You would often hear the terms cross-site scripting and JavaScript used simultaneously. JavaScript is a client-side scripting language introduced by Netscape in 1995. The main purpose of JavaScript was to make the web browser perform some tasks at the client side. Although JavaScript can be used for other purposes too, it is most commonly used in web browsers to implement client-side scripts that can be used to alter the web page displayed on the browser, for example, displaying a popup error message dialog box when a wrong value is entered by the user or showing ads on the web page.

Some hackers soon found out that using JavaScript, they could read data from web pages loaded in adjacent windows or frames. Thus, a malicious website could cross the boundary and interact with contents loaded on an entirely different web page that is not related to its domain. This trick was named as cross-site scripting attack. To block this attack, Netscape introduced the...