A system and method for authenticating users of a data processing platform stores a mapping of a unique user platform identifier to multiple user identity provider identifiers associated with multiple realms for a same user. In some examples, the method includes receiving a request from a client device to establish an access session to perform one or more actions on data of the data processing platform and receiving, from at least one of the first external identity provider of the first realm or the second external identity provider of the second realm, a user identity provider identifier associated with the request. In certain examples, the method includes granting permission to perform the one or more actions on the data of the data processing platform based at least in part on the received user identity provider identifier.