An access site allows a client application to access a server application on behalf of a subscriber who has an account at the client site. A client application registers with the access site and receives a certificate for the client application. A subscriber is directed to the access site upon an indication that she would like to use the features of the client application that integrate with the server application. The subscriber specifies access rights to the access site, and issues a validation token in association with the specified access rights. When the client site needs the server application to process subscriber data, it forwards the validation token to the access site, using the certificate. The access site validates this information, and where appropriate the server application processes the subscriber data and returns the results to the client application.