In this challenge, we need to exploit quickjs engine, which is a lightweight JavaScript engine, and this is actually my first time to exploit this engine. The vulnerability we need to exploit is that when variable is copied to promise result, the reference counter is not incremented, so that use-after-free problem can be triggered. We trigger such UAF using ArrayBuffer instance so that we can manipulate baking storage of ArrayBuffer after it is freed. We utilize this to leak libc address and to rewrite backing store pointer of another TypedArray to achieve arbitrary write that rewrites \_\_free_hook to system to get the shell.

Original writeup (https://mem2019.github.io/jekyll/update/2021/09/27/TCTF2021-Promise.html).