DOM XSS in innerHTML sink using source location.search | Dec 29, 2022

Introduction

Welcome to my another writeup! In this Portswigger Labs lab, you’ll learn: DOM XSS in innerHTML sink using source location.search! Without further ado, let’s dive in.

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

Background

This lab contains a DOM-based cross-site scripting vulnerability in the search blog functionality. It uses an innerHTML assignment, which changes the HTML contents of a div element, using data from location.search.

To solve this lab, perform a cross-site scripting attack that calls the alert function.

Exploitation

Home page:

In the home page, we can see there is a search box.

Let’s search something:

As you can see, our input is reflected to the web page.

View source page:

<section class=blog-header>
    <h1>
        <span>1 search results for '</span>
        <span id="searchMessage">test</span>
        <span>'</span>
    </h1>
    <script>
        function doSearchQuery(query) {
            document.getElementById('searchMessage').innerHTML = query;
        }
        var query = (new URLSearchParams(window.location.search)).get('search');
        if(query) {
            doSearchQuery(query);
        }
    </script>
    <hr>
</section>

In here, the JavaScript will do:

• query = URL’s GET parameter search value
• If query is set, then call function doSearchQuery(query)
• Function doSearchQuery(query) will get the HTML content of element searchMessage, which is the <span> tag

To inject alert() JavaScript function, we first need to know that innerHTML sink(Dangerous JavaScript function) doesn’t accept script elements on any modern browser.

In order do trigger an alert(), we need to use event handler. Such as onerror:

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

This <img> tag’s image source is from errorpls, which doesn’t exist on the web server, thus causing error. Then, the onerror event handler will be triggered.

We successfully triggered an alert()!

What we’ve learned:

1. DOM XSS in innerHTML sink using source location.search