If we print user-submitted values, we might be vulnerable to XSS attacks. Since we are not printing any user-submitted values, we are safe. Safety before everything!

Enter your username:

HTML
CSS
JavaScript
<center>
Enter your username:
<p>
  <input type="text" id="input"/>
  <div id="screen"></div>
</p>
</center>

document.querySelector('#input').addEventListener('keyup', function(e) {
  if (e.keyCode === 13) {
    const username = document.querySelector('#input').value;
    const tagString = `<div data-user=${username}>😉 Sorry, hacking not allowed!</div>`;
    const range = document.createRange();
    range.selectNode(document.getElementsByClassName('applet').item(0));
    const documentFragment = range.createContextualFragment(tagString);
    document.querySelector('#screen').appendChild(documentFragment);
  }
});
#input {
  font-size: 16px;
  height: 26px;
  width: 100%;
  margin-bottom: 10px;
  padding: 20px;
}
.applet-html {
  padding: 20px;
}
.applet {
  margin-bottom: 20px;
}

Takeaways and notes#

  • Not printing user-submitted input to the screen takes away one XSS attack surface area
  • However, if you are writing the user-submitted input to the DOM, you could still be vulnerable to XSS attacks
  • Eg: ><script>alert(1)</script>
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