Resolving CloudFront CORS Errors for Font Loading

Encountering a Cross-Origin Resource Sharing (CORS) error can be frustrating, especially when it only affects specific resources like fonts while other files load without issues. This guide will help you understand the cause of these errors and how to resolve them in an AWS CloudFront scenario.

When using AWS CloudFront in conjunction with S3 to serve assets, you might run into an error that prevents fonts from loading. This typically manifests as a CORS issue because the request to load a font comes from a different origin than where the font is hosted. The problem often arises due to missing or incorrect CORS headers.

Understanding the CORS Error

The error states that a font from a CloudFront distribution is blocked because there is no ‘Access-Control-Allow-Origin’ header on the requested resource. This header is essential for browsers to determine if a cross-origin request is allowed.

Fixing the CORS Issue

Step 1: Update S3 CORS Configuration

Ensure that your S3 bucket is correctly configured to handle CORS requests, especially for fonts. To do this, you need to update the CORS configuration in your S3 bucket:

1. Go to your AWS Management Console.
2. Navigate to Amazon S3.
3. Select the bucket hosting your fonts.
4. Under the Permissions tab, locate CORS configuration.

Apply the following CORS configuration:

<CORSConfiguration>
   <CORSRule>
      <AllowedOrigin>https://your-domain.example</AllowedOrigin>
      <AllowedMethod>GET</AllowedMethod>
      <AllowedMethod>HEAD</AllowedMethod>
      <AllowedHeader>*</AllowedHeader>
   </CORSRule>
</CORSConfiguration>

Replace https://your-domain.example with your specific domain to restrict access to only your site. Use the asterisk (*) if you wish to allow all domains, but this is not recommended due to security reasons. The AllowedMethod specifies the HTTP methods that are allowed, and AllowedHeader lets you allow specific headers.

Step 2: Setting Headers via CloudFront

If you manage HTTP headers at the CloudFront distribution level, ensure the Access-Control-Allow-Origin header is correctly set. This is useful if you’re serving dynamic content and need to control headers directly from your server-side logic, which might involve some web server configurations like .htaccess in Apache.

For Apache, consider these lines in your .htaccess file:

Header add Access-Control-Allow-Origin "https://your-domain.example"

Again, specify your domain instead of using a wildcard unless absolutely necessary.

Step 3: Validate Configuration

1. Clear Cache: After making changes, clear your browser cache or use an incognito window to test the changes.
2. Test Load: Try loading the font again in your application to see if the issue persists.
3. Check Headers: Use your browser’s developer tools (typically under the “Network” tab) to verify if the Access-Control-Allow-Origin header is correctly set in the response.

Understanding CORS is crucial for any developer dealing with web assets. CORS is a security feature implemented by browsers that allows or restricts resources to be requested by web pages from different origins. Proper configuration is critical to ensure your applications work securely and correctly without unnecessary exposure.

By following these steps, you can resolve common CORS issues associated with font loading in a CloudFront distribution working with an S3 bucket. Always tailor configurations to your needs, balancing usability with security.