Why is my cross-domain POST request being preflighted with an OPTIONS request?

Question

According to the Mozilla Developer Center HTTP access control article, cross-site POST requests can be "simple" -- i.e. require no preflighting -- if the request's Content-Type is application/x-www-form-urlencoded.

I'm not getting this behavior in Firefox, and I'm not understanding at all why this is so. Here's my setup code:

function makeXDomainRequest(url, method, data) {
    var req =
        typeof XDomainRequest !== "undefined" ?
        new XDomainRequest() : new XMLHttpRequest();

    req.open(method || "GET", url, true);

    if (typeof req.onload !== "undefined") {
        req.onload = onResponseLoad;
        req.onerror = onRequestError;
    } else {
        req.onreadystatechange = onRequestStateChange;
    }

    if (data && typeof req.setRequestHeader === "function") {
        req.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
    } else {
        // no way to set Content-Type req header in IE's XDomainRequest:
        // http://blogs.msdn.com/b/ieinternals/archive/2010/05/13/xdomainrequest-restrictions-limitations-and-workarounds.aspx
    }

    req.send(data || null);
}

function onResponseLoad() {
    alert("Response!\n" + this.responseText);
}

function onRequestError(args) {
    alert("Error!");
}

function onRequestStateChange() {
    if (this.readyState === 4) {
        if (this.status === 200) {
            onResponseLoad.apply(this);
        } else {
            onRequestError.apply(this);
        }
    }
}

And here's the server I'm pinging:

// thanks to http://saltybeagle.com/cors/ for having this demo endpoint:
var URL = "http://ucommbieber.unl.edu/CORS/cors.php";

Now if I do a simple POST request -- with data sent as application/x-www-form-urlencoded in the code above -- the request is preflighted in Firefox with an OPTIONS request. It is not preflighted in Chrome. Open Fiddler before running this to see for yourself:

makeXDomainRequest(URL, "POST", "name=foobar");
// alerts "Response! Hello CORS [...] You sent a POST request. Your name is foobar"

Here is the preflight OPTIONS request in Fiddler (notice the Access-Control-Request-Method: POST header even though I specified a supposedly safe Content-Type and no custom headers):

OPTIONS http://ucommbieber.unl.edu/CORS/cors.php HTTP/1.1
Host: ucommbieber.unl.edu
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.2.8) Gecko/20100722 Firefox/3.6.8
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 115
Connection: keep-alive
Origin: http://localhost
Access-Control-Request-Method: POST

What's going on? Is this a bug in Firefox, or am I doing something wrong? Thanks!

Answer 1

It indeed turned out to be a Firefox bug. It ended up getting fixed for FF4b6: https://bugzilla.mozilla.org/show_bug.cgi?id=588920