Wednesday, May 27, 2015

[Service Workers] Secure Open Redirect becomes XSS Demo

This is the shortest delay between blog posts I've had in a while, but I figured that since my  last post had some confusing stuff, it might make sense to add a short demo. The demo application has three things that enable the attack:

  1. An open redirect. Available at /cgi-bin/redirect?continue=.
  2. A Cache Service Worker. Available at /sw.js.
  3. A page that embeds images via <img crossorigin="anonymous" src="" />.
And the attacker's site has one thing:
  1. A CORS enabled attack page. Available at /cgi-bin/attack.
Let's do the attack then! For it to work we need two things to happen:
  • The service worker must be installed.
  • A request to our attack page must be cached with mode=cors.
Our index.html page will do both of those things for us.
Poison Cache

Image URL:

When you click submit above the following things will happen:
  1. A service worker will be installed for the whole origin.
  2. An image tag pointing to the open redirect will be created.
  3. The service worker will cache the request with the CORS response of the attacker.
If all went well, the page above should be "poisoned", and forever (or until the SW updates it's cache) you will get the XSS payload. Try it (note that you must have first "poisoned" the response above, if you click the button below here before poisoning the cache first, you will ruin the demo for ever since an opaque response will get cached ;):
If the demo doesn't work for you, that probably means you navigated to the attack page before caching the CORS response. If that's the case, to clear the cache.

Note you need Chrome 43 or later for this to work (or a nightly version of Firefox with the flags flipped). Hope this clarifies things a bit!

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