Cross-site scripting (XSS) is a type of security vulnerability that can be found in some web applications. It occurs when an attacker injects malicious scripts into web pages viewed by other users. The attacker can use a web application to send malicious code, generally in the form of a browser-side script, to a different end-user. Flaws that allow these attacks to succeed are quite widespread and occur anywhere a web application uses input from a user within the output it generates without validating or encoding it.

There are several types of XSS attacks, including stored/persistent XSS, reflected/non-persistent XSS, and DOM-based XSS. Reflected XSS is the most common type of cross-site scripting vulnerability, where the attacker must deliver the payload to the victim. The attacker uses phishing and other social engineering methods to lure victims to inadvertently make a request to the web server that includes the XSS payload script.

To prevent cross-site scripting, it is best to eliminate application security flaws that enable it. Some best practices include validating and encoding user input, filtering user input, and using security tools such as web application firewalls. It is also important to perform manual penetration testing or use a vulnerability scanner to discover cross-site scripting vulnerabilities.

In summary, cross-site scripting is a type of security vulnerability that allows attackers to inject malicious scripts into web pages viewed by other users. It can be prevented by eliminating application security flaws and using security tools such as web application firewalls.