Cross-Site Scripting (XSS) is a cyberattack method where malicious actors inject harmful scripts into trusted websites. When users visit these compromised sites, the malicious code runs in their browsers without their knowledge. This can allow attackers to steal sensitive data such as login credentials, session cookies, or personal information. XSS attacks exploit vulnerabilities in web applications that fail to properly validate or sanitize user input. Protecting against XSS involves secure coding practices, input validation, and using security features like Content Security Policy (CSP).