Cross-site scripting (XSS) is a type of cyber attack where malicious code is injected into websites to steal user data. It can be used to gain access to administrative accounts, redirect users to other pages or inject malicious content onto the website. XSS is most easily deployed by sending malicious code via links or web forms and can be prevented with proper validation and sanitization techniques. XSS attacks are extremely dangerous as they can allow attackers to control your website and access confidential information.