ProCTF - Cybrics Quals 2019

For this challenge, we are provided with the username and the password for a machine.

The ssh session gives us a strange shell:

By trying some commands we run into an error message:

Searching the error message on Google tells us that it's probably Prolog. The prolog's documentation tells us that the syntax to run a system command is:

shell('command').

Let's try:

Searching through the directories we find the flag file in the user's home:

After I solved the challenge, reading the documentation, I found out that running the shell command without parameters would have gave me the default system shell, making it easier to move through the system.

Flag

cybrics{feeling_like_a_PRO?_that_sounds_LOGical_to_me!____g3t_it?_G37_1T?!?!_ok_N3v3Rm1nd...}