Developing an XSS Prevention Strategy

Cross-Site Scripting (XSS) is a security vulnerability that allows attackers to inject malicious scripts into web applications. This lesson aims to develop a comprehensive strategy to prevent XSS vulnerabilities in front-end development.

XSS vulnerabilities occur when an application includes untrusted data in a web page without proper validation or escaping. This can lead to a range of security issues, including data theft and session hijacking.

3.1 Stored XSS

Malicious scripts are stored on the server and retrieved by users later.

3.2 Reflected XSS

Malicious scripts are reflected off a web server and executed immediately.

3.3 DOM-based XSS

Scripts are executed as a result of modifying the DOM in the browser.

4.1 Input Validation

Always validate input on both the client and server sides. Use a whitelist approach for input validation.

// Example: Input validation using regex
const isValidInput = (input) => /^[a-zA-Z0-9]+$/.test(input);
            

4.2 Output Encoding

Encode data before rendering it in the browser to prevent script execution.

// Example: Encoding output in JavaScript
const encodeHTML = (str) => str.replace(/&/g, "&").replace(//g, ">");
            

4.3 Content Security Policy (CSP)

Implement a strong CSP to restrict the sources from which scripts can be loaded.

Content-Security-Policy: default-src 'self';
            

4.4 HTTPOnly and Secure Cookies

Set cookies with the HttpOnly and Secure flags to prevent access via JavaScript.

• Use HTTPS to protect data in transit.
• Regularly update libraries and dependencies.
• Conduct security audits and penetration testing.
• Educate developers about secure coding practices.