WordPress at a glance
action is not described

wp_authenticate action-hook . WP 1.5.1

Fires before the user is authenticated.

The variables passed to the callbacks are passed by reference, and can be modified by callback functions.

Usage

add_action( 'wp_authenticate', 'action_function_name_66', 10, 2 );
function action_function_name_66( $user_login, $user_password ){
	// action...
}
$user_login(string)
Username (passed by reference).
$user_password(string)
User password (passed by reference).

Where the hook is called

wp_signon()
wp-includes/user.php 63
do_action_ref_array( 'wp_authenticate', array( &$credentials['user_login'], &$credentials['user_password'] ) );

Where the hook is used (in WP core)

Does not used.