Frontend Security Checklist (2025): 10 Critical Vulnerabilities & How to Fix Them

1. Cross-Site Scripting (XSS)

What it is:
XSS occurs when attackers inject malicious scripts into your site through unescaped user input, compromising user data or hijacking sessions.

How to fix it:

• Sanitize all user inputs using libraries like DOMPurify.

• Avoid using innerHTML. Use textContent or safe rendering frameworks like React or Vue.

• Implement Content Security Policy (CSP) headers to block unauthorized scripts.

2. Clickjacking

What it is:
Clickjacking tricks users into clicking hidden elements by overlaying invisible frames over trusted UI components.

How to fix it:

• Add X-Frame-Options: DENY or SAMEORIGIN to your HTTP headers.

• Use CSP’s frame-ancestors directive to control who can embed your site in iframes.

3. Insecure Local Storage

What it is:
Storing sensitive data (like JWTs) in localStorage or sessionStorage makes it vulnerable to XSS attacks.

How to fix it:

• Use HttpOnly cookies to store authentication tokens securely.

• If using localStorage, make sure your app is completely protected from XSS.

4. Exposed API Keys or Secrets

What it is:
Frontend code is visible to users, and hardcoding API keys exposes them to theft through browser dev tools or source inspection.

How to fix it:

• Never hardcode secrets in frontend files.

• Store them on the server side and proxy sensitive requests through a backend route.

5. Open Redirects

What it is:
Open redirects let attackers redirect users to malicious websites by manipulating unvalidated URL parameters.

How to fix it:

• Whitelist allowed redirect destinations.

• Use relative routes or IDs instead of full external URLs in query strings.

6. Weak Input Validation

What it is:
Inadequate input validation opens the door to XSS, form manipulation, and injection attacks.

How to fix it:

• Validate inputs both on the client and server.

• Use form libraries like Yup, Zod, or native HTML validation attributes.

7. CORS Misconfigurations

What it is:
Improper CORS (Cross-Origin Resource Sharing) settings can expose APIs to unauthorized access.

How to fix it:

• Always define specific trusted origins in Access-Control-Allow-Origin.

• Avoid using * unless the API is completely public and doesn’t require auth.

8. Sensitive Data in URLs

What it is:
Putting emails, tokens, or passwords in query strings makes them visible in browser history, logs, and referrer headers.

How to fix it:

• Use POST requests instead of GET for sensitive data.

• Store sensitive info in secure cookies or headers—not in the URL.

9. DOM-Based Vulnerabilities

What it is:
DOM-based vulnerabilities arise when JavaScript directly manipulates the DOM using untrusted input—without server involvement.

How to fix it:

• Don’t trust values from window.location, document.URL, or URL fragments.

• Avoid using document.write() and always sanitize dynamic content.

10. Missing Security Headers

What it is:
Lack of proper HTTP security headers leaves your app exposed to several attacks like XSS, clickjacking, and MIME sniffing.

How to fix it:

• Set a strict Content-Security-Policy (CSP).

• Use X-Content-Type-Options: nosniff to block MIME sniffing.

• Enforce HTTPS with Strict-Transport-Security (HSTS).

• Control referrer data using Referrer-Policy.