Fortunately, it doesn't appear as if credit card information is at stake, although some users' personal information may be:
We are assessing the full extent of this potential breach to determine what data, if any, was compromised and will provide more details as soon as possible. While some personal information may have been accessed, we can confirm that there is no possibility of any credit card information leak from this incident, since the server in question stores no credit card information. We estimate that the suspension will continue for a few days until we complete our investigation and counter-measures. We will update you as we learn more.Square Enix Members is a loyalty program that dishes out rewards in exchange for players who register Square-Enix games with the service. The website does store users' names, addresses, usernames, and passwords.