You would not use legitimate interests to cover off your processing of data in connection with letting a user log in to your site, if it is a requirement of usi

You would not use legitimate interests to cover off your processing of data in connection with letting a user log in to your site, if it is a requirement of using the service that you are logged in, for example to authenticate who you are. The correct processing basis here would be to process data to provide a service, not under legitimate interests. If you were processing someone's data to, for example, ensure the safety of your network/detect unauthorised login attempts, then that would likely fall under legitimate interests, because it is processing that is not necessary to provide the underlying service, but is in the users' interests to ensure the protection of their personal data.