IFrame can access mashlibs window causing XSS

Summary

When a user opens a html file with mashlib, this file can access the window of mashlib and thus among other things the authenticated fetch at UI.authn.session.fetch.

Reproducing

Steps to reproduce (on NSS):

1. open yourpod.solidcommunity.net
2. login (required precondition for the exploit to work)
3. create a file test.html at any location (in a real exploit the attacker with append/write permissions would create this)
4. Write the html below into the file. Change the url with a private file
5. Display the html with mashlib (click on the "d" symbol)
6. Observe one alert with your webid and one with the file contents

<script>
  const main = async () => {
    mashlibWindow = window.parent;
    alert('Logged in as ' + await mashlibWindow.UI.authn.checkUser())
    res = await mashlibWindow.UI.authn.session.fetch('https://yourpod.solidcommunity.net/private/secret.txt')
    alert('Secret: ' + await res.text())
  }
  main()
</script>

For CSS, the steps are similar, however I think the test.html file would need to be publicly readable to be loaded in the iframe. In NSS this is not necessary because it also uses cookies.

Impact

Opening a malicious html file can result in the attacker gaining control over the whole pod.

Fix

Use an iframe with the sandbox attribute which may not include allow-same-origin. See for instance this article on sandboxing: https://web.dev/sandboxed-iframes/

This would also be fixed, if pods don't serve or sandbox html files (see https://forum.solidproject.org/t/is-it-secure-for-pods-to-serve-html-files/6379/2)
EDIT: after looking at the code, I don't think it matters if pods sandbox the html, as they are not directly included with <iframe src=".../test.html"> but fetched manually and then added as a blob.
EDIT 2: it's more complicated than I thought, SolidOS has 3 different ways of creating iframes. See here for a small discussion)

Start by reproducing the exploit on NSS, then trace SolidOS's three iframe-creation paths referenced in the issue. The work is done when an HTML file displayed by mashlib cannot access the parent mashlib window or authenticated fetch session.