This challenge has no description, so the solution must be on the site itself.
Clicking on a problem makes a request for some HTML to embed in the modal that pops up. If we take a look at this request, this header comes along with it:
This isn’t the final flag, because four of the hex characters are missing. However, taking another look at the code reveals something useful.
Before submitting an answer, it checks your submission against a hash of the correct answer to see if you’re right. The hash itself also comes with the request.
So to find the hash, we have to loop through all possible replacements of those four missing hex characters in the flag (16^4, or 65536 combinations), and test them against the hash until we find the correct flag. A quick Python script will do the trick.