Fixing CORS Issues with AWS API Gateway for Browser Access

If you’re working with AWS API Gateway and running into issues where the Access-Control-Allow-Origin header is giving you a headache, you’re not alone. Imagine setting up CORS in API Gateway, and yet, your browser insists there’s no Access-Control-Allow-Origin header when you try to call your API. Meanwhile, tools like Postman have no trouble making the same call.

Let’s dive into how you can resolve this issue and ensure your API can be accessed from the browser without hassles.

Understanding CORS in API Gateway

Cross-Origin Resource Sharing (CORS) is a security feature that restricts JavaScript code running in a browser from making requests to a different origin than the one that served the web page. When you’re using API Gateway, configuring CORS correctly is crucial, especially if your API is being consumed by a web application. You need to properly set up CORS headers to respond to preflight requests effectively.

The Access-Control-Allow-Origin header specifies which domains can access the API. A wildcard (*) can be used to allow all origins, but for security, consider restricting this to known domains.

Setting Up CORS in API Gateway

Here’s a step-by-step guide to ensure your CORS configuration is correct and effective:

1. API Gateway Console Configuration:

   • On the API Gateway console, navigate to the Resource tab for your API.
   • Choose Enable CORS for the resource method that should accept cross-origin requests.
   • Specify which headers, methods, and origins are allowed. Set the Access-Control-Allow-Origin to * or a specific domain.

2. Lambda Function Responses:

   • If you’re using Lambda proxy integration, ensure your Lambda function includes CORS headers in its response.

   Example implementation in Node.js for a Lambda function:

   "use strict";
   
   module.exports.handler = async (event) => {
     const response = {
       statusCode: 200,
       headers: {
         "Access-Control-Allow-Origin": "*",  // Required for CORS
         "Access-Control-Allow-Credentials": true, // Required for cookies, authorization headers with HTTPS
       },
       body: JSON.stringify({ message: "Hello from Lambda!" }),
     };
   
     return response;
   };
   

3. Preflight Request Setup:

   • Ensure that OPTIONS requests are handled. This is crucial because the browser sends a preflight OPTIONS request to check if the actual request is safe to send.
   • You can create an OPTIONS resource in API Gateway, which includes relevant CORS headers in its response.

Debugging and Testing CORS

• Use Browser Developer Tools: Utilize the Network tab in Chrome DevTools to inspect requests and responses. Look for the presence of correct CORS headers.

• Testing with Postman: While Postman doesn’t run into CORS issues due to how browsers enforce them, seeing proper headers in Postman validates that your server-side headers are set correctly.

• Use a Proxy: As a temporary workaround for browser development, you might consider using a proxy server to bypass CORS issues. This isn’t a production solution but can be handy for debugging.

By ensuring that your API’s responses include all necessary CORS headers and that your OPTIONS preflight requests are configured correctly, you should be able to alleviate most, if not all, CORS-related issues when accessing your API through a web browser.

For additional information, refer to AWS’s documentation on setting up CORS in API Gateway.