Purple Belt Generic: Lab - File Inclusion Vulnerabilities

Hi,

hope someone could help me out with the last question of this mentioned lab

It says: "Exploit the final path traversal vulnerability on Yellow. You'll need to find, and run, a custom binary to get the token."

I tried several things - was able to read the etc/passwd and several other logs. Unfortunately I´m a little stuck. 

Could anyone please give a hint to point me into the right direction? I´d really like to finish this lab and to learn how this works.

Thanks a lot.