A controller for user authentication and access control, configured to: store data representing a graph having: nodes representing data elements associated with accesses made using an access token; and links among the nodes representing connections between the data elements identified in details of the accesses. In response to receiving details of an access made using the access token, the controller updates the graph according to the details and identifies a new connection in the graph resulting from update. The controller communicates with an identity service to verify the association of data elements corresponding to the new connection in the graph. Based on a result of the verification, the controller authenticates the user of the access and/or controls the access.