In this attack an attacker injects malicious piece of code into an otherwise benign site. There are two types of XSS attack:

• Reflected XSS

• Stored XSS

The first is used when the attacker exploits a vulnerable Web Application that display back to the browser input parameters. Usually this happens publishing URLs that contains the attack string. Next is shown an example[8]:

When someone clicks on this URL, he is forwarded to the trusted.com page, but, because of the vulnerability of this web application, the code contained in the URL will be executed. Let's read this code:

This simple script makes a request to the attacker's web application, sending the user cookie as information. This way, the attacker collects personal cookies that could contain reserved informations.

Stored XSS uses the same strategy, but, instead of URLs, it uses the possibility to save content that will be shown in future by the web application, example of these applications are a wiki or a forum.

SOLUTION: XSS is an attack that can be easy avoided if the user input is sanitized. However this is not so simple to take care of all different sources of threat, and so, this is not so simple to known how to sanitize input. An interesting XSS cheat list is in [14].