API Gateway CORS: no "Access-Control-Allow-Origin" header

Solving the API Gateway CORS Issue: The Missing 'Access-Control-Allow-Origin' Header

So you've set up the CORS (Cross-Origin Resource Sharing) configuration in API Gateway, and you've added the 'Access-Control-Allow-Origin' header to allow cross-origin requests. However, you're still encountering the frustrating error message:

XMLHttpRequest cannot load http://XXXXX.execute-api.us-west-2.amazonaws.com/beta/YYYYY. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'null' is therefore not allowed access. The response had HTTP status code 403.

Don't worry! You're not alone in facing this issue. In this blog post, we'll dive into common causes for this error and provide you with simple solutions to get rid of it once and for all.

Understanding the Problem

When you try to make a cross-origin request (i.e., a request made from one website to a different domain), the browser's security mechanism kicks in. It makes a pre-flight request to the server to check if the requested resource supports cross-origin requests.

The server responds to this pre-flight request with the appropriate headers, including the 'Access-Control-Allow-Origin' header. This header specifies the domains allowed to access the requested resource.

Possible Causes

There are a few potential causes for the missing 'Access-Control-Allow-Origin' header issue:

1. Misconfigured CORS settings: Double-check that you have correctly configured the CORS settings in your API Gateway. Make sure you have specified the correct 'Access-Control-Allow-Origin' value, allowing the specific domain or '*' for any domain.

2. Incorrect values in requests: Ensure that the value of the 'Origin' header in your AJAX request matches the allowed origin specified in the 'Access-Control-Allow-Origin' header. If they don't match, the browser will block the request.

Easy Solutions

Now let's explore two simple solutions to fix the missing 'Access-Control-Allow-Origin' header issue:

Solution 1: Wildcard '*' Approach

If you want to allow requests from any domain, you can set the 'Access-Control-Allow-Origin' header value to '*'. Keep in mind that this approach is less secure since it allows any domain to access your API.

Access-Control-Allow-Origin: *

Solution 2: Specific Domain Approach

If you only want to allow specific domains to access your API, you need to set the 'Access-Control-Allow-Origin' header value to the specific domain(s). For example:

Access-Control-Allow-Origin: https://www.example.com

Replace https://www.example.com with the actual domain(s) you want to allow access.

Verify Your Configuration

To verify that your changes have been applied correctly, you can use tools like Postman, mentioned in the original question, to send requests and inspect the response headers.

Going Beyond JSONP

Sometimes, to bypass the CORS limitations imposed by browsers, people resort to using JSONP (JSON with Padding) as a workaround. However, this can lead to its own set of security risks and limitations.

Instead of resorting to JSONP, with the solutions provided above, you can call your API directly from the browser while adhering to the proper CORS standards.

Your Turn to Take Action!

Now that you have a clear understanding of the API Gateway CORS issue and its solutions, it's time to put this knowledge into action. Here's what you can do:

1. Double-check your CORS configuration in API Gateway.

2. Verify that the 'Access-Control-Allow-Origin' header is correctly set.

3. Make sure your AJAX request includes the correct 'Origin' header value.

4. Apply the appropriate solution: either the wildcard ('*') approach or the specific domain approach.

5. Test your API by sending requests from your website or application.

6. Share this blog post with your fellow developers who might be facing the same issue.

By actively applying these steps, you'll overcome the missing 'Access-Control-Allow-Origin' header issue and build robust and secure cross-origin APIs.

Happy coding! 🚀👩‍💻👨‍💻