A method, apparatus, and computer instructions for authorizing a user to access grid resources. A request is received from the user to access a resource on the data processing system. This request includes a certificate. An authentication process is performed using the certificate when the request is received. In response to successfully authenticating the user in the authentication process, a first host name for the certificate is requested from a trusted source. A reply containing the first host name is received. Access to the resource is provided if the first host name returned by the trusted source matches a second host name for the user from which the request originated.