AJAX requests sent from different origins to AWS S3 may encounter CORS errors on occasion

My current objective is to access publicly available files stored in S3.

The CORS configuration for my S3 setup is as follows:

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

My JavaScript code implementation looks like this:

const response = await fetch(url, {
    mode: 'cors',
});

const blob = await response.blob();

The code works most of the time, however, I encounter an error in the console occasionally:

Access to XMLHttpRequest at '' from origin '' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.

Interestingly, after refreshing the page a few times, or sometimes just once, the error disappears and I can successfully retrieve the response object.

Answer №1

Hey everyone, it's me.

The other day, I was investigating successful and unsuccessful requests in Chrome developer tools. It caught my eye that unsuccessful requests were returning a Status code of: 200 OK (from disk cache), while successful ones had a Status code of: 200 OK.

To resolve this issue, I decided to disable caching by using cache: 'no-cache', which seemed to do the trick.

const response = await fetch(url, {
    method: 'GET',
    mode: 'cors',
    cache: 'no-cache',
    headers: {
        Origin: window.location.origin,
    },
});

Although I'm still puzzled as to why cached requests would encounter CORS problems, at least I found a workaround to address it.


// UPDATE:

It turns out I'm not the only one facing this issue. Check out this post for more information: Cached non CORS response conflicts with new CORS request

Answer №2

It appears that the

<AllowedMethod>Head</AllowedMethod>

tag is missing from your configuration file.

This could be causing the issues you are experiencing.

Ensure that in the Allowed Http Methods section, you have selected Get, Head, Options, and also include Access-Control-Allow-Methods, Access-Control-Allow-Origin, and Origin on your whitelist.

Give that a try!

Best of luck

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