Building sophisticated websites is pretty easy these days. Whatever you want (blog, forum, eCommerce solution, picture gallery, video sharing site, or even your own social network) – there is a free third party script that you can use to build your site in a matter of hours.
However this ease comes at a price. Unfortunately, no software is perfect. Hackers have a great incentive to find vulnerabilities in popular scripts – if they find a security hole, they can exploit thousands (and sometimes even millions) of websites that use the buggy script. And the fact that most popular script are free Open Source software helps hackers immensely.