Think you've protected your web applications from cross-site scripting (XSS) vulnerabilities? The odds are against you. Roughly 90 per cent of web applications have this problem, and it's getting worse as web applications and web services share more and more data.
Many frameworks and libraries are encoding, decoding, and re-encoding with all kinds of schemes and sending data through new protocols. Ajax and other "rich" applications are complicating this situation.
XSS happens any time your application uses input from an HTTP request directly in HTML output. This covers everything in the HTTP request, including the query string, form fields, hidden fields, pull-down menus, check boxes, cookies, and headers. And, it doesn't matter if you immediately send the input back to the user who sent it - you get something called "Reflected XSS" - or store it for a while and send it later to someone else - that's "Stored XSS".
Fortunately, there are steps application developers can take to protect applications:
Validate input: One way to keep code out of user input is "whitelist input validation." All you have to do is verify that each input matches a strict definition of what you expect, like a tight regular expression. Attempting to take a shortcut and apply a global filter for attacks is known as a "blacklist" approach and never works. Unfortunately, even the tightest input validation can't completely defeat XSS, as some input fields require the same characters that are significant in HTML - a you can see here:
?name=O'Connell&comment=I "like" your website; it's great!