Wednesday, 9 February 2011

Ajax Authentication in Firefox and Safari

We've recently been tripped up by a difference between the way that Firefox and Mozilla handle authentication in Ajax requests.  We have a jQuery Ajax call whose essential elements are like this:

        type:         "GET",
        url:          "/admiral-test/datasets/"+datasetName,
        username:     username,
        password:     password,
        cache:        false
        success:      function (data, status, xhr)
        error:        function (xhr, status) 

The username and password here are credentials needed for accessing non-public information on the Databank repository server.  We find this works fine with Firefox, but when accessing some Databank services using Safari (and possibly IE) we get HTTP 403 Forbidden responses, despite the fact that we provide correct credentials.

We diagnosed the problem using Wireshark to monitor the HTTP protocol exchanges.  It is worth noting that Wireshark can trace encrypted HTTPS traffic if a copy of the server private key is provided.  A summary of our investigations is at

What we observed was that when credentials are supplied in the Ajax call, Firefox always includes an appropriate HTTP Authorization (sic) header. Safari, on the other hand, does not initially include this, but instead re-sends the request with an Authorization header in response to an HTTP 401 Unauthorized status return.  Both behaviours are correct within the HTTP specification. Our problem was caused by the fact that the Databank service was responding to a request without the Authorization header  with a 403 instead of a 401 response.  The 403 response explicitly indicates that re-issuing the request with credentials will not make any difference (in our case, incorrectly).

There is a separate issue about whether we actually need to provide credentials in the Ajax call: in other parts of our system, we have found that the browser (well, Firefox, anyway) will intelligently pop up a credentials box if an Ajax request needs authentication credentials - this clearly depends on getting a 401 response to the original request, so is something that should be tested when the Databank server is fixed.

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