Access control mechanisms have been designed and implemented to limit the access, authenticate and authorize single users, either in fixed networks, or in mobile environments. With the recent advances in network mobility support in IPv6 (NEMO), access networks will soon be deployed in public transportation such as buses or trains and will in turn provide access to mobile nodes and even mobile networks. However, access control mechanisms have not been designed for users located in such nested mobile environments, that is, when the access network in which they get access to is itself mobile. An actual deployment scenario comprising a bus offering both Internet and local services to its passengers is used to illustrate the needs and the issues from a security and authentication point of view. An authentication architecture based on the latest access control mechanisms and protocols is then proposed to offer basic authentication in nested mobile environments.