This is a very in-depth writeup that explains everything you need to know to write an exploit for this challenge.
While I was doing this challenge, I found that none of the other writeups for it really explained everything that was required to understand their writeups.
I thought I would do a writeup that is extremely detailed so that even someone that has never done browser exploitation will understand it.
Hope it helps, feel free to DM me on twitter @farazsth98 with any questions.