If you’re using SSH on your Raspberry Pi (or another computer running a Debian GNU/Linux distro), you may be vulnerable to brute force attacks. The fail2ban package can help prevent malicious behaviour. Here’s a handy guide I stumbled across recently, should you wish to set it up.

https://www.digitalocean.com/community/tutorials/how-to-protect-ssh-with-fail2ban-on-ubuntu-12-04