This content originally appeared on DEV Community and was authored by NoobDev
Introduction
If you've ever built a web application with a separate frontend and backend, chances are you've encountered the infamous CORS error. It's one of those issues that seem simple at first but can turn into a nightmare if not configured correctly.
Recently, while working on a personal project with FastAPI (backend) and React (frontend), I ran into a CORS issue. Everything worked fine individually in production—frontend on Vercel, backend, Redis, and PostgreSQL on Render. But when my frontend made a simple request to the backend, I was greeted with the dreaded message:
Access to fetch at 'http://backend-url' from origin 'http://frontend-url' has been blocked by CORS policy.
At first, I thought, "Okay, I just need to tweak my CORS configuration and I'll be good to go." Turns out, the real issue was a missing https:// in my environment variable.
✅ STEP 1: My CORS configuration looked Fine
I had already configured CORS middleware in FastAPI
from fastapi.middleware.cors import CORSMiddleware
app.add_middleware(
CORSMiddleware,
allow_origins=["https://frontend-url.com"],
allow_credentials=True,
allow_methods=["*"],
allow_headers=["*"],
)
Everything seemed fine, but the CORS error persisted.
🔍 STEP 2: Debugging Everything But Finding Nothing 🤯
I started checking different things:
✅ Ensured the frontend was sending the correct Origin header.
✅ Confirmed that preflight (OPTIONS) requests were handled properly.
✅ Checked if WebSockets were being blocked due to CORS.
✅ Looked at browser network logs and backend logs.
Yet, nothing changed. The same error haunted me.
🤦♂️ Step 3: The Silly Mistake That Wasted Hours 🤦♂️
After hours of debugging, I finally noticed the issue:
👉 I had added frontend-url.com in my environment variable on Render WITHOUT the https:// protocol.
This is how I set up the env variable:
ALLOWED_ORIGIN=frontend-url.com # Missing https://
But my fronted was making a request from:
Origin: https://frontend-url.com
Since CORS policies are strict about exact matches, my backend wasn't recognizing the request's Origin header, causing it to be blocked.
🚀 Step 4: The Simple Fix
Once I included https:// protocol in the env variable, everything worked instantly:
ALLOWED_ORIGINS=https://frontend-url.com
and after restarting my backend server, the CORS issue was completely resolved! 🎉
Key Takeaways 🚀
-
CORS is case-sensitive and scheme sensitive (
http://≠https://). - Always check how your environment variables are stored and ensure that they match the frontend exactly.
- Debugging CORS? Start simple. Check your headers, preflight requests, and environment configuration first.
- Sometimes, the hardest bugs are caused by the smallest mistakes.
Ever lost hours over a tiny mistake? Share your worst debugging nightmares below! 👇
This content originally appeared on DEV Community and was authored by NoobDev
NoobDev | Sciencx (2025-01-31T19:37:56+00:00) CORS Nightmares: The One Missing Protocol That Broke Everything 😅. Retrieved from https://www.scien.cx/2025/01/31/cors-nightmares-the-one-missing-protocol-that-broke-everything-%f0%9f%98%85/
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