This lab demonstrates a simple web-message vulnerability. To solve this lab, use the exploit server to post a message to the website that exploits an XSS vulnerability and alerts document.cookie.
When the iframe loads, the postMessage() method sends a web message to the home page. The event listener, which is intended to serve ads, takes the content of the web message and inserts it into the div with the ID ads. However, in this case it inserts our img tag, which contains an invalid src attribute. This throws an error, which causes the onerror event handler to execute our payload.