Fixed a security issue in the CSRF component. Disclosure and new release forthcoming.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15464 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/middleware/csrf.py

@@ -101,6 +101,7 @@ class CsrfViewMiddleware(object):
         return _get_failure_view()(request, reason=reason)
 
     def process_view(self, request, callback, callback_args, callback_kwargs):
+
         if getattr(request, 'csrf_processing_done', False):
             return None
 
@@ -134,31 +135,6 @@ class CsrfViewMiddleware(object):
                 # any branches that call reject()
                 return self._accept(request)
 
-            if request.is_ajax():
-                # .is_ajax() is based on the presence of X-Requested-With.  In
-                # the context of a browser, this can only be sent if using
-                # XmlHttpRequest.  Browsers implement careful policies for
-                # XmlHttpRequest:
-                #
-                #  * Normally, only same-domain requests are allowed.
-                #
-                #  * Some browsers (e.g. Firefox 3.5 and later) relax this
-                #    carefully:
-                #
-                #    * if it is a 'simple' GET or POST request (which can
-                #      include no custom headers), it is allowed to be cross
-                #      domain.  These requests will not be recognized as AJAX.
-                #
-                #    * if a 'preflight' check with the server confirms that the
-                #      server is expecting and allows the request, cross domain
-                #      requests even with custom headers are allowed. These
-                #      requests will be recognized as AJAX, but can only get
-                #      through when the developer has specifically opted in to
-                #      allowing the cross-domain POST request.
-                #
-                # So in all cases, it is safe to allow these requests through.
-                return self._accept(request)
-
             if request.is_secure():
                 # Suppose user visits http://example.com/
                 # An active network attacker,(man-in-the-middle, MITM) sends a
@@ -222,6 +198,10 @@ class CsrfViewMiddleware(object):
 
             # check incoming token
             request_csrf_token = request.POST.get('csrfmiddlewaretoken', '')
+            if request_csrf_token == "":
+                # Fall back to X-CSRFToken, to make things easier for AJAX
+                request_csrf_token = request.META.get('HTTP_X_CSRFTOKEN', '')
+
             if not constant_time_compare(request_csrf_token, csrf_token):
                 if cookie_is_new:
                     # probably a problem setting the CSRF cookie

docs/ref/contrib/csrf.txt

@@ -81,6 +81,47 @@ The utility script ``extras/csrf_migration_helper.py`` can help to automate the
 finding of code and templates that may need to be upgraded.  It contains full
 help on how to use it.
 
+AJAX
+----
+
+While the above method can be used for AJAX POST requests, it has some
+inconveniences: you have to remember to pass the CSRF token in as POST data with
+every POST request. For this reason, there is an alternative method: on each
+XMLHttpRequest, set a custom `X-CSRFToken` header to the value of the CSRF
+token. This is often easier, because many javascript frameworks provide hooks
+that allow headers to be set on every request. In jQuery, you can use the
+``beforeSend`` hook as follows:
+
+.. code-block:: javascript
+
+    $.ajaxSetup({
+        beforeSend: function(xhr, settings) {
+            function getCookie(name) {
+                var cookieValue = null;
+                if (document.cookie && document.cookie != '') {
+                    var cookies = document.cookie.split(';');
+                    for (var i = 0; i < cookies.length; i++) {
+                        var cookie = jQuery.trim(cookies[i]);
+                        // Does this cookie string begin with the name we want?
+                        if (cookie.substring(0, name.length + 1) == (name + '=')) {
+                            cookieValue = decodeURIComponent(cookie.substring(name.length + 1));
+                            break;
+                        }
+                    }
+                }
+                return cookieValue;
+            }
+            if (!(/^http:.*/.test(settings.url) || /^https:.*/.test(settings.url))) {
+                // Only send the token to relative URLs i.e. locally.
+                xhr.setRequestHeader("X-CSRFToken", getCookie('csrftoken'));
+            }
+        }
+    });
+
+Adding this to a javascript file that is included on your site will ensure that
+AJAX POST requests that are made via jQuery will not be caught by the CSRF
+protection.
+
 The decorator method
 --------------------
 
@@ -261,10 +302,6 @@ in the same module.  These disable the view protection mechanism
 (``CsrfResponseMiddleware``) respectively.  They can be used individually if
 required.
 
-You don't have to worry about doing this for most AJAX views. Any request sent
-with "X-Requested-With: XMLHttpRequest" is automatically exempt. (See the `How
-it works`_ section.)
-
 Subdomains
 ----------
 
@@ -342,24 +379,6 @@ request ought to be harmless.
 response, and only pages that are served as 'text/html' or
 'application/xml+xhtml' are modified.
 
-AJAX
-----
-
-The middleware tries to be smart about requests that come in via AJAX. Most
-modern JavaScript toolkits send an "X-Requested-With: XMLHttpRequest" HTTP
-header; these requests are detected and automatically *not* handled by this
-middleware.  We can do this safely because, in the context of a browser, the
-header can only be added by using ``XMLHttpRequest``, and browsers already
-implement a same-domain policy for ``XMLHttpRequest``.
-
-For the more recent browsers that relax this same-domain policy, custom headers
-like "X-Requested-With" are only allowed after the browser has done a
-'preflight' check to the server to see if the cross-domain request is allowed,
-using a strictly 'opt in' mechanism, so the exception for AJAX is still safe—if
-the developer has specifically opted in to allowing cross-site AJAX POST
-requests on a specific URL, they obviously don't want the middleware to disallow
-exactly that.
-
 .. _9.1.1 Safe Methods, HTTP 1.1, RFC 2616: http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html
 
 Caching

tests/regressiontests/csrf_tests/tests.py

@@ -284,12 +284,12 @@ class CsrfMiddlewareTest(TestCase):
         req2 = CsrfMiddleware().process_view(req, csrf_exempt(post_form_view), (), {})
         self.assertEquals(None, req2)
 
-    def test_ajax_exemption(self):
+    def test_csrf_token_in_header(self):
         """
-        Check that AJAX requests are automatically exempted.
+        Check that we can pass in the token in a header instead of in the form
         """
         req = self._get_POST_csrf_cookie_request()
-        req.META['HTTP_X_REQUESTED_WITH'] = 'XMLHttpRequest'
+        req.META['HTTP_X_CSRFTOKEN'] = self._csrf_id
         req2 = CsrfMiddleware().process_view(req, post_form_view, (), {})
         self.assertEquals(None, req2)