I’m confused. Are you saying a website might actually spit out your current password or what you had just entered when attempting to change it? The former should never, ever be the case; the latter shouldn’t be the case, although it does happen from time to time.

Attack scenario would be a website loading CSS that is controllable by someone malicious. This could be due to ad code or custom themes for part of the site. That CSS would include selectors that would trigger different remote image requests for different partial matches on the value of the input. Based upon what remote URLs were triggered, one could reconstruct all or part of a password.

But I get your point. The website should not know the plaintext of your password for an overlap check unless their security practices are really bad. And if they are that bad, hopefully it is a throwaway password anyway. A duplicate check could still be done with hashes, but partial hash leaks are NBD.

Personally, I've had this happen though on password change prompts, which makes me think that the website is storing the value I just entered temporarily in the session. That's still bad even if it isn't being persisted beyond that page post though.

They could store the hashes of all the prefixes of the password and send them to the client. It's almost as bad as sending the password, because it's easy to brute force the cleartext.