I first checked the source code and found this line: <!-- include("admin/pass.html") --> So, i tried to go there, and got this:

Got a rick rolled? ;)
Don't worry, you're not the last :p

Just search

From the name of the level, I guessed directory indexing is enabled, let’s try to go to http://challenge01.root-me.org/web-serveur/ch4/admin/. There, we can see a directory backup, and in this directory, we can find admin.txt, which has this: Password: LINUX

Flag: LINUX