Corridor

https://tryhackme.com/room/corridor

We get quite some help from the description of this challenges, as we are ask to look for "the hexadecimal values you find (they look an awful lot like a hash, don't they?)".

I solved this by viewing the source code of the main page and extracting all the hashes from there. Then if we put those into https://crackstation.net/, we see that they are the hashes of the numbers 1, 2, ... , 12, 13.

Simply make a MD5 hash of the value '0' and access the sub-site to find the flag.

MD5(0) = cfcd208495d565ef66e7dff9f98764da