While browsing the I came across an article that deals with web security.
As it is commonly rejected and left aside by most webdevelopers because it takes too much time and there are more important issues... Web Security IS a big deal and needs to be taken into account to prevent your site from becoming a toy for hackers or people who just like to have fun at the cost of others.
I came across a very interesting article on the web that adresses these issues, and what to do about them. It was written by Dr. Johannes Ullrich from the SANS Technology Institute.
You can find the article here: http://www.sans.edu/resources/securitylab/audit_web_apps.php