CORS is a browser mechanism that allows servers to specify which domains can access their resources via HTTP requests. This policy is in place to prevent malicious websites from making unauthorized requests to other servers on behalf of a user.
By default, browsers block requests from origins (e.g., your app’s domain) that differ from the server’s origin unless the server explicitly allows it by sending an Access-Control-Allow-Origin header.
Why does the API endpoint work in a browser but not my app?
When you directly access the API endpoint via your browser, it’s a simple GET request made by the browser, not constrained by the same-origin policy since it doesn’t involve JavaScript. However, when your JavaScript application makes the request, the browser enforces the CORS policy, and the server does not allow your origin.
How to Bypass CORS
1. Use a proxy server that your app communicates with. The proxy server forwards the request to the API and adds the appropriate CORS headers to the response. You can find several CORS Anywhere services by searching the web.
2. Create an API wrapper on your server that uses a CURL request to retrieve the data. In your app, make requests to the API wrapper (e.g., https://yoursite.com/api.wrapper.php) instead of directly to the API.
Key Points to Remember
1. CORS is a Browser-Specific Security Feature
CORS enforcement happens only in browsers. It is designed to protect users by preventing malicious JavaScript from accessing resources from a different origin without proper authorization.
Server-side requests (like those made with cURL) are not subject to CORS because they are not executed in a browser environment. The server doesn’t care about the origin of the request—it simply processes it as it would any other HTTP request.
2. The Key Difference Between Client-Side and Server-Side Requests
Client-Side (JavaScript in Browser)
- When your app makes a request using fetch or XMLHttpRequest in the browser, the browser adds an Origin header to the request.
- The server checks the Origin header to decide whether it should allow the request. If the server doesn’t respond with a CORS policy (like Access-Control-Allow-Origin), the browser blocks the response.
Server-Side (with cURL)
When you make a request using cURL, there’s no browser involved. The request doesn’t include the Origin header unless you explicitly set it. As a result:
- The server doesn’t reject the request based on origin.
- The response is directly returned to your backend code without any CORS checks.
3. Why Browsers Enforce CORS
CORS is part of the same-origin policy, which is a critical security mechanism in browsers. It prevents unauthorized JavaScript on one website from reading sensitive data from another website, like an API or a private user dashboard, which could compromise security.
Server-side requests don’t need this level of security because:
- They are typically controlled by the app developer.
- There’s no risk of a malicious website manipulating the request, as the server-side code runs in a secure environment (e.g., your backend).
How to Use cURL as a Proxy
Since a web server can utilize cURL to make the request without CORS restrictions, you can leverage it as a proxy to fetch the data and pass it to your JavaScript app. Here’s how: How to Create a ViewBits API Backend for Your Website
