User authentication and authorization is provided by the auth module.
The auth module is included with Kohana, but needs to be enabled before you can use it. To enable, open your application/bootstrap.php file and modify the call to Kohana::modules by including the auth module like so:
Kohana::modules(array(
...
'auth' => MODPATH.'auth',
...
));
Next, you will then need to configure the auth module.
The auth module provides the Auth::File driver for you. There is also an auth driver included with the ORM module.
As your application needs change you may need to find another driver or develop your own.