Cross-site Scripting
Cross-site Scripting (XSS) is an attack technique that forces a web site to echo attacker-supplied executable code, which loads in a user's browser. The code itself is usually written in HTML/JavaScript, but may also extend to VBScript, ActiveX, Java, Flash, or any other browser-supported technology.

When an attacker gets a user's browser to execute his code, the code will run within the security context (or zone) of the hosting web site. With this level of privilege, the code has the ability to read, modify and transmit any sensitive data accessible by the browser. A Cross-site Scripted user could have his account hijacked (cookie theft), their browser redirected to another location, or possibly shown fraudulent content delivered by the web site they are visiting. Cross- site Scripting attacks essentially compromise the trust relationship between a user and the web site.

There are two types of Cross-site Scripting attacks, non-persistent and persistent. Non-persistent attacks require a user to visit a specially crafted link laced with malicious code. Upon visiting the link, the code embedded in the URL will be echoed and executed within the user's web browser. Persistent attacks occur when the malicious code is submitted to a web site where it's stored for a period of time. Examples of an attacker's favorite targets often include message board posts, web mail messages, and web chat software. The unsuspecting user is not required to click on any link, just simply view the web page containing the code.


Example

Persistent Attack
Many web sites host bulletin boards where registered users may post messages. A registered user is commonly tracked using a session ID cookie authorizing them to post. If an attacker were to post a message containing a specially crafted JavaScript, a user reading this message could have their cookies and their account compromised.