Debugging CORS Errors

Step 1 — Read the Error Message

Open DevTools → Console. CORS errors look like:

Access to fetch at 'https://api.example.com/data' from origin 'https://app.example.com'
has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present
on the requested resource.

The key parts:

• Blocked origin — your frontend URL (app.example.com)
• Target URL — the API you're calling (api.example.com)
• Reason — what's missing or wrong

Step 2 — Identify the Request Type

CORS behaves differently for simple vs preflighted requests.

Is it a preflight? Check the Network tab. If you see an OPTIONS request before your actual request, it's preflighted.

Step 3 — Check the Server Response

In DevTools → Network → click the failing request → Response Headers. Look for:

Access-Control-Allow-Origin: https://app.example.com

• Missing entirely → Server is not configured for CORS. Add the header.
• Wrong origin → Server is returning a hardcoded origin that doesn't match. Fix the allowlist.
• Wildcard with credentials → Access-Control-Allow-Origin: * cannot be used with credentials: 'include'. Use a specific origin.

Step 4 — Fix by Framework

Express.js

const cors = require('cors')

// Allow a single origin
app.use(cors({ origin: 'https://app.example.com' }))

// Allow multiple origins dynamically
const ALLOWED = new Set(['https://app.example.com', 'https://admin.example.com'])
app.use(cors({
  origin(origin, cb) {
    if (!origin || ALLOWED.has(origin)) return cb(null, true)
    cb(new Error('Not allowed by CORS'))
  },
  credentials: true
}))
    
Next.js App Router
    
// app/api/data/route.ts
export async function GET(request: Request) {
  return Response.json({ data: [] }, {
    headers: {
      'Access-Control-Allow-Origin': 'https://app.example.com',
      'Access-Control-Allow-Credentials': 'true'
    }
  })
}

export async function OPTIONS() {
  return new Response(null, {
    status: 204,
    headers: {
      'Access-Control-Allow-Origin': 'https://app.example.com',
      'Access-Control-Allow-Methods': 'GET, POST, PUT, DELETE',
      'Access-Control-Allow-Headers': 'Content-Type, Authorization',
      'Access-Control-Max-Age': '86400'
    }
  })
}
    
nginx
    
location /api/ {
    add_header 'Access-Control-Allow-Origin' 'https://app.example.com' always;
    add_header 'Access-Control-Allow-Credentials' 'true' always;

    if ($request_method = 'OPTIONS') {
        add_header 'Access-Control-Allow-Methods' 'GET, POST, PUT, DELETE';
        add_header 'Access-Control-Allow-Headers' 'Content-Type, Authorization';
        add_header 'Access-Control-Max-Age' 86400;
        return 204;
    }
}

    
Step 5 — Common Mistakes
    
Mistake: Setting CORS headers on the wrong server. If your frontend is behind a reverse proxy (nginx, Cloudflare), the CORS headers must come from the API server, not the proxy — unless the proxy is the one handling the cross-origin request.
    
Mistake: Using * with credentials.
    
// ❌ This will fail when credentials: 'include' is set
res.set('Access-Control-Allow-Origin', '*')
res.set('Access-Control-Allow-Credentials', 'true')

// ✅ Use specific origin
res.set('Access-Control-Allow-Origin', req.headers.origin)
res.set('Vary', 'Origin')
res.set('Access-Control-Allow-Credentials', 'true')
    
Mistake: Forgetting the Vary header. When you dynamically set Access-Control-Allow-Origin based on the request origin, add Vary: Origin to prevent CDNs from caching the wrong origin's response.
    
Mistake: CORS on the client side. CORS is enforced by the browser. You cannot bypass it from JavaScript. The fix must be on the server. If you control neither server, use a server-side proxy.

    
Checklist
    

      
Server sends Access-Control-Allow-Origin matching your frontend origin
      
If using credentials: specific origin (not *) + Access-Control-Allow-Credentials: true
      
Preflight OPTIONS returns Access-Control-Allow-Methods and Access-Control-Allow-Headers
      
Vary: Origin is set when origin is dynamic
      
CORS headers are on the API server, not just the proxy