A Cross-Site Scripting (XSS) vulnerability is a security flaw in a web application that allows attackers to inject malicious scripts into a trusted website. This occurs when the application does not properly validate or encode user input before rendering it in the generated web page.
The risks of an XSS attack include stealing sensitive information, hijacking user accounts, changing the content or functionality of the website, and more.
XSS vulnerabilities can be classified into various types, such as stored, reflected, and DOM-based XSS. They are included the OWASP Top 10 as critical types of injection vulnerabilities.