Over 100,000 compromised servers and personal computers are being used to brute force Wordpress login pages. A brute force attack tries to keep entering passwords till the correct password is found out and the installation is compromised. Wordpress by default has little security against a brute force attack.
Attacks at the rate of 30 per second per installation, causing high server loads
Protect your Wordpress Blog by Changing the Passwords and Limit Login Attempts by installing http://wordpress.org/extend/plugins/limit-login-attempts/.
http://blog.hostgator.com/2013/04/11/global-wordpress-brute-force-flood/
http://blog.cloudflare.com/patching-the-internet-fixing-the-wordpress-br
http://blog.sucuri.net/2013/04/protecting-against-wordpress-brute-force-attacks.html