What is CORS?
Cross-Origin Resource Sharing (CORS) is a browser security mechanism that controls which domains can make requests to your API, preventing unauthorized cross-origin access.
CORS Explained
CORS (Cross-Origin Resource Sharing) is a security mechanism built into web browsers that restricts web pages from making requests to a different domain than the one that served the page. When your React frontend at app.example.com needs to call your API at api.example.com, CORS headers tell the browser whether to allow this cross-origin request. Without proper CORS configuration, browsers will block these requests for security reasons. CORS is one of the most common issues developers encounter when building web applications with separate frontend and backend services. At M3L Software, we configure CORS correctly from the start, using FastAPI's built-in CORSMiddleware to specify allowed origins, methods, and headers.
Key Features
Common Use Cases
Frequently Asked Questions
Why do I get CORS errors?
CORS errors occur when your frontend tries to call an API on a different origin (domain, port, or protocol) and the API doesn't include the proper CORS headers. Fix it by configuring your API to include Access-Control-Allow-Origin headers.
Should I allow all origins (*)?
Only in development. In production, whitelist specific origins (your frontend domain). Allowing all origins with credentials is a security risk.
Does CORS protect my API?
CORS only protects browser-based requests. API calls from servers, mobile apps, or tools like Postman bypass CORS entirely. Always implement proper authentication and rate limiting regardless of CORS.
Related Terms
API
An Application Programming Interface (API) is a set of rules and protocols that allows different software applications to communicate with each other, enabling data exchange and functionality sharing.
Read moreREST API
A REST API (Representational State Transfer) is an architectural style for building web APIs that uses standard HTTP methods (GET, POST, PUT, DELETE) to perform operations on resources identified by URLs.
Read moreFastAPI
FastAPI is a modern, high-performance Python web framework for building APIs, known for its speed, automatic documentation, and type-hint-based validation using Pydantic.
Read moreSSL/TLS
SSL/TLS (Secure Sockets Layer/Transport Layer Security) is the cryptographic protocol that encrypts data transmitted between web browsers and servers, indicated by HTTPS and the padlock icon.
Read moreNeed Help with CORS?
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