A method of controlling access to network services enables an authorized proxy client to access a service on behalf of a user. To permit the client to function as a proxy, the user registers proxy authorization information with a trusted security server. The proxy authorization information identifies the proxy client and specifies the extent of proxy authority granted to the proxy client. When the proxy client wants to access a target service on behalf of the user, it sends a proxy request to the trusted security server. The trusted security server checks the proxy authorization information of the user to verify whether the request is within the proxy authority granted to the proxy client. If so, the trusted security server returns to the proxy client a data structure containing information recognizable by the target service to authenticate the proxy client for accessing the target service on behalf of the user.