Fixed #14445 - Use HMAC and constant-time comparison functions where needed.

All adhoc MAC applications have been updated to use HMAC, using SHA1 to
generate unique keys for each application based on the SECRET_KEY, which is
common practice for this situation. In all cases, backwards compatibility
with existing hashes has been maintained, aiming to phase this out as per
the normal deprecation process. In this way, under most normal
circumstances the old hashes will have expired (e.g. by session expiration
etc.) before they become invalid.

In the case of the messages framework and the cookie backend, which was
already using HMAC, there is the possibility of a backwards incompatibility
if the SECRET_KEY is shorter than the default 50 bytes, but the low
likelihood and low impact meant compatibility code was not worth it.

All known instances where tokens/hashes were compared using simple string
equality, which could potentially open timing based attacks, have also been
fixed using a constant-time comparison function.

There are no known practical attacks against the existing implementations,
so these security improvements will not be backported.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14218 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/contrib/auth/tests/tokens.py

@@ -50,3 +50,25 @@ class TokenGeneratorTest(TestCase):
 
         p2 = Mocked(date.today() + timedelta(settings.PASSWORD_RESET_TIMEOUT_DAYS + 1))
         self.assertFalse(p2.check_token(user, tk1))
+
+    def test_django12_hash(self):
+        """
+        Ensure we can use the hashes generated by Django 1.2
+        """
+        # Hard code in the Django 1.2 algorithm (not the result, as it is time
+        # dependent)
+        def _make_token(user):
+            from django.utils.hashcompat import sha_constructor
+            from django.utils.http import int_to_base36
+
+            timestamp = (date.today() - date(2001,1,1)).days
+            ts_b36 = int_to_base36(timestamp)
+            hash = sha_constructor(settings.SECRET_KEY + unicode(user.id) +
+                                   user.password + user.last_login.strftime('%Y-%m-%d %H:%M:%S') +
+                                   unicode(timestamp)).hexdigest()[::2]
+            return "%s-%s" % (ts_b36, hash)
+
+        user = User.objects.create_user('tokentestuser', 'test2@example.com', 'testpw')
+        p0 = PasswordResetTokenGenerator()
+        tk1 = _make_token(user)
+        self.assertTrue(p0.check_token(user, tk1))

django/contrib/auth/tokens.py

@@ -1,6 +1,9 @@
 from datetime import date
+
 from django.conf import settings
+from django.utils.hashcompat import sha_constructor
 from django.utils.http import int_to_base36, base36_to_int
+from django.utils.crypto import constant_time_compare, salted_hmac
 
 class PasswordResetTokenGenerator(object):
     """
@@ -30,7 +33,11 @@ class PasswordResetTokenGenerator(object):
             return False
 
         # Check that the timestamp/uid has not been tampered with
-        if self._make_token_with_timestamp(user, ts) != token:
+        if not constant_time_compare(self._make_token_with_timestamp(user, ts), token):
+            # Fallback to Django 1.2 method for compatibility.
+            # PendingDeprecationWarning <- here to remind us to remove this in
+            # Django 1.5
+            if not constant_time_compare(self._make_token_with_timestamp_old(user, ts), token):
                 return False
 
         # Check the timestamp is within limit
@@ -50,7 +57,16 @@ class PasswordResetTokenGenerator(object):
         # last_login will also change), we produce a hash that will be
         # invalid as soon as it is used.
         # We limit the hash to 20 chars to keep URL short
-        from django.utils.hashcompat import sha_constructor
+        key_salt = "django.contrib.auth.tokens.PasswordResetTokenGenerator"
+        value = unicode(user.id) + \
+            user.password + user.last_login.strftime('%Y-%m-%d %H:%M:%S') + \
+            unicode(timestamp)
+        hash = salted_hmac(key_salt, value).hexdigest()[::2]
+        return "%s-%s" % (ts_b36, hash)
+
+    def _make_token_with_timestamp_old(self, user, timestamp):
+        # The Django 1.2 method
+        ts_b36 = int_to_base36(timestamp)
         hash = sha_constructor(settings.SECRET_KEY + unicode(user.id) +
                                user.password + user.last_login.strftime('%Y-%m-%d %H:%M:%S') +
                                unicode(timestamp)).hexdigest()[::2]

django/contrib/comments/forms.py

@@ -6,6 +6,7 @@ from django.forms.util import ErrorDict
 from django.conf import settings
 from django.contrib.contenttypes.models import ContentType
 from models import Comment
+from django.utils.crypto import salted_hmac, constant_time_compare
 from django.utils.encoding import force_unicode
 from django.utils.hashcompat import sha_constructor
 from django.utils.text import get_text_list
@@ -46,7 +47,12 @@ class CommentSecurityForm(forms.Form):
         }
         expected_hash = self.generate_security_hash(**security_hash_dict)
         actual_hash = self.cleaned_data["security_hash"]
-        if expected_hash != actual_hash:
+        if not constant_time_compare(expected_hash, actual_hash):
+            # Fallback to Django 1.2 method for compatibility
+            # PendingDeprecationWarning <- here to remind us to remove this
+            # fallback in Django 1.5
+            expected_hash_old = self._generate_security_hash_old(**security_hash_dict)
+            if not constant_time_compare(expected_hash_old, actual_hash):
                 raise forms.ValidationError("Security hash check failed.")
         return actual_hash
 
@@ -82,7 +88,17 @@ class CommentSecurityForm(forms.Form):
         return self.generate_security_hash(**initial_security_dict)
 
     def generate_security_hash(self, content_type, object_pk, timestamp):
+        """
+        Generate a HMAC security hash from the provided info.
+        """
+        info = (content_type, object_pk, timestamp)
+        key_salt = "django.contrib.forms.CommentSecurityForm"
+        value = "-".join(info)
+        return salted_hmac(key_salt, value).hexdigest()
+
+    def _generate_security_hash_old(self, content_type, object_pk, timestamp):
         """Generate a (SHA1) security hash from the provided info."""
+        # Django 1.2 compatibility
         info = (content_type, object_pk, timestamp, settings.SECRET_KEY)
         return sha_constructor("".join(info)).hexdigest()
 

django/contrib/formtools/preview.py

@@ -9,6 +9,7 @@ from django.http import Http404
 from django.shortcuts import render_to_response
 from django.template.context import RequestContext
 from django.utils.hashcompat import md5_constructor
+from django.utils.crypto import constant_time_compare
 from django.contrib.formtools.utils import security_hash
 
 AUTO_ID = 'formtools_%s' # Each form here uses this as its auto_id parameter.
@@ -67,11 +68,33 @@ class FormPreview(object):
         else:
             return render_to_response(self.form_template, context, context_instance=RequestContext(request))
 
+    def _check_security_hash(self, token, request, form):
+        expected = self.security_hash(request, form)
+        if constant_time_compare(token, expected):
+            return True
+        else:
+            # Fall back to Django 1.2 method, for compatibility with forms that
+            # are in the middle of being used when the upgrade occurs. However,
+            # we don't want to do this fallback if a subclass has provided their
+            # own security_hash method - because they might have implemented a
+            # more secure method, and this would punch a hole in that.
+
+            # PendingDeprecationWarning <- left here to remind us that this
+            # compatibility fallback should be removed in Django 1.5
+            FormPreview_expected = FormPreview.security_hash(self, request, form)
+            if expected == FormPreview_expected:
+                # They didn't override security_hash, do the fallback:
+                old_expected = security_hash(request, form)
+                return constant_time_compare(token, old_expected)
+            else:
+                return False
+
     def post_post(self, request):
         "Validates the POST data. If valid, calls done(). Else, redisplays form."
         f = self.form(request.POST, auto_id=AUTO_ID)
         if f.is_valid():
-            if self.security_hash(request, f) != request.POST.get(self.unused_name('hash')):
+            if not self._check_security_hash(request.POST.get(self.unused_name('hash'), ''),
+                                             request, f):
                 return self.failed_hash(request) # Security hash failed.
             return self.done(request, f.cleaned_data)
         else:

django/contrib/formtools/test_urls.py

@@ -1,10 +0,0 @@
-"""
-This is a URLconf to be loaded by tests.py. Add any URLs needed for tests only.
-"""
-
-from django.conf.urls.defaults import *
-from django.contrib.formtools.tests import *
-
-urlpatterns = patterns('',
-                       (r'^test1/', TestFormPreview(TestForm)),
-                      )

django/contrib/formtools/tests/__init__.py

@@ -1,23 +1,37 @@
+import os
+
 from django import forms
 from django import http
+from django.conf import settings
 from django.contrib.formtools import preview, wizard, utils
 from django.test import TestCase
 from django.utils import unittest
 
 success_string = "Done was called!"
 
+
 class TestFormPreview(preview.FormPreview):
 
     def done(self, request, cleaned_data):
         return http.HttpResponse(success_string)
 
+
 class TestForm(forms.Form):
     field1 = forms.CharField()
     field1_ = forms.CharField()
     bool1 = forms.BooleanField(required=False)
 
+
+class UserSecuredFormPreview(TestFormPreview):
+    """
+    FormPreview with a custum security_hash method
+    """
+    def security_hash(self, request, form):
+        return "123"
+
+
 class PreviewTests(TestCase):
-    urls = 'django.contrib.formtools.test_urls'
+    urls = 'django.contrib.formtools.tests.urls'
 
     def setUp(self):
         # Create a FormPreview instance to share between tests
@@ -102,6 +116,39 @@ class PreviewTests(TestCase):
         response = self.client.post('/test1/', self.test_data)
         self.assertEqual(response.content, success_string)
 
+    def test_form_submit_django12_hash(self):
+        """
+        Test contrib.formtools.preview form submittal, using the hash function
+        used in Django 1.2
+        """
+        # Pass strings for form submittal and add stage variable to
+        # show we previously saw first stage of the form.
+        self.test_data.update({'stage':2})
+        response = self.client.post('/test1/', self.test_data)
+        self.failIfEqual(response.content, success_string)
+        hash = utils.security_hash(None, TestForm(self.test_data))
+        self.test_data.update({'hash': hash})
+        response = self.client.post('/test1/', self.test_data)
+        self.assertEqual(response.content, success_string)
+
+
+    def test_form_submit_django12_hash_custom_hash(self):
+        """
+        Test contrib.formtools.preview form submittal, using the hash function
+        used in Django 1.2 and a custom security_hash method.
+        """
+        # Pass strings for form submittal and add stage variable to
+        # show we previously saw first stage of the form.
+        self.test_data.update({'stage':2})
+        response = self.client.post('/test2/', self.test_data)
+        self.assertEqual(response.status_code, 200)
+        self.failIfEqual(response.content, success_string)
+        hash = utils.security_hash(None, TestForm(self.test_data))
+        self.test_data.update({'hash': hash})
+        response = self.client.post('/test2/', self.test_data)
+        self.failIfEqual(response.content, success_string)
+
+
 class SecurityHashTests(unittest.TestCase):
 
     def test_textfield_hash(self):
@@ -127,10 +174,41 @@ class SecurityHashTests(unittest.TestCase):
         hash2 = utils.security_hash(None, f2)
         self.assertEqual(hash1, hash2)
 
+
+class FormHmacTests(unittest.TestCase):
+    """
+    Same as SecurityHashTests, but with form_hmac
+    """
+
+    def test_textfield_hash(self):
+        """
+        Regression test for #10034: the hash generation function should ignore
+        leading/trailing whitespace so as to be friendly to broken browsers that
+        submit it (usually in textareas).
+        """
+        f1 = HashTestForm({'name': 'joe', 'bio': 'Nothing notable.'})
+        f2 = HashTestForm({'name': '  joe', 'bio': 'Nothing notable.  '})
+        hash1 = utils.form_hmac(f1)
+        hash2 = utils.form_hmac(f2)
+        self.assertEqual(hash1, hash2)
+
+    def test_empty_permitted(self):
+        """
+        Regression test for #10643: the security hash should allow forms with
+        empty_permitted = True, or forms where data has not changed.
+        """
+        f1 = HashTestBlankForm({})
+        f2 = HashTestForm({}, empty_permitted=True)
+        hash1 = utils.form_hmac(f1)
+        hash2 = utils.form_hmac(f2)
+        self.assertEqual(hash1, hash2)
+
+
 class HashTestForm(forms.Form):
     name = forms.CharField()
     bio = forms.CharField()
 
+
 class HashTestBlankForm(forms.Form):
     name = forms.CharField(required=False)
     bio = forms.CharField(required=False)
@@ -139,20 +217,38 @@ class HashTestBlankForm(forms.Form):
 # FormWizard tests
 #
 
+
 class WizardPageOneForm(forms.Form):
     field = forms.CharField()
 
+
 class WizardPageTwoForm(forms.Form):
     field = forms.CharField()
 
+
+class WizardPageThreeForm(forms.Form):
+    field = forms.CharField()
+
+
 class WizardClass(wizard.FormWizard):
-    def render_template(self, *args, **kw):
+
-        return http.HttpResponse("")
+    def get_template(self, step):
+        return 'formwizard/wizard.html'
 
     def done(self, request, cleaned_data):
         return http.HttpResponse(success_string)
 
+
+class UserSecuredWizardClass(WizardClass):
+    """
+    Wizard with a custum security_hash method
+    """
+    def security_hash(self, request, form):
+        return "123"
+
+
 class DummyRequest(http.HttpRequest):
+
     def __init__(self, POST=None):
         super(DummyRequest, self).__init__()
         self.method = POST and "POST" or "GET"
@@ -160,22 +256,87 @@ class DummyRequest(http.HttpRequest):
             self.POST.update(POST)
         self._dont_enforce_csrf_checks = True
 
+
 class WizardTests(TestCase):
+    urls = 'django.contrib.formtools.tests.urls'
+
+    def setUp(self):
+        self.old_TEMPLATE_DIRS = settings.TEMPLATE_DIRS
+        settings.TEMPLATE_DIRS = (
+            os.path.join(
+                os.path.dirname(__file__),
+                'templates'
+            ),
+        )
+        # Use a known SECRET_KEY to make security_hash tests deterministic
+        self.old_SECRET_KEY = settings.SECRET_KEY
+        settings.SECRET_KEY = "123"
+
+    def tearDown(self):
+        settings.TEMPLATE_DIRS = self.old_TEMPLATE_DIRS
+        settings.SECRET_KEY = self.old_SECRET_KEY
+
     def test_step_starts_at_zero(self):
         """
         step should be zero for the first form
         """
-        wizard = WizardClass([WizardPageOneForm, WizardPageTwoForm])
+        response = self.client.get('/wizard/')
-        request = DummyRequest()
+        self.assertEquals(0, response.context['step0'])
-        wizard(request)
-        self.assertEquals(0, wizard.step)
 
     def test_step_increments(self):
         """
         step should be incremented when we go to the next page
         """
-        wizard = WizardClass([WizardPageOneForm, WizardPageTwoForm])
+        response = self.client.post('/wizard/', {"0-field":"test", "wizard_step":"0"})
-        request = DummyRequest(POST={"0-field":"test", "wizard_step":"0"})
+        self.assertEquals(1, response.context['step0'])
-        response = wizard(request)
-        self.assertEquals(1, wizard.step)
 
+    def test_bad_hash(self):
+        """
+        Form should not advance if the hash is missing or bad
+        """
+        response = self.client.post('/wizard/',
+                                    {"0-field":"test",
+                                     "1-field":"test2",
+                                     "wizard_step": "1"})
+        self.assertEquals(0, response.context['step0'])
+
+    def test_good_hash_django12(self):
+        """
+        Form should advance if the hash is present and good, as calculated using
+        django 1.2 method.
+        """
+        # We are hard-coding a hash value here, but that is OK, since we want to
+        # ensure that we don't accidentally change the algorithm.
+        data = {"0-field": "test",
+                "1-field": "test2",
+                "hash_0": "2fdbefd4c0cad51509478fbacddf8b13",
+                "wizard_step": "1"}
+        response = self.client.post('/wizard/', data)
+        self.assertEquals(2, response.context['step0'])
+
+    def test_good_hash_django12_subclass(self):
+        """
+        The Django 1.2 method of calulating hashes should *not* be used as a
+        fallback if the FormWizard subclass has provided their own method
+        of calculating a hash.
+        """
+        # We are hard-coding a hash value here, but that is OK, since we want to
+        # ensure that we don't accidentally change the algorithm.
+        data = {"0-field": "test",
+                "1-field": "test2",
+                "hash_0": "2fdbefd4c0cad51509478fbacddf8b13",
+                "wizard_step": "1"}
+        response = self.client.post('/wizard2/', data)
+        self.assertEquals(0, response.context['step0'])
+
+    def test_good_hash_current(self):
+        """
+        Form should advance if the hash is present and good, as calculated using
+        current method.
+        """
+        data = {"0-field": "test",
+                "1-field": "test2",
+                "hash_0": "7e9cea465f6a10a6fb47fcea65cb9a76350c9a5c",
+                "wizard_step": "1"}
+        response = self.client.post('/wizard/', data)
+        self.assertEquals(2, response.context['step0'])

django/contrib/formtools/tests/templates/formwizard/wizard.html

@@ -0,0 +1,9 @@
+<p>Step {{ step }} of {{ step_count }}</p>
+<form action="." method="post">{% csrf_token %}
+<table>
+{{ form }}
+</table>
+<input type="hidden" name="{{ step_field }}" value="{{ step0 }}" />
+{{ previous_fields|safe }}
+<input type="submit">
+</form>

django/contrib/formtools/tests/urls.py

@@ -0,0 +1,17 @@
+"""
+This is a URLconf to be loaded by tests.py. Add any URLs needed for tests only.
+"""
+
+from django.conf.urls.defaults import *
+from django.contrib.formtools.tests import *
+
+urlpatterns = patterns('',
+                       (r'^test1/', TestFormPreview(TestForm)),
+                       (r'^test2/', UserSecuredFormPreview(TestForm)),
+                       (r'^wizard/$', WizardClass([WizardPageOneForm,
+                                                   WizardPageTwoForm,
+                                                   WizardPageThreeForm])),
+                       (r'^wizard2/$', UserSecuredWizardClass([WizardPageOneForm,
+                                                               WizardPageTwoForm,
+                                                               WizardPageThreeForm]))
+                      )

django/contrib/formtools/utils.py

@@ -4,8 +4,10 @@ except ImportError:
     import pickle
 
 from django.conf import settings
-from django.utils.hashcompat import md5_constructor
 from django.forms import BooleanField
+from django.utils.crypto import salted_hmac
+from django.utils.hashcompat import md5_constructor
+
 
 def security_hash(request, form, *args):
     """
@@ -15,7 +17,9 @@ def security_hash(request, form, *args):
     order, pickles the result with the SECRET_KEY setting, then takes an md5
     hash of that.
     """
-
+    import warnings
+    warnings.warn("security_hash is deprecated; use form_hmac instead",
+                  PendingDeprecationWarning)
     data = []
     for bf in form:
         # Get the value from the form data. If the form allows empty or hasn't
@@ -37,3 +41,23 @@ def security_hash(request, form, *args):
 
     return md5_constructor(pickled).hexdigest()
 
+
+def form_hmac(form):
+    """
+    Calculates a security hash for the given Form instance.
+    """
+    data = []
+    for bf in form:
+        # Get the value from the form data. If the form allows empty or hasn't
+        # changed then don't call clean() to avoid trigger validation errors.
+        if form.empty_permitted and not form.has_changed():
+            value = bf.data or ''
+        else:
+            value = bf.field.clean(bf.data) or ''
+        if isinstance(value, basestring):
+            value = value.strip()
+        data.append((bf.name, value))
+
+    pickled = pickle.dumps(data, pickle.HIGHEST_PROTOCOL)
+    key_salt = 'django.contrib.formtools'
+    return salted_hmac(key_salt, pickled).hexdigest()

django/contrib/formtools/wizard.py

@@ -8,12 +8,13 @@ import cPickle as pickle
 
 from django import forms
 from django.conf import settings
+from django.contrib.formtools.utils import security_hash, form_hmac
 from django.http import Http404
 from django.shortcuts import render_to_response
 from django.template.context import RequestContext
+from django.utils.crypto import constant_time_compare
 from django.utils.hashcompat import md5_constructor
 from django.utils.translation import ugettext_lazy as _
-from django.contrib.formtools.utils import security_hash
 from django.utils.decorators import method_decorator
 from django.views.decorators.csrf import csrf_protect
 
@@ -53,6 +54,27 @@ class FormWizard(object):
         # hook methods might alter self.form_list.
         return len(self.form_list)
 
+    def _check_security_hash(self, token, request, form):
+        expected = self.security_hash(request, form)
+        if constant_time_compare(token, expected):
+            return True
+        else:
+            # Fall back to Django 1.2 method, for compatibility with forms that
+            # are in the middle of being used when the upgrade occurs. However,
+            # we don't want to do this fallback if a subclass has provided their
+            # own security_hash method - because they might have implemented a
+            # more secure method, and this would punch a hole in that.
+
+            # PendingDeprecationWarning <- left here to remind us that this
+            # compatibility fallback should be removed in Django 1.5
+            FormWizard_expected = FormWizard.security_hash(self, request, form)
+            if expected == FormWizard_expected:
+                # They didn't override security_hash, do the fallback:
+                old_expected = security_hash(request, form)
+                return constant_time_compare(token, old_expected)
+            else:
+                return False
+
     @method_decorator(csrf_protect)
     def __call__(self, request, *args, **kwargs):
         """
@@ -72,7 +94,7 @@ class FormWizard(object):
         # TODO: Move "hash_%d" to a method to make it configurable.
         for i in range(current_step):
             form = self.get_form(i, request.POST)
-            if request.POST.get("hash_%d" % i, '') != self.security_hash(request, form):
+            if not self._check_security_hash(request.POST.get("hash_%d" % i, ''), request, form):
                 return self.render_hash_failure(request, i)
             self.process_step(request, form, i)
 
@@ -95,6 +117,21 @@ class FormWizard(object):
                 # Validate all the forms. If any of them fail validation, that
                 # must mean the validator relied on some other input, such as
                 # an external Web site.
+
+                # It is also possible that validation might fail under certain
+                # attack situations: an attacker might be able to bypass previous
+                # stages, and generate correct security hashes for all the
+                # skipped stages by virtue of:
+                #  1) having filled out an identical form which doesn't have the
+                #     validation (and does something different at the end),
+                #  2) or having filled out a previous version of the same form
+                #     which had some validation missing,
+                #  3) or previously having filled out the form when they had
+                #     more privileges than they do now.
+                #
+                # Since the hashes only take into account values, and not other
+                # other validation the form might do, we must re-do validation
+                # now for security reasons.
                 for i, f in enumerate(final_form_list):
                     if not f.is_valid():
                         return self.render_revalidation_failure(request, i, f)
@@ -155,7 +192,7 @@ class FormWizard(object):
         Subclasses may want to take into account request-specific information,
         such as the IP address.
         """
-        return security_hash(request, form)
+        return form_hmac(form)
 
     def determine_step(self, request, *args, **kwargs):
         """

django/contrib/messages/storage/cookie.py

@@ -1,11 +1,9 @@
-import hmac
-
 from django.conf import settings
 from django.contrib.messages import constants
 from django.contrib.messages.storage.base import BaseStorage, Message
 from django.http import CompatCookie
 from django.utils import simplejson as json
-from django.utils.hashcompat import sha_hmac
+from django.utils.crypto import salted_hmac, constant_time_compare
 
 
 class MessageEncoder(json.JSONEncoder):
@@ -111,8 +109,8 @@ class CookieStorage(BaseStorage):
         Creates an HMAC/SHA1 hash based on the value and the project setting's
         SECRET_KEY, modified to make it unique for the present purpose.
         """
-        key = 'django.contrib.messages' + settings.SECRET_KEY
+        key_salt = 'django.contrib.messages'
-        return hmac.new(key, value, sha_hmac).hexdigest()
+        return salted_hmac(key_salt, value).hexdigest()
 
     def _encode(self, messages, encode_empty=False):
         """
@@ -139,7 +137,7 @@ class CookieStorage(BaseStorage):
         bits = data.split('$', 1)
         if len(bits) == 2:
             hash, value = bits
-            if hash == self._hash(value):
+            if constant_time_compare(hash, self._hash(value)):
                 try:
                     # If we get here (and the JSON decode works), everything is
                     # good. In any other case, drop back and return None.

django/contrib/sessions/backends/base.py

@@ -12,6 +12,7 @@ except ImportError:
 from django.conf import settings
 from django.core.exceptions import SuspiciousOperation
 from django.utils.hashcompat import md5_constructor
+from django.utils.crypto import constant_time_compare, salted_hmac
 
 # Use the system (hardware-based) random number generator if it exists.
 if hasattr(random, 'SystemRandom'):
@@ -83,24 +84,46 @@ class SessionBase(object):
     def delete_test_cookie(self):
         del self[self.TEST_COOKIE_NAME]
 
+    def _hash(self, value):
+        key_salt = "django.contrib.sessions" + self.__class__.__name__
+        return salted_hmac(key_salt, value).hexdigest()
+
     def encode(self, session_dict):
         "Returns the given session dictionary pickled and encoded as a string."
         pickled = pickle.dumps(session_dict, pickle.HIGHEST_PROTOCOL)
-        pickled_md5 = md5_constructor(pickled + settings.SECRET_KEY).hexdigest()
+        hash = self._hash(pickled)
-        return base64.encodestring(pickled + pickled_md5)
+        return base64.encodestring(hash + ":" + pickled)
 
     def decode(self, session_data):
         encoded_data = base64.decodestring(session_data)
-        pickled, tamper_check = encoded_data[:-32], encoded_data[-32:]
-        if md5_constructor(pickled + settings.SECRET_KEY).hexdigest() != tamper_check:
-            raise SuspiciousOperation("User tampered with session cookie.")
         try:
+            # could produce ValueError if there is no ':'
+            hash, pickled = encoded_data.split(':', 1)
+            expected_hash = self._hash(pickled)
+            if not constant_time_compare(hash, expected_hash):
+                raise SuspiciousOperation("Session data corrupted")
+            else:
                 return pickle.loads(pickled)
+        except Exception:
+            # ValueError, SuspiciousOperation, unpickling exceptions
+            # Fall back to Django 1.2 method
+            # PendingDeprecationWarning <- here to remind us to
+            # remove this fallback in Django 1.5
+            try:
+                return self._decode_old(session_data)
+            except Exception:
                 # Unpickling can cause a variety of exceptions. If something happens,
                 # just return an empty dictionary (an empty session).
-        except:
                 return {}
 
+    def _decode_old(self, session_data):
+        encoded_data = base64.decodestring(session_data)
+        pickled, tamper_check = encoded_data[:-32], encoded_data[-32:]
+        if not constant_time_compare(md5_constructor(pickled + settings.SECRET_KEY).hexdigest(),
+                                     tamper_check):
+            raise SuspiciousOperation("User tampered with session cookie.")
+        return pickle.loads(pickled)
+
     def update(self, dict_):
         self._session.update(dict_)
         self.modified = True

django/contrib/sessions/tests.py

@@ -1,4 +1,6 @@
+import base64
 from datetime import datetime, timedelta
+import pickle
 import shutil
 import tempfile
 
@@ -12,6 +14,7 @@ from django.contrib.sessions.models import Session
 from django.core.exceptions import ImproperlyConfigured
 from django.test import TestCase
 from django.utils import unittest
+from django.utils.hashcompat import md5_constructor
 
 
 class SessionTestsMixin(object):
@@ -237,6 +240,24 @@ class SessionTestsMixin(object):
         finally:
             settings.SESSION_EXPIRE_AT_BROWSER_CLOSE = original_expire_at_browser_close
 
+    def test_decode(self):
+        # Ensure we can decode what we encode
+        data = {'a test key': 'a test value'}
+        encoded = self.session.encode(data)
+        self.assertEqual(self.session.decode(encoded), data)
+
+    def test_decode_django12(self):
+        # Ensure we can decode values encoded using Django 1.2
+        # Hard code the Django 1.2 method here:
+        def encode(session_dict):
+            pickled = pickle.dumps(session_dict, pickle.HIGHEST_PROTOCOL)
+            pickled_md5 = md5_constructor(pickled + settings.SECRET_KEY).hexdigest()
+            return base64.encodestring(pickled + pickled_md5)
+
+        data = {'a test key': 'a test value'}
+        encoded = encode(data)
+        self.assertEqual(self.session.decode(encoded), data)
+
 
 class DatabaseSessionTests(SessionTestsMixin, TestCase):
 

django/middleware/csrf.py

@@ -15,6 +15,7 @@ from django.utils.cache import patch_vary_headers
 from django.utils.hashcompat import md5_constructor
 from django.utils.log import getLogger
 from django.utils.safestring import mark_safe
+from django.utils.crypto import constant_time_compare
 
 _POST_FORM_RE = \
     re.compile(r'(<form\W[^>]*\bmethod\s*=\s*(\'|"|)POST(\'|"|)\b[^>]*>)', re.IGNORECASE)
@@ -216,8 +217,8 @@ class CsrfViewMiddleware(object):
                 csrf_token = request.META["CSRF_COOKIE"]
 
             # check incoming token
-            request_csrf_token = request.POST.get('csrfmiddlewaretoken', None)
+            request_csrf_token = request.POST.get('csrfmiddlewaretoken', '')
-            if request_csrf_token != csrf_token:
+            if not constant_time_compare(request_csrf_token, csrf_token):
                 if cookie_is_new:
                     # probably a problem setting the CSRF cookie
                     logger.warning('Forbidden (%s): %s' % (REASON_NO_CSRF_COOKIE, request.path),

django/utils/crypto.py

@@ -0,0 +1,45 @@
+"""
+Django's standard crypto functions and utilities.
+"""
+import hmac
+
+from django.conf import settings
+from django.utils.hashcompat import sha_constructor
+
+
+def salted_hmac(key_salt, value, secret=None):
+    """
+    Returns the HMAC-SHA1 of 'value', using a key generated from key_salt and a
+    secret (which defaults to settings.SECRET_KEY).
+
+    A different key_salt should be passed in for every application of HMAC.
+    """
+    if secret is None:
+        secret = settings.SECRET_KEY
+
+    # We need to generate a derived key from our base key.  We can do this by
+    # passing the key_salt and our base key through a pseudo-random function and
+    # SHA1 works nicely.
+
+    key = sha_constructor(key_salt + secret).digest()
+
+    # If len(key_salt + secret) > sha_constructor().block_size, the above
+    # line is redundant and could be replaced by key = key_salt + secret, since
+    # the hmac module does the same thing for keys longer than the block size.
+    # However, we need to ensure that we *always* do this.
+
+    return hmac.new(key, msg=value, digestmod=sha_constructor)
+
+
+def constant_time_compare(val1, val2):
+    """
+    Returns True if the two strings are equal, False otherwise.
+
+    The time taken is independent of the number of characters that match.
+    """
+    if len(val1) != len(val2):
+        return False
+    result = 0
+    for x, y in zip(val1, val2):
+        result |= ord(x) ^ ord(y)
+    return result == 0

docs/internals/deprecation.txt

@@ -114,6 +114,10 @@ their deprecation, as per the :ref:`Django deprecation policy
           :class:`~django.test.simple.DjangoTestRunner` will be removed in
           favor of using the unittest-native class.
 
+        * The undocumented function
+          :func:`django.contrib.formtools.utils.security_hash`
+          is deprecated, in favour of :func:`django.contrib.formtools.utils.form_hmac`
+
     * 2.0
         * ``django.views.defaults.shortcut()``. This function has been moved
           to ``django.contrib.contenttypes.views.shortcut()`` as part of the

docs/ref/contrib/formtools/form-wizard.txt

@@ -240,7 +240,7 @@ Advanced ``FormWizard`` methods
     Calculates the security hash for the given request object and
     :class:`~django.forms.Form` instance.
 
-    By default, this uses an MD5 hash of the form data and your
+    By default, this generates a SHA1 HMAC using your form data and your
     :setting:`SECRET_KEY` setting. It's rare that somebody would need to
     override this.
 

tests/regressiontests/comment_tests/tests/comment_form_tests.py

@@ -2,6 +2,7 @@ import time
 from django.conf import settings
 from django.contrib.comments.models import Comment
 from django.contrib.comments.forms import CommentForm
+from django.utils.hashcompat import sha_constructor
 from regressiontests.comment_tests.models import Article
 from regressiontests.comment_tests.tests import CommentTestCase
 
@@ -43,6 +44,23 @@ class CommentFormTests(CommentTestCase):
     def testObjectPKTampering(self):
         self.tamperWithForm(object_pk="3")
 
+    def testDjango12Hash(self):
+        # Ensure we can use the hashes generated by Django 1.2
+        a = Article.objects.get(pk=1)
+        d = self.getValidData(a)
+
+        content_type = d['content_type']
+        object_pk = d['object_pk']
+        timestamp = d['timestamp']
+
+        # The Django 1.2 method hard-coded here:
+        info = (content_type, object_pk, timestamp, settings.SECRET_KEY)
+        security_hash = sha_constructor("".join(info)).hexdigest()
+
+        d['security_hash'] = security_hash
+        f = CommentForm(a, data=d)
+        self.assertTrue(f.is_valid(), f.errors)
+
     def testSecurityErrors(self):
         f = self.tamperWithForm(honeypot="I am a robot")
         self.assert_("honeypot" in f.security_errors())