Can Amazon S3 CORS Solve Firefox's Cross-Domain Font Loading Problems?

Amazon S3 CORS and Firefox Cross-Domain Font Loading

Background

Firefox has historically encountered difficulties loading fonts from origins distinct from the current webpage, particularly when fonts are stored on CDNs. While various solutions have been proposed, the introduction of Amazon S3 Cross-Origin Resource Sharing (CORS) raises the question of whether this issue can be addressed using CORS.

CORS Configuration

An Amazon S3 CORS configuration that has been successfully implemented to address this issue is provided below:

<?xml version="1.0" encoding="UTF-8"?>
<CORSConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
    <CORSRule>
        <AllowedOrigin>https://mydomain.com</AllowedOrigin>
        <AllowedMethod>GET</AllowedMethod>
        <MaxAgeSeconds>3000</MaxAgeSeconds>
        <AllowedHeader>Content-*</AllowedHeader>
        <AllowedHeader>Host</AllowedHeader>
    </CORSRule>
    <CORSRule>
        <AllowedOrigin>https://*.mydomain.com</AllowedOrigin>
        <AllowedMethod>GET</AllowedMethod>
        <MaxAgeSeconds>3000</MaxAgeSeconds>
        <AllowedHeader>Content-*</AllowedHeader>
        <AllowedHeader>Host</AllowedHeader>
    </CORSRule>
</CORSConfiguration>

The significance of the AllowedMethod (GET) and AllowedHeader (Content-*) settings in this configuration is crucial.

Query String Workaround

In certain cases, developers may encounter issues due to Cloudfront caching the Access-Control-Allow-Origin header. To address this, AWS staff recommends using a query string to differentiate between requests from various domains.

By using a unique query string for each domain, such as "?https_a.domain.com" and "?http_b.domain.com", Cloudfront will return different Access-Control-Allow-Origin values, allowing font loading across domains.

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