In the Source-Code of the site we can find a hint
so i just guessed based on the hind and because its a site where you should upload you homwork `web.zh3r0.ml:7777?page=upload`
which leads to an upload formular
lets upload a php shell and try to access it
it works! lets find the flag!
After a bit tweaking i got a extendet shell running which i could access by `http://web.zh3r0.ml:7777/?page=pwny`
To see what we are dealing with `ls -la` and we see a bunch of weirdly named folders..
lets search them with `grep -nr 'zh3r0*'` and voilà the flag