New user added (possibly by hacking)

Jan 26 at 3:09 PM
Hi, I am using BlogEngine for, just today I realized that another user login was created and that user created a spam blog inside it.

Currently I have delete that but if you search "lokinshah" on bing the page comes 4th.

What are the ways he might have done this? How can I prevent it?
Jan 26 at 3:12 PM
Jan 26 at 3:13 PM
Jan 26 at 3:14 PM
Thanks, I will upgrade it ASAP..