The presently disclosed subject matter includes a system, a method and a non-transitory program storage device configured for authorizing access of a user device to a service provider server. Responsive to a request to authenticate the identity of a user attempting to access an SP server via a user device (UD), an authentication server is configured to initiate at least one authentication operation using a second UD; in the event of a failure to receive a response to the at least one authentication operation from the second UD, the authentication server is configured to proceed according to an alternative authentication method which does not involve the second UD.