How to modify Cookie from Ajax call

Question

I have this code:

window.onload = function() {
        document.cookie = 'foo=bar; expires=Sun, 01 Jan 2012 00:00:00 +0100; path=/';
        var xhr = new XMLHttpRequest();
        xhr.open("GET", "/showcookie.php",true);
        xhr.setRequestHeader("Cookie", "foo=quux");

        xhr.setRequestHeader("Foo", "Bar");
        xhr.setRequestHeader("Foo", "Baz");

        xhr.withCredentials = true;
        var pre = document.getElementById('output');
        xhr.onreadystatechange = function() {
            if (4 == xhr.readyState) {
                pre.innerHTML += xhr.responseText + "\n";
            }
        };
        xhr.send(null);
    };

and this /showcookie.php

<?php

print_r($_COOKIE);

?>

and it always show

Array
(
    [Host] => localhost
    [User-Agent] => 
    [Accept] => 
    [Accept-Language] => pl,en-us;q=0.7,en;q=0.3
    [Accept-Encoding] => gzip,deflate
    [Accept-Charset] => ISO-8859-2,utf-8;q=0.7,*;q=0.7
    [Keep-Alive] => 115
    [Connection] => keep-alive
    [foo] => Baz
    [Referer] =>
    [Cookie] => foo=bar
)

Array
(
    [foo] => bar
)

I'm using Firefox 3.6.13, Opera 11.00 and Chromium 9.0 on Ubuntu.

Is anybody have the same problem or maybe it's impossible to modify Cookie header.

Answer 1

The Cookie header is one of several which cannot be modified in an XMLHttpRequest. From the specification:

Terminate [execution of the setRequestHeader method] if header is a case-insensitive match for one of the following headers:

• Accept-Charset
• Accept-Encoding
• Connection
• Content-Length
• Cookie
• Cookie2
• Content-Transfer-Encoding
• Date
• Expect
• Host
• Keep-Alive
• Referer
• TE
• Trailer
• Transfer-Encoding
• Upgrade
• User-Agent
• Via

… or if the start of header is a case-insensitive match for Proxy- or Sec- (including when header is just Proxy- or Sec-).

The above headers are controlled by the user agent to let it control those aspects of transport. This guarantees data integrity to some extent. Header names starting with Sec- are not allowed to be set to allow new headers to be minted that are guaranteed not to come from XMLHttpRequest.