Techniques for proxing services with a single sign on are provided. A principal authenticates to a first identity service. The first identity service is in a trusted relationship with a second identity service. An authentication request is sent to the second identity service and the request includes an authentication response supplied by the first identity service in response to successful authentication of the principal to the first identity service. In response to the authentication request and the accompanying response, the principal is authenticated for access to the second identity service. Furthermore, targeted services accessible to the second identity service are proxied from and to the principal during interactions between the principal and an external service of that principal.