How can you remediate cross-site scripting vulnerabilities in a web app?

1 Identify XSS vulnerabilities

The first step to remediate XSS vulnerabilities is to identify where they exist in your web app. You can use various tools and techniques to scan your web app for XSS vulnerabilities, such as static code analysis, dynamic testing, fuzzing, or manual inspection. You should look for any user input that is reflected or stored in the web page without proper validation or encoding, such as in forms, URLs, cookies, headers, or scripts. You should also check for any third-party components or libraries that may introduce XSS vulnerabilities.

2 Validate user input

The second step to remediate XSS vulnerabilities is to validate user input before processing it on the server or the client. You should implement strict and consistent input validation rules that only accept expected and legitimate data, and reject or discard any unexpected or malicious data. You should use a whitelist approach that defines what is allowed, rather than a blacklist approach that defines what is not allowed. You should also avoid relying on client-side validation alone, as it can be easily bypassed or manipulated by attackers.

3 Sanitize user output

The third step to remediate XSS vulnerabilities is to sanitize user output before rendering it on the web page. You should remove or encode any potentially dangerous characters or tags that may trigger XSS attacks, such as <, >, ", ', &, or /. You should use a context-aware encoding method that matches the output format, such as HTML, JavaScript, CSS, or URL. You should also use a trusted and well-tested sanitization library that covers all possible XSS scenarios, rather than writing your own custom code.

4 Escape user data

The fourth step to remediate XSS vulnerabilities is to escape user data before inserting it into the web page. You should use a safe and standard method to insert user data into the web page, such as document.createElement or textContent in JavaScript, rather than using innerHTML or eval. You should also avoid using user data as part of dynamic code execution, such as in script src, style, or event handlers. You should also use a secure and modern framework that automatically escapes user data, such as React or Angular.

By following these four steps, you can remediate XSS vulnerabilities in your web app and protect your app and your users from XSS attacks. However, you should also keep in mind that XSS remediation is an ongoing process that requires regular testing, monitoring, and updating of your web app and its components. You should also follow the best practices and guidelines for web security, such as using HTTPS, CSP, SRI, and HSTS.

5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?