Dream Diary: Chapter 1

Hi HTB Team,

Any guide on Pwn Challenge: Dream Diary : Chapter 1… still my brain is out-of-nowhere here on this challenge. all i know is the Out-Of-Bound vulnerability in Ubuntu 16.04 : Xenial Xerus if i’m not mistaken. can someone help me out?

iam still waiting for the clue/hint
it’s difficult to do primitive write-what-where when there’s no uaf,oob, or even double free