Cross-Site Scripting (XSS) is a critical security vulnerability prevalent in web applications. It occurs when an application includes untrusted data in a web page without proper validation or escaping, allowing an attacker to inject and execute malicious scripts—typically JavaScript—in the victim's web browser. Because the browser trusts the script as if it originated from the legitimate website, the script can access sensitive information stored in the browser, such as session cookies, tokens, or personal data.

There are three primary types of XSS:

Stored (Persistent) XSS: The malicious script is permanently stored on the target server (e.g., in a database, in a comment field). When a victim views the page, the script executes.

Reflected XSS: The script is "reflected" off a web application to the victim's browser, usually through a link containing the payload (e.g., in a URL parameter).

DOM-based XSS: The vulnerability exists in the client-side code rather than the server-side code, where the script is executed by modifying the Document Object Model (DOM) environment.

Managing the threat of XSS involves implementing strict input validation and output encoding. Developers must ensure that any data provided by users is treated as "untrusted" and filtered to remove executable code before it is rendered on a page. From an ethical hacking perspective, identifying XSS is a key part of web application penetration testing. A successful XSS attack can lead to account hijacking, website defacement, or the redirection of users to malicious websites. By understanding how malicious scripts are executed in the context of other users' browsers, security professionals can better protect the integrity of web services and the privacy of their users.