A system for preventing fraud at a service provider's website has a fake credential distributor for creating a set of valid but fake credentials to access such website and distributing such set to a plurality of predefined websites, a fake user database including said set of fake credentials and a monitor for real-time monitoring any user login attempt from any origin network location to the website and comparing the credentials used in such user login attempt with the set of fake credentials in the fake user database. If the credentials used in such a login attempt coincide with any fake credential in the fake user database, such origin network location is added to a tainted connections list; and if there is another login attempt of a user from a network location in said tainted connections list, said user access request is refused even when it is using real credentials.