I wanted to integrate Facebook's authentication with my WordPress site's user login system. This had led me stuck for a couple of days because of all the authentication routes that I've known in WordPress would require a correct password to complete authentication. But, in my case, I just needed to authenticate a user without actually user providing a password. That means, I needed a method by which I could authenticate users using just their username or user ID. So after a lot of hacking and reading, I finally found this method that works out for me. It basically deals with using filters, precisely authenticate filter. The functions added to this filter are executed in wp_authenticate function which is defined in pluggable.php and is the authentication end point in WordPress. If you watch the code in user.php, there you would find that two functions added to this authenticate filter, which are wp_authenticate_username_password with priority 20 and wp_authenticate_cookie with priority 30. wp_authenticate_cookie is not that we want to care about. It's wp_authenticate_username_password that we need to deal with. So when you want to bypass this username and password combined authentication you basically need to bypass the execution of this function and return the user before it. So we'll hook our own method to this filter with a priority less than 20 by the following line of code.

add_filter('authenticate','forceLogin',0,3);

Now suppose you want to define an authentication that doesn't really need a password, just write a function forceLogin() like the following.

function forceLogin($user,$username,$password)
{
  if(username_exists($username))
  {
    $user=get_user_by('login',$username);
    remove_action('authenticate', 'wp_authenticate_username_password', 20);
    return $user;
  }
}

Thats it, now when you call the wp_signon() with just a username in the credentials, the user with the username in the credentials will be authenticated without actually requiring the password. Thats how its done. One can integrate OAuth and construct customized logins for their WordPress websites.