangerous function), and our source (user input) is directly parsed to that sink without HTML encoded, escaped, sanitized.

That being said, if the back-end didn’t do any input validations, it’s very likely to be vulnerable to DOM-based XSS (Cross-Site Scripting).

Let’s test for XSS in the name field:

<img src=errorpls onerror=alert(document.domain)>

Nice! We successfully exploited DOM-based XSS.

Now, we can combine DOM-based XSS and clickjacking.

First, we need to prepopulate the XSS payload via providing a GET parameter name and other required parameters:

/feedback?name=<img src=errorpls onerror=print()>&email=attacker@evil.com&subject=subject&message=message

Then, we can craft the HTML payload that tricks users to click on the “Submit feedback” button.

However, instead of crafting it manually, we can use Burp Suite’s Clickbandit:

Click “Start”:

Click the “Submit feedback” button:

Then click the “Finish” button:

After that, turn off transparency, and click “Save”:

Now we can test it works or not:

It worked!

Finally, go to exploit server to host our clickjacking payload, and deilver to victim:

Nice!

What we’ve learned:

1. Exploiting clickjacking vulnerability to trigger DOM-based XSS