Where only users in Active Directory can login to the Craft Control Panel.
There's a feature request here that would allow custom login forms for the Craft Control Panel login page you might want to add a comment to.
In the meantime, there is a
userSession.onBeforeAuthenticate event a plugin can listen to to perform any 3rd party authentication.
You can see how a similar plugin did this for front-end LDAP logins here: https://github.com/thepitagroup/adldap_craftcms