We just posted an article on the Magento Security Center that shares best practices for protecting stores from brute-force password guessing attacks. We’ve recently been made aware of a rise in these attacks, so we strongly recommend that you review the approaches outlined in this article with your developer and hosting provider immediately and implement the ones that are best suited to your unique situation. Learn more at https://magento.com/security/best-practices/protect-your-magento-installation-password-guessing 