Introduction to Web Security
http://www.extropia.com/tutorials/security/index.html

Extropia

Abstract:

Computer security is a give and take situation. You can never be safe so long as you offer services. However, without offering services you may as well not have the computer in the first place. Thus, security becomes more about acceptable risk and emergency recovery than impregnability. It is your job to make sure that the cons of a break have far less impact than the pros of having a web site.