1. A computer-implemented method, comprising: under the control of one or more computer systems configured with executable instructions, receiving, by a service provider, a request to provision one or more virtual machine instances, the request associated with a policy, the policy specified by a user and indicating which of the one or more virtual machine instances is authorized to obtain credentials; receiving, by the service provider, an authorization specification indicating one or more privileges corresponding to the one or more virtual machine instances, the one or more privileges for accessing one or more services utilizing the one or more virtual machine instances, the one or more services operated by the service provider; after receiving the authorization specification, newly generating, based at least in part on the one or more privileges indicated by the authorization specification, one or more credentials corresponding to the one or more virtual machine instances, the one or more credentials utilizable by the one or more virtual machine instances to make authenticable web service requests to the one or more services operated by the service provider; causing the one or more credentials to be sent to one or more server computers configured to: host the one or more virtual machine instances; and make the one or more credentials available to the one or more virtual machine instances via one or more web services running on the one or more server computers; and allowing, based at least in part on the authorization specification, the one or more virtual machine instances access to the one or more services operated by the service provider.