How to protect against CSRF when using Backbone.js to post data?

Question

Backbone.js handles posting data to server under the hood, so there is no easy way to insert a CSRF token in the payload. How can I protect my site against CSRF in this situation?

In this SO answer: https://stackoverflow.com/a/10386412/954376, the suggestion is to verify the x-Requested-By header to be XMLHTTPRequest. Is this enough to block all CSRF attempts?

In Django docs, the suggestion is to add CSRF token in another custom header in every AJAX request: https://docs.djangoproject.com/en/1.5/ref/contrib/csrf/#ajax. Is this necessary?

I understand if the attack uses hidden form, I am safe by just assuring the request is from XMLHTTPRequest. But is there any CSRF attack tricks that can forge the header?

Answer 1

Setting a global CSRF-token for all jQuery.ajax calls:

$(function(){ 
  $.ajaxSetup({
    headers: {'X-CSRFToken': CSRF_TOKEN}
  });
})

Setting the token just for Backbone by overriding Backbone.sync:

var oldSync = Backbone.sync;
Backbone.sync = function(method, model, options){
  options.beforeSend = function(xhr){
    xhr.setRequestHeader('X-CSRFToken', CSRF_TOKEN);
  };
  return oldSync(method, model, options);
};

EDIT: Fixed a typo Kadam points at in comments

Answer 2

You can use a prefilter to add the token to all requests:

$.ajaxPrefilter(function(opts) {
    if (opts.data) {
        opts.data += "&";
    }
    opts.data += "csrfToken=" + token;
});

You may need to add additional logic if you don't always send the token.

Answer 3

Here's an updated version, based in Django 1.7 (using the jQuery cookie plugin)

oldSync = Backbone.sync
Backbone.sync = (method, model, options) ->

    csrfSafeMethod = (method) ->
        # these HTTP methods do not require CSRF protection
        /^(GET|HEAD|OPTIONS|TRACE)$/.test method

    options.beforeSend = (xhr, settings) ->
        if !csrfSafeMethod(settings.type) and !@crossDomain
            xhr.setRequestHeader 'X-CSRFToken', $.cookie('csrftoken')
        return
    oldSync method, model, options