Since there was no answer in this topic, I can leave an answer for other people who may look for something similar. I created a new registration flow definition with additional form action at beginning of it. My custom form action contains validation rules working on top of form data. Based on received attributes validation fails registration attempt if external system client do not contain matching user record. Thanks to that no database write attempt is made and there is no need for implementing user storage provider. I attach pseudo code for reference. There are lots of missing pieces, but it should give general idea how problem can be approached. public class InternalUserFormAction implements FormAction { public static final String PROVIDER_ID = “internal-user-form-action"; @Override public void validate(ValidationContext context) { AuthenticatorConfigModel config = context.getAuthenticatorConfig(); // form action have one option which is external system uri, which should be called in order to check if user who is attempted to made account exists. String externalSystemURI = Constants.DEFAULT_SYSTEM_URI; if (config != null && config.getConfig() != null) { externalSystemURI = config.getConfig().getOrDefault(Constants.SYSTEM_URI_OPTION_NAME, Constants.DEFAULT_SYSTEM_URI); } MultivaluedMap<String, String> formData = context.getHttpRequest().getDecodedFormParameters(); List<FormMessage> errors = new ArrayList<>(); String eventError = Errors.INVALID_REGISTRATION; Optional<String> firstName = validateAndGet(formData, errors, RegistrationPage.FIELD_FIRST_NAME, Messages.MISSING_FIRST_NAME); Optional<String> lastName = validateAndGet(formData, errors, RegistrationPage.FIELD_LAST_NAME, Messages.MISSING_LAST_NAME); Optional<String> email = validateAndGet(formData, errors, RegistrationPage.FIELD_EMAIL, Messages.MISSING_EMAIL); Optional<String> phone = validateAndGet(formData, errors, Constants.MOBILE_NUMBER_ATTRIBUTE, "missing_phone_number"); if (errors.size() > 0) { fail(context, formData, eventError, errors); return; } else { ExternalSystemClient client = new ExternalSystemClient(externalSystemURI); Optional<InternalUser> internalUser = ex.find( firstName.orElse(null), lastName.orElse(null), email.orElse(null), phone.orElse(null) )); if (!internalUser.isPresent()) { // final check if all above data is valid errors.add(new FormMessage(“generic", "unknown_member")); fail(context, formData, eventError, errors); return; } context.success(); } } } Cheers, Łukasz — Twitter: ldywicki Blog: http://dywicki.pl Code-House - http://code-house.org