How implement a single login system for drupal and another CMS

Question

I am trying to implement a login share between a Drupal site and another CMS - kayako in particular. Are you aware of any guide or reference? Thanks.

Answer 1

Drupal 6 supports OpenID out of the box.

If kayako provides an API for user authentication, you can also implements an external authentication module for Drupal. This module will have to implements hook_form_alter() to edit the user_login and user_login_block forms and replace user_login_authenticate_validate() in their $form['#validate'] by the module's own validation callback that should authenticate the user.