I received a mail to change my password. At first i was skeptical but it seems to be no fraud.
Ubisoft's servers are hacked and emails, usernames and passwords are stolen. For security reasons Ubisoft suggest to change the password. And also change the password on all sites that use the same or a similar password.
Payment information was not stolen cause it is not on these servers. Ubisoft says that the bug is fixed and the site is secure again.
Every user should have received a mail with instructions how to reset his password. More information in the Ubisof FAQ.