What you see here is the message that would pop up if you entered an insecure password to login to the site. What is NIST Special Publication 800-63b § 18.104.22.168? Well, NIST is the National Institute of Standards and Technology, and Special Publication 800-63b are the latest guidelines on digital authentication. There is some interesting stuff in there. For example, did you know that it is considered a poor security practice to require users to change their passwords periodically? Anyway, section 22.214.171.124 says that passwords should be checked against a list of passwords “obtained from previous breach corpuses.” There are a lot of these, and we use the haveibeenpwned API to check against all known breached passwords each and every time you log in.
We should be clear that your password is never transmitted to a 3rd party. Rather, we send the first 2.5 bytes of the SHA-1 hash of the password and search the results for the remaining part of the hash.
The result is that, if you are using a password that is secure one day, but becomes insecure the next because it was publicized in a breach, we will be able to notify you next time you log in.