Comment by daguava

You can list what problems you've solved by showing an image generated for you.

Ex) https://projecteuler.net/profile/daguava.png

But you can also use this to quickly test the status of accounts.

For example, I was able to find Euler is an admin account by trying

https://projecteuler.net/profile/euler.png

It tells you it's admin in the image, why?

Edit: Wonder if they're exposing some vulnerability with the HTTP 300 Multiple Files they're returning.

If you try something like this: https://projecteuler.net/profile/.wat

the page confirms a .htaccess file exists at https://projecteuler.net/profile/.htaccess we also find one at https://projecteuler.net/.htaccess

While currently inaccessible, this is significant information leak

All directories allow this, so you can do some digging to find what files exist.

Edit 2:while logged in, you can enumerate all usernames with a skill level attached by using URLs like

https://projecteuler.net/level=1

If you try changing the level to a period, the page conveniently tells you there are over 118k users in total (listing the first 10k), and MAY even show accounts without levels, but I'm not sure.

Combine this with the profile image URLs above and you may be able to find more admin account usernames if they have levels associated with them.