Extensible account authentication system

Systems and methods directed at enhancing the capability of a federated authentication system by configuring the system with extensibility points for adding new account stores and customizing claim transformations. The federated authentication system includes accounts stores, a security token service (STS), and custom claim transformation modules. The account stores are configured to maintain data associated with accounts and to provide security claims in an intermediate format. The STS is configured to retrieve the security claims provided by the account stores and includes built-in transformations for transforming each security claim from the intermediate format to formats associated with resource providers. The STS is further configured to provide extensibility points for custom claim transformations that are not available from the built-in transformations. The custom claim transformation modules are configured to perform at least one custom claim transformation. Each custom claim transformation module is further configured to interact with the STS through at least one of the extensibility points. The STS may be configured to provide extensibility points for interacting with account stores that the STS does not explicitly recognize.