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Cross-site scripting (XSS) is a security vulnerability that allows attackers to inject malicious scripts into web pages that are then viewed by other users. This occurs when a web application does not properly validate or sanitize user input before displaying it in the browser. As a result, an attacker can craft a malicious payload that can be executed in the context of the victim’s browser, potentially leading to data theft, session hijacking, or the spread of malware.

For instance, if a user submits a comment form on a website and the application displays that comment without any filtering, an attacker could include a script that executes when other users view the comment. This could allow the attacker to steal cookies, redirect users to malicious sites, or perform other harmful actions without the user’s consent.

Understanding XSS is crucial for web developers and security professionals to implement appropriate defenses, such as input validation, output encoding, and content security policies. By recognizing how XSS operates, one can better protect both web applications and users from exploitation.