WordPress at a glance
action is not described

auth_cookie_malformed action-hook . WP 2.7.0

Fires if an authentication cookie is malformed.

Usage

add_action( 'auth_cookie_malformed', 'action_function_name_2598', 10, 2 );
function action_function_name_2598( $cookie, $scheme ){
	// action...
}
$cookie(string)
Malformed auth cookie.
$scheme(string)
Authentication scheme. Values include 'auth', 'secure_auth', or 'logged_in'.

Where the hook is called

wp_validate_auth_cookie()
auth_cookie_malformed
wp-includes/pluggable.php 613
do_action( 'auth_cookie_malformed', $cookie, $scheme );

Where the hook is used (in WP core)

wp-includes/default-filters.php 273
add_action( 'auth_cookie_malformed', 'rest_cookie_collect_status' );