The username of an account can be ..

How it was found

Hello,

Description:

The username of an account can be set to ... This makes it impossible to view the public profile of this account.

POC:

I have claimed the username .. on the demo.weblate.org site. It is impossible to view this account's public profile page. Here is the public profile page: https://demo.weblate.org/user/../

Mitigation

I recommend you filtering usernames to prevent them from starting with ..

Thanks!