CORS Explained: Why Your API Requests Get Blocked (And How to Fix It)
Raise your hand if you've ever seen this: "Blocked by CORS policy." That red error that ruins your day. You copy-paste some Stack Overflow fix, it works, and you move on.
But what actually happened? Why does the browser block perfectly valid requests? And what other security headers are you missing?
In this video, I'll demystify CORS and show you the security headers every developer needs to know.
📖 *What You'll Learn:*
00:00 - The CORS Error Everyone Hates
00:45 - Same-Origin Policy Explained (The Foundation)
02:00 - How CORS Actually Works (Headers Deep Dive)
03:30 - Simple Requests vs Preflight (OPTIONS)
05:00 - Live Debug: CORS Errors in Network Tab
06:30 - Common CORS Mistakes (With Fixes)
08:00 - Essential Security Headers You're Missing
09:30 - CSP, HSTS, X-Frame-Options Explained
11:00 - Code: Proper CORS + Security Headers Setup
12:30 - Security Checklist for Your APIs
13:30 - Key Takeaways & Next Series
🔧 *Code Examples:*
Express.js CORS configuration (correct way)
Handling preflight OPTIONS requests
Security headers middleware
Nginx CORS configuration
Debugging CORS in DevTools
🌐 *Key Concepts:*
*Same-Origin Policy:*
Origin = protocol + domain + port
https://example.com vs https://api.example.com = DIFFERENT
Prevents malicious sites from accessing your data
*CORS Headers:*
Access-Control-Allow-Origin: Who's allowed
Access-Control-Allow-Methods: Which HTTP methods
Access-Control-Allow-Headers: Which custom headers
Access-Control-Allow-Credentials: Allow cookies/auth
Access-Control-Max-Age: Cache preflight results
*Request Types:*
Simple: GET, POST (with simple content types)
Preflight: OPTIONS request first for PUT, DELETE, JSON, custom headers
🛡️ *Security Headers Cheat Sheet:*
```
Content-Security-Policy: default-src 'self'
X-Frame-Options: DENY
Strict-Transport-Security: max-age=31536000
X-Content-Type-Options: nosniff
Referrer-Policy: strict-origin-when-cross-origin
Permissions-Policy: geolocation=(), camera=()
```
✅ *CORS Best Practices:*
Specific origins, not wildcards in production
Handle OPTIONS requests properly
Use Vary: Origin for CDN caching
Cache preflight with reasonable Max-Age
Never use * with credentials
❌ *Common Mistakes:*
Forgetting preflight for PUT/DELETE
Missing Vary: Origin (caching issues)
Overly permissive CSP with 'unsafe-inline'
No HSTS (users vulnerable to SSL stripping)
Missing X-Frame-Options (clickjacking risk)
#cors #securityheaders #websecurity #webdevelopment #javascript #api #express #programming #csp #hsts
💬 *Discussion:* What's the most frustrating CORS error you've debugged? How long did it take to fix?
🔔 *Subscribe* for more "Under the Hood" content. Next series: Authentication & Security - JWT Deep Dive, OAuth 2.0, and more!
📚 *Resources:*
MDN CORS Docs: https://developer.mozilla.org/en-US/d...
CSP Evaluator: https://csp-evaluator.withgoogle.com
Security Headers Checker: https://securityheaders.com
CORS Express.js Middleware: https://github.com/expressjs/cors