By default if you type your username or password incorrectly when trying to login, Wordpress lets you know what mistake you’ve made. This can provide a hacker with confirmation that they know one half of the login equation.
Edit the functions.php file in your /wp-content/themes folder and add this code to prevent these errors from showing up:
<?php add_filter(’login_errors’,create_function(’$a’, “return null;”)); ?>
Note: This can be done automatically with the Login Lockdown plugin.
[...] 4. Hide Login Errors [...]