A method for authorization of a user to access a computer system locally at a site is described. The computer system determines whether a network connection to a remote authentication source is available. If the network connection is available, the computer system authenticates the user by interaction with the remote authentication source. If the network connection is not available, the computer system authenticates the user against a credential provided by the user. In this case, the credential will have been provided by or validated by the remote authentication source less than a predetermined time prior to the authenticating step, and the credential is a certificate issued by a certificate authority already trusted by the computer system and valid for a predetermined period of time. A suitable computer system is also described.