Prevention measure against bot registering fake users

[thomaschiozzi-tndigit]

Currently, when in a realm the password + internal provider are set and enabled, the password registration form (when enabled) does not include any mitigation against bot attempting to register a multitude of users in an automated way.
While the newly created users are not confirmed and usable, they might do still occupy backend resources for some amount of time.

A possible solution against such a scenario could be to optionally include a captcha in the registration form, where the option to include the captcha or not is provided in the admin console for that provider.