jQuery AJAX call results in error status 403

Question

I\'m making a query to a web service using jQuery AJAX. My query looks like this:

var serviceEndpoint = \'http://example.com/object/details?version=1.1\';
$.         


        

Answer 1

The reason of 403 error is you are not sending headers. Since you are making a CORS request, you cannot send any custom headers unless server enables these header by adding Access-Control-Allow-Headers to the response.

In a preflighted-request, client makes 2 requests to the server. First one is preflight (with OPTIONS method) and the second one is the real request. The server sends Access-Control-Allow-Headers header as a response of the preflight request. So it enables some headers to be sent. By this way your POST request can work because the POST request is a preflight-request. But for a GET request, there is no preflight to gather Access-Control-Allow-Headers header and browser doesn't send your custom headers in this case.

A workaround for this issue:

As a workaround, set your dataType and contentType to json as the following:

var serviceEndpoint = 'http://example.com/object/details?version=1.1';
$.ajax({
  type: 'GET', 
  url: serviceEndpoint,
  dataType: 'json',
  contentType: 'json',
  headers: { 'api-key':'myKey' },
  success: onSuccess,
  error: onFailure
});

By this way, your get request will be a preflighted request. If your server enables the api-key with Access-Control-Allow-Headers header, it will work.

Sample server configuration for the above request (written in express.js):

res.setHeader('Access-Control-Allow-Origin', '*');
res.setHeader('Access-Control-Allow-Methods', '*');
res.setHeader('Access-Control-Allow-Headers', 'api-key,content-type');
res.setHeader('Access-Control-Allow-Credentials', true);

ADDED:

Actually, contentType should be either application/javascript or application/json while doing a jsonp request. There is no contentType as jsonp.

Answer 2

If you look at the API page for jQuery's Ajax call, it mentions the following in the Content-Type section:

Note: For cross-domain requests, setting the content type to anything other than application/x-www-form-urlencoded, multipart/form-data, or text/plain will trigger the browser to send a preflight OPTIONS request to the server.

That page doesn't really mention what a "preflight OPTIONS request" is, but I found some interesting links when looking that phrase up online:

• Preflight Blob Request
• HTML 5 Rocks Using CORS
• W3C's Cross-Origin Resource Sharing spec
• Using CORS for Cross-Domain Ajax Requests

What's intersting is the code example & the CORS image at the HTML5Rocks page. The image shows how the Ajax calls are being made from the JavaScript code to the browser to the server & how the responses are round-tripping between all 3 of those.

We tend to think of JavaScript + Browser = Client, but in the illustration the author is explaining the difference between the web developer's code & the browser developer's code, where the former is written in JavaScript code, but the latter was written using C, C++ or C# code.

A good packet analyzer tool is Fiddler, which would be similar to Wireshark. Either one of those tools, should show you the pre-flight requests which are being sent from the browser to the server. Most likely, that's where your Ajax request is being blocked at by the server with a 403 Forbidden error.