A very long paper on web application security threats has been released by honeynet.org. If you're curious about web application security this document is a good place to start for the overall picture.
"With the constant growth of the Internet, more and more web applications are being deployed. Web applications offer services such as bulletin boards, mail services such as SquirrelMail, online shops, or database administration tools like PhpMyAdmin. They significantly increase the exposed surface area by which a system can be exploited. By their nature, web applications are often widely accessible to the Internet as a whole meaning a very large number of potential attackers. All these factors have caused web applications to become a very attractive target for attackers and the emergence of new attacks. This KYE paper focuses on application threats against common web applications. After reviewing the fundamentals of a typical attack, we will go on to describe the trends we have observed and to describe the research methods that we currently use to observe and monitor these threats. In Appendix A, we give actual examples of a bot (a variant of PERL/Shellbot), the Lupper worm and an attack against a web Content Management System (CMS) as examples that show how web application threats actually act and propagate."
Article Link: http://www.honeynet.org/papers/webapp/index.html