On PHPBuilder.com, there's a new tutorial about validating user sessions - ensuring that data in your user's sessions is valid and isn't an attack trying to sneak in.
In a nutshell, sessions are the way that we "maintain state" from one page to the next, that is, how we identify specific users across multiple page requests. The ability to track users as they go from one page to the next using sessions allows us a number of options, such as tracking where they are going (web statistics) or to verify credentials for a specific section of the site.
First, there's a little mini-intro to sessions for those unsure on their use. It's followed by a look at some session vulnerabilities that could be introduces by malicious users looking to break things on your site (including HTML/Javascript injection, as their example shows).
They recommend a three step plan to get started with the validation of your user's sessions - making it easy for users to log out when they want to, use unique aspects of the remote machine to define the session and to validate all user input, especially things that will be put into a session variable.