1. A method implemented by data processing apparatus, the method comprising: receiving a plurality of requests for long-term security tokens from a host machine, each request comprising authentication information for a respective service account; providing one or more of the long-term security tokens to the host machine based on the plurality of requests, wherein the one or more long-term security tokens can be used to generate short-term security tokens for virtual machines executing on the host machine, wherein the virtual machines are hardware virtualizations on the host machine and cannot access the one or more long-term security token provided to the host machine; and generating by a process executing in a host operating system of the host machine a short-term security token based on a long-term security token of the one or more long-term security tokens for use by a virtual machine executing on the host machine to access one of the respective service accounts, wherein the short-term security token is different than the long-term security token and the short-term security token is useable for a pre-determined amount of time.