The password you are trying to use has been seen in a data breach. While you can still register with it, it is highly recommended not to, as it severely decreases the security of your account. Please consider using another password. This message will go away when a safe password is detected.
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On Passwords
This site uses the Have I Been Pwned? API to check if passwords have been seen in a data breach. Your password itself is never sent to a third party during this check. Read about validating leaked passwords with K-Anonimity here.
I very much prefer giving users a choice when possible, so while I do check if a password is breached, I only notify you of it. You can still register with a breached password, though I strongly recommend against it, and recommend using a Password Manager.