Trouble Enabling CORS on S3

Question

I set up CORS on an S3 bucket like so:

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

Here's a screenshot of the AWS console: https://dzwonsemrish7.cloudfront.net/items/341y0o1n1X2a0O1X2s38/Screen%20Shot%202012-10-09%20at%209.59.44%20PM.png?v=2478ad83

When I point my color-thief javascript at an image hosted on the same domain, everything works as expected, but when I point to an asset in my S3 bucket, regardless of whether I run my script from localhost, lvh.me (which points to 127.0.0.1), or from the real interwebs, I get errors like this in Chrome 22:

Unable to get image data from canvas because the canvas has been tainted by cross-origin data.
Uncaught Error: SECURITY_ERR: DOM Exception 18

and this in Firefox 15:

SecurityError: The operation is insecure.

Here's what the headers look like in the Google Chrome Network Inspector:

Request URL:https://s3.amazonaws.com/assets-zeke.heroku.com/addons-zeke.heroku.com/catalogs/58/original.png
Request Method:GET
Status Code:304 Not Modified

Request Headers
Accept:*/*
Accept-Charset:ISO-8859-1,utf-8;q=0.7,*;q=0.3
Accept-Encoding:gzip,deflate,sdch
Accept-Language:en-US,en;q=0.8
Cache-Control:max-age=0
Connection:keep-alive
Host:s3.amazonaws.com
If-Modified-Since:Tue, 09 Oct 2012 22:52:57 GMT
If-None-Match:"6de1a52294934c5e288894b84100d99b"
Referer:http://localhost:5000/marketplace/addons/sendgrid/edit
User-Agent:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_4) AppleWebKit/537.4 (KHTML, like Gecko) Chrome/22.0.1229.79 Safari/537.4

Response Headers
HTTP/1.1 304 Not Modified
x-amz-id-2: qGvoGlvpKeSSzelanWsIPDF3zd5wQzHTr27NZoUbhNqAM1QzmKcWHnTqIkKVxF/m
x-amz-request-id: FD24FB8CA244E327
Date: Wed, 10 Oct 2012 05:20:53 GMT
Last-Modified: Tue, 09 Oct 2012 22:52:57 GMT
ETag: "6de1a52294934c5e288894b84100d99b"
Server: AmazonS3

And here's what the headers look like after I changed the URL structure to {bucket}.s3.amazonaws.com (and removed the period) from my bucket name.

Request URL:http://assets-zeke.s3.amazonaws.com/addons-zeke.heroku.com/catalogs/58/original.png
Request Method:GET
Status Code:200 OK
Request Headers

Request Headers
Accept:*/*
Accept-Charset:ISO-8859-1,utf-8;q=0.7,*;q=0.3
Accept-Encoding:gzip,deflate,sdch
Accept-Language:en-US,en;q=0.8
Cache-Control:no-cache
Connection:keep-alive
Host:assets-zeke.s3.amazonaws.com
Pragma:no-cache
Referer:http://lvh.me:5000/marketplace/addons/airbrake/edit
User-Agent:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_4) AppleWebKit/537.4 (KHTML, like Gecko) Chrome/22.0.1229.79 Safari/537.4

Response Headers
Accept-Ranges:bytes
Content-Length:6696
Content-Type:image/png
Date:Wed, 10 Oct 2012 17:56:17 GMT
ETag:"6de1a52294934c5e288894b84100d99b"
Last-Modified:Wed, 10 Oct 2012 17:50:38 GMT
Server:AmazonS3
x-amz-id-2:UGVKQ9VQbJ82DLDxR53uDP0ZUMgla+e0GU5vO9yLr6MsY8wijl9KnM7fOyDlT+ta
x-amz-request-id:8A16CF1E02A0106C

Shouldn't I be seeing Access-Control-Allow-Origin: * here? Does the 304 mean that Amazon is caching the response?

Answer 1

This can be caused by not having enabled CORS in your image element before it has loaded:

imgEl.crossOrigin = '';

Answer 2

Request URL:https://s3.amazonaws.com/assets-zeke.heroku.com/addons-zeke.heroku.com/catalogs/58/original.png

Well, there's your problem. Because of the way that CORS and other cross-domain things work, you need to use DNS-style addressing to access your buckets.

Assuming your original URL is correct (it doesn't look like it, but I could totally be wrong), you'd want to use this URL instead:

http://assets-zeke.heroku.com.s3.amazonaws.com/addons-zeke.heroku.com/catalogs/58/original.png

In other words:

http://{bucket}.s3.amazonaws.com/path/object.ext

Check out: http://docs.amazonwebservices.com/AmazonS3/latest/dev/cors.html for more explanation.