XSS (Cross-Site Scripting) is a web security vulnerability that allows attackers to inject malicious scripts into content delivered to users. These scripts are executed in the context of the victim’s browser and can be used to steal cookies, session tokens, or even redirect the user to malicious sites.

There are three primary types of XSS:

• Stored XSS: The malicious script is permanently stored on the target server (e.g., in a database) and executed when a user views the affected page.
• Reflected XSS: The script is reflected off a web server, e.g., via a search result or error message, and sent to the victim via URL.
• DOM-based XSS: The vulnerability is in the client-side code and triggered by modifying the DOM environment in the browser.

To prevent XSS attacks, always sanitize and validate user input. Use libraries like DOMPurify for client-side protection and ensure your server escapes special characters properly. Consider implementing a Content Security Policy (CSP) to reduce the impact of XSS by restricting script sources.

Example of vulnerable code:

<input type="text" value="<script>alert('XSS')</script>" />

Secure version:

<input type="text" value="<!-- user input escaped properly -->" />

By understanding and mitigating XSS, developers can build safer applications and protect users from common web attacks.