CORS (Cross-Origin Resource Sharing) is a critical security mechanism that allows or restricts cross-origin HTTP requests. Here's a concise breakdown:

🧠 What is CORS?

CORS enables browsers to relax the Same-Origin Policy by specifying which origins are permitted to access resources. It's essential for APIs to communicate with frontend applications across domains.

📝 Key Concepts

  • Simple Requests: GET/POST with headers like Content-Type: application/json (no preflight)
  • Preflight Requests: OPTIONS method checks CORS headers before actual requests
  • CORS Headers:
    • Access-Control-Allow-Origin (specifies allowed domains)
    • Access-Control-Allow-Methods (defines permitted HTTP methods)
    • Access-Control-Allow-Headers (lists allowed headers)

📚 Example Scenarios

Scenario CORS Behavior
Same origin No issues
Different origin Requires proper headers
With credentials Access-Control-Allow-Credentials: true needed

⚠️ Common Issues

  • 403 Forbidden: Missing Access-Control-Allow-Origin header
  • Pre-flight Failure: Mismatched methods/headers
  • Caching Problems: Use Access-Control-Max-Age for preflight caching

For deeper implementation details, check our CORS Implementation Guide.

CORS_Understanding