What is Cross-Site Scripting (XSS) and how can it be prevented in a web application?

Cross-Site Scripting (XSS) is a type of security vulnerability typically found in web applications. It occurs when an attacker is able to inject malicious scripts into web pages viewed by other users. These scripts can then execute in the browsers of other users, potentially causing harm such as stealing sensitive information or session hijacking.

Prevention of Cross-Site Scripting (XSS) in Web Applications:

1. Input Validation: Ensure that all user inputs are validated on the server-side before being processed or stored.
2. Output Encoding: Encode user-supplied data when displaying it on a web page to prevent script execution.
3. Content Security Policy (CSP): Implement CSP to restrict the sources from which certain types of content can be loaded on your web page.
4. HTTPOnly and Secure Flags for Cookies: Set these flags on cookies to prevent access by client-side scripts in the case of a successful XSS attack.
5. Sanitization Libraries: Utilize libraries that can help automatically sanitize user inputs to prevent XSS attacks.