Stripe is running a web security capture the flag - a series of increasingly difficult web security exploit challenges. I've finished it and had a lot of fun. Working on a web browser I knew the theory of these various web based attacks, but this was my first chance to put theory into practice with:
Here's a blog post on the CTF behind the scenes setup which has many impressive features including phantom users that can be XSS/CSRF'ed.
I'll have another post on my difficulties and answers for the CTF levels after the contest is over on Wed, but if you're looking for hints, try out the CTF chatroom or the level specific CTF chatroom.