XSS, or Cross-Site Scripting, is a type of security vulnerability commonly found in web applications. It occurs when an attacker is able to inject malicious scripts into content that is then delivered to other users. These scripts are typically executed in the context of the victim s browser, allowing attackers to steal sensitive information, hijack user sessions, or manipulate website content. Preventing XSS involves properly validating and encoding user input to ensure that untrusted data cannot be executed as code within web pages.