As the popularity of your WordPress blog increases, it gets the attention of more and more hackers and malicious persons who would like to sabotage your blogging efforts. In order to avoid or deter such attacks you need to make your WordPress installation stronger, and you should also take regular backups of your WordPress database.
The most common mistake that most WordPress blog publishers commit is that they never change the default “admin” login ID WordPress creates after the blog is installed. This makes it very easy for the hackers. Another mistake is, publicly displaying the version number of your WordPress installation. Ideally you should have the latest version of WordPress installed but if you don’t, don’t let the wannabe hackers know your version easily, because there are many version-specific vulnerabilities that can be easily exploited. So upgrade your installation to the latest version now, as the newer versions are created to take care of vulnerabilities.
Read more about how to make your WordPress installation more secure in this blog post.