Remove Error Message on the Login Page
Whenever your users enter an incorrect login name or password, an error message is displayed on the login page alerting them to an incorrect piece of information. If a hacker were to correctly guess one of those pieces of information, the error message would help them identify what he/she got right.
Block hackers by adding the following filter:
add_filter('login_errors',create_function('$a', "return null;"));
This hack isn’t recommended for sites with multiple authors.