Lessons learned from Google Capture The Flag 2024: [WEB] sappy

On challenges where we need XSS and for example host is checked, we can do the following using data url schema:

let url = new URL("data://osorin.net/,<script>alert()</script>");
 
fetch(url)
   .then(response => response.text())
   .then((response) => {
       console.log(response)
   })
   .catch(err => console.log(err));
 
console.log(url.host);

Other write-up(s) with different solutions: https://adragos.ro/google-ctf-2024/#sappy