It is very important that you take all measures to protect your site against attacks. A very important tool that will allow you to add more security to your Joomla website is Alarm System.
Alarm System is Joomla extension that adds an extra layer of security by asking users to enter a pin number after log in. Alarm System is a pretty handy extension, since the users that do not enter the pin will be logged out of the website.
The installation is pretty straightforward. You can set the how many seconds you want to allow the user to enter the pin, and also how many attempts are allowed. Not only that, this tool will log any failed attempt so that you can check who tried to log into your Joomla Site.
You can download Alarm System Extension here
For installation instructions, read this article