How to use LDAP for authentication in Craft CP (back end) 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