[keycloak-user] Server-side validation of custom user attributes

Hello, Chapter 32 of the Keycloak user manual describes how custom user attributes can be used. Is there a way to validate the user attribute values server-sided (as opposed to in the theme / client-sided)? In our case, we'd like to require our users to supply a particular value, which must match one of many pre-defined values. We do not want to expose the entire list of valid values publicly though. Regards, Guus