The problem
When you fail to log into a WordPress blog, the CMS displays some info telling you what went wrong. This is good if you’ve forgotten your password, but it might also be good for people who want to hack your blog. So, why not prevent WordPress from displaying error messages on failed log-ins?

The solution
To remove log-in error messages, simply open your theme’s functions.php file, and paste the following code:

add_filter('login_errors',create_function('$a', "return null;"));

Save the file, and see for yourself: no more messages are displayed if you fail to log in.

Please note that there are several functions.php files. Be sure to change the one in your wp-content directory.

Code explanation
With this code, we’ve added a simple hook to overwrite the login_errors() function. Because the custom function that we created returns only null, the message displayed will be a blank string.