Wicked Cool PHP

#20: Preventing Basic XSS Attacks

XSS stands for cross-site scripting. Unlike most attacks, this exploit works on the client side. The most basic form of XSS is to put some JavaScript in user-submitted content to steal the data in a user's cookie. Since most sites use cookies and sessions to identify visitors, the stolen data can then be used to impersonate that user—which is deeply troublesome when it's a typical user account, and downright disastrous if it's the administrative account. If you don't use cookies or session IDs on your site, your users aren't vulnerable, but you should still be aware of how this attack works.

Unlike MySQL injection attacks, XSS attacks are difficult to prevent. Yahoo!, eBay, Apple, and Microsoft have all been ...