Lab: Reflected XSS in a JavaScript URL with some characters blocked

EXPERT

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.

Access the lab

Solution

Visit the following URL, replacing YOUR-LAB-ID with your lab ID:

https://YOUR-LAB-ID.web-security-academy.net/post?postId=5&%27},x=x=%3E{throw/**/onerror=alert,1337},toString=x,window%2b%27%27,{x:%27

The lab will be solved, but the alert will only be called if you click "Back to blog" at the bottom of the page.

The exploit uses exception handling to call the alert function with arguments. The throw statement is used, separated with a blank comment in order to get round the no spaces restriction. The alert function is assigned to the onerror exception handler.

As throw is a statement, it cannot be used as an expression. Instead, we need to use arrow functions to create a block so that the throw statement can be used. We then need to call this function, so we assign it to the toString property of window and trigger this by forcing a string conversion on window.