Reflected XSS in a JavaScript URL with some characters blocked | Jan 1, 2023

Introduction

Welcome to my another writeup! In this Portswigger Labs lab, you’ll learn: Reflected XSS in a JavaScript URL with some characters blocked! Without further ado, let’s dive in.

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

Background

This lab reflects your input in a JavaScript URL, but all is not as it seems. This initially seems like a trivial challenge; however, the application is blocking some characters in an attempt to prevent XSS attacks.

To solve the lab, perform a cross-site scripting attack that calls the alert function with the string 1337 contained somewhere in the alert message.

Exploitation

Home page:

In the home page, we can view one of those posts:

View source page:

<div class="is-linkback">
    <a href="javascript:fetch('/analytics', {method:'post',body:'/post%3fpostId%3d2'}).finally(_ => window.location = '/')">Back to Blog</a>
</div>

In the Back to Blog <a> tag link, it’s using a JavaScript code, which sends a POST request to /analytics with parameter /post?postId=2.

Let’s try to inject JavaScript code in /post?postId=2 GET parameter:

Hmm… Looks like we first need to bypass the Invalid blog post ID.

To do so, I’ll try to close the JavaScript URL via ', with HTML encoding:

&%27;

Let’s try to inject a XSS payload:

However, the parentheses (()) are missing. Looks like the application removes them.

To bypass that, we can:

2&%27},x=x=>{throw/**/onerror=alert,1337},toString=x,window+'',{x:'

Result:

javascript:fetch('/analytics', {method:'post',body:'/post/postId=2'},x=x=>{throw/**/onerror=alert,1337},toString=x,window+'',{x:''}).finally(_ => window.location = '/')

In here, we can use throw statement with an exception handler. This enables you to pass arguments to a function without using parentheses.

The {throw/**/onerror=alert,1337} code is to throw an exception, which is a JavaScript comment and it’ll trigger an error. Then, when an error occurred, assign function alert(), with argument 1337 to onerror exception handler.

Finally, the ,toString=x,window+'',{x:' code is to assign the toString property of window and trigger this by forcing a string conversion on window.

Let’s try it!

When we click the Back to Blog link, it’ll trigger an alert box!

What we’ve learned:

Reflected XSS in a JavaScript URL with some characters blocked