Is an X-Requested-With header server check sufficient to protect against a CSRF for an ajax-driven application?

I’m working on a completely ajax-driven application where all requests pass through what basically amounts to a main controller which, at its bare bones, looks something like this:

if(strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest') {
    fetch($page);
}

Is this generally sufficient to protect against cross-site request forgeries?

It’s rather inconvenient to have a rotating token when the entire page isn’t refreshed with each request.

I suppose I could pass and update unique token as a global javascript variable with every request — but somehow that feels clumsy and seems inherently unsafe anyway.

EDIT – Perhaps a static token, like the user’s UUID, would be better than nothing?

EDIT #2 – As The Rook pointed out, this might be a hair-splitting question. I’ve read speculation both ways and heard distant whispers about older versions of flash being exploitable for this kind of shenanigans. Since I know nothing about that, I’m putting up a bounty for anyone who can explain how this is a CSRF risk. Otherwise, I’m giving it to Artefacto. Thanks.

Answer

I’d say it’s enough. If cross-domain requests were permitted, you’d be doomed anyway because the attacker could use Javascript to fetch the CSRF token and use it in the forged request.

A static token is not a great idea. The token should be generated at least once per session.

EDIT2 Mike is not right after all, sorry. I hadn’t read the page I linked to properly. It says:

A simple cross-site request is one that: […] Does not set custom headers with the HTTP Request (such as X-Modified, etc.)

Therefore, if you set X-Requested-With, the request has to be pre-flown, and unless you respond to pre-flight OPTIONS request authorizing the cross-site request, it won’t get through.

EDIT Mike is right, as of Firefox 3.5, cross-site XMLHttpRequests are permitted. Consequently, you also have to check if the Origin header, when it exists, matches your site.

if (array_key_exists('HTTP_ORIGIN', $_SERVER)) {
    if (preg_match('#^https?://myserver.com$#', $_SERVER['HTTP_ORIGIN'])
        doStuff();
}
elseif (array_key_exists('HTTP_X_REQUESTED_WITH', $_SERVER) &&
        (strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest'))
    doStuff();