Fixed #20922 -- Allowed customizing the serializer used by contrib.sessions

Added settings.SESSION_SERIALIZER which is the import path of a serializer
to use for sessions.

Thanks apollo13, carljm, shaib, akaariai, charettes, and dstufft for reviews.

django/conf/global_settings.py

@@ -475,6 +475,7 @@ SESSION_SAVE_EVERY_REQUEST = False                      # Whether to save the se
 SESSION_EXPIRE_AT_BROWSER_CLOSE = False                 # Whether a user's session cookie expires when the Web browser is closed.
 SESSION_ENGINE = 'django.contrib.sessions.backends.db'  # The module to store session data
 SESSION_FILE_PATH = None                                # Directory to store session files if using the file session module. If None, the backend will use a sensible default.
+SESSION_SERIALIZER = 'django.contrib.sessions.serializers.JSONSerializer'  # class to serialize session data
 
 #########
 # CACHE #

django/contrib/messages/storage/session.py

@@ -1,4 +1,8 @@
+import json
+
 from django.contrib.messages.storage.base import BaseStorage
+from django.contrib.messages.storage.cookie import MessageEncoder, MessageDecoder
+from django.utils import six
 
 
 class SessionStorage(BaseStorage):
@@ -20,14 +24,23 @@ class SessionStorage(BaseStorage):
         always stores everything it is given, so return True for the
         all_retrieved flag.
         """
-        return self.request.session.get(self.session_key), True
+        return self.deserialize_messages(self.request.session.get(self.session_key)), True
 
     def _store(self, messages, response, *args, **kwargs):
         """
         Stores a list of messages to the request's session.
         """
         if messages:
-            self.request.session[self.session_key] = messages
+            self.request.session[self.session_key] = self.serialize_messages(messages)
         else:
             self.request.session.pop(self.session_key, None)
         return []
+
+    def serialize_messages(self, messages):
+        encoder = MessageEncoder(separators=(',', ':'))
+        return encoder.encode(messages)
+
+    def deserialize_messages(self, data):
+        if data and isinstance(data, six.string_types):
+            return json.loads(data, cls=MessageDecoder)
+        return data

django/contrib/messages/tests/base.py

@@ -61,6 +61,7 @@ class BaseTests(object):
             MESSAGE_TAGS    = '',
             MESSAGE_STORAGE = '%s.%s' % (self.storage_class.__module__,
                                          self.storage_class.__name__),
+            SESSION_SERIALIZER = 'django.contrib.sessions.serializers.JSONSerializer',
         )
         self.settings_override.enable()
 

django/contrib/messages/tests/test_session.py

@@ -11,13 +11,13 @@ def set_session_data(storage, messages):
     Sets the messages into the backend request's session and remove the
     backend's loaded data cache.
     """
-    storage.request.session[storage.session_key] = messages
+    storage.request.session[storage.session_key] = storage.serialize_messages(messages)
     if hasattr(storage, '_loaded_data'):
         del storage._loaded_data
 
 
 def stored_session_messages_count(storage):
-    data = storage.request.session.get(storage.session_key, [])
+    data = storage.deserialize_messages(storage.request.session.get(storage.session_key, []))
     return len(data)
 
 

django/contrib/sessions/backends/base.py

@@ -3,11 +3,6 @@ from __future__ import unicode_literals
 import base64
 from datetime import datetime, timedelta
 import logging
-
-try:
-    from django.utils.six.moves import cPickle as pickle
-except ImportError:
-    import pickle
 import string
 
 from django.conf import settings
@@ -17,6 +12,7 @@ from django.utils.crypto import get_random_string
 from django.utils.crypto import salted_hmac
 from django.utils import timezone
 from django.utils.encoding import force_bytes, force_text
+from django.utils.module_loading import import_by_path
 
 from django.contrib.sessions.exceptions import SuspiciousSession
 
@@ -42,6 +38,7 @@ class SessionBase(object):
         self._session_key = session_key
         self.accessed = False
         self.modified = False
+        self.serializer = import_by_path(settings.SESSION_SERIALIZER)
 
     def __contains__(self, key):
         return key in self._session
@@ -86,21 +83,21 @@ class SessionBase(object):
         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)
-        hash = self._hash(pickled)
-        return base64.b64encode(hash.encode() + b":" + pickled).decode('ascii')
+        "Returns the given session dictionary serialized and encoded as a string."
+        serialized = self.serializer().dumps(session_dict)
+        hash = self._hash(serialized)
+        return base64.b64encode(hash.encode() + b":" + serialized).decode('ascii')
 
     def decode(self, session_data):
         encoded_data = base64.b64decode(force_bytes(session_data))
         try:
             # could produce ValueError if there is no ':'
-            hash, pickled = encoded_data.split(b':', 1)
-            expected_hash = self._hash(pickled)
+            hash, serialized = encoded_data.split(b':', 1)
+            expected_hash = self._hash(serialized)
             if not constant_time_compare(hash.decode(), expected_hash):
                 raise SuspiciousSession("Session data corrupted")
             else:
-                return pickle.loads(pickled)
+                return self.serializer().loads(serialized)
         except Exception as e:
             # ValueError, SuspiciousOperation, unpickling exceptions. If any of
             # these happen, just return an empty dictionary (an empty session).

django/contrib/sessions/backends/signed_cookies.py

@@ -1,26 +1,9 @@
-try:
-    from django.utils.six.moves import cPickle as pickle
-except ImportError:
-    import pickle
-
 from django.conf import settings
 from django.core import signing
 
 from django.contrib.sessions.backends.base import SessionBase
 
 
-class PickleSerializer(object):
-    """
-    Simple wrapper around pickle to be used in signing.dumps and
-    signing.loads.
-    """
-    def dumps(self, obj):
-        return pickle.dumps(obj, pickle.HIGHEST_PROTOCOL)
-
-    def loads(self, data):
-        return pickle.loads(data)
-
-
 class SessionStore(SessionBase):
 
     def load(self):
@@ -31,7 +14,7 @@ class SessionStore(SessionBase):
         """
         try:
             return signing.loads(self.session_key,
-                serializer=PickleSerializer,
+                serializer=self.serializer,
                 # This doesn't handle non-default expiry dates, see #19201
                 max_age=settings.SESSION_COOKIE_AGE,
                 salt='django.contrib.sessions.backends.signed_cookies')
@@ -91,7 +74,7 @@ class SessionStore(SessionBase):
         session_cache = getattr(self, '_session_cache', {})
         return signing.dumps(session_cache, compress=True,
             salt='django.contrib.sessions.backends.signed_cookies',
-            serializer=PickleSerializer)
+            serializer=self.serializer)
 
     @classmethod
     def clear_expired(cls):

django/contrib/sessions/models.py

@@ -5,7 +5,7 @@ from django.utils.translation import ugettext_lazy as _
 class SessionManager(models.Manager):
     def encode(self, session_dict):
         """
-        Returns the given session dictionary pickled and encoded as a string.
+        Returns the given session dictionary serialized and encoded as a string.
         """
         return SessionStore().encode(session_dict)
 

django/contrib/sessions/serializers.py

@@ -0,0 +1,20 @@
+from django.core.signing import JSONSerializer as BaseJSONSerializer
+try:
+    from django.utils.six.moves import cPickle as pickle
+except ImportError:
+    import pickle
+
+
+class PickleSerializer(object):
+    """
+    Simple wrapper around pickle to be used in signing.dumps and
+    signing.loads.
+    """
+    def dumps(self, obj):
+        return pickle.dumps(obj, pickle.HIGHEST_PROTOCOL)
+
+    def loads(self, data):
+        return pickle.loads(data)
+
+
+JSONSerializer = BaseJSONSerializer

django/contrib/sessions/tests.py

@@ -285,6 +285,10 @@ class SessionTestsMixin(object):
 
 
     def test_actual_expiry(self):
+        # this doesn't work with JSONSerializer (serializing timedelta)
+        with override_settings(SESSION_SERIALIZER='django.contrib.sessions.serializers.PickleSerializer'):
+            self.session = self.backend()  # reinitialize after overriding settings
+
             # Regression test for #19200
             old_session_key = None
             new_session_key = None

docs/ref/settings.txt

@@ -2403,7 +2403,7 @@ SESSION_ENGINE
 
 Default: ``django.contrib.sessions.backends.db``
 
-Controls where Django stores session data. Valid values are:
+Controls where Django stores session data. Included engines are:
 
 * ``'django.contrib.sessions.backends.db'``
 * ``'django.contrib.sessions.backends.file'``
@@ -2446,6 +2446,28 @@ Whether to save the session data on every request. If this is ``False``
 (default), then the session data will only be saved if it has been modified --
 that is, if any of its dictionary values have been assigned or deleted.
 
+.. setting:: SESSION_SERIALIZER
+
+SESSION_SERIALIZER
+------------------
+
+Default: ``'django.contrib.sessions.serializers.JSONSerializer'``
+
+.. versionchanged:: 1.6
+
+    The default switched from
+    :class:`~django.contrib.sessions.serializers.PickleSerializer` to
+    :class:`~django.contrib.sessions.serializers.JSONSerializer` in Django 1.6.
+
+Full import path of a serializer class to use for serializing session data.
+Included serializers are:
+
+* ``'django.contrib.sessions.serializers.PickleSerializer'``
+* ``'django.contrib.sessions.serializers.JSONSerializer'``
+
+See :ref:`session_serialization` for details, including a warning regarding
+possible remote code execution when using
+:class:`~django.contrib.sessions.serializers.PickleSerializer`.
 
 Sites
 =====

docs/releases/1.6.txt

@@ -727,6 +727,29 @@ the ``name`` argument so it doesn't conflict with the new url::
 You can remove this url pattern after your app has been deployed with Django
 1.6 for :setting:`PASSWORD_RESET_TIMEOUT_DAYS`.
 
+Default session serialization switched to JSON
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Historically, :mod:`django.contrib.sessions` used :mod:`pickle` to serialize
+session data before storing it in the backend. If you're using the :ref:`signed
+cookie session backend<cookie-session-backend>` and :setting:`SECRET_KEY` is
+known by an attacker, the attacker could insert a string into his session
+which, when unpickled, executes arbitrary code on the server. The technique for
+doing so is simple and easily available on the internet. Although the cookie
+session storage signs the cookie-stored data to prevent tampering, a
+:setting:`SECRET_KEY` leak immediately escalates to a remote code execution
+vulnerability.
+
+This attack can be mitigated by serializing session data using JSON rather
+than :mod:`pickle`. To facilitate this, Django 1.5.3 introduced a new setting,
+:setting:`SESSION_SERIALIZER`, to customize the session serialization format.
+For backwards compatibility, this setting defaulted to using :mod:`pickle`
+in Django 1.5.3, but we've changed the default to JSON in 1.6. If you upgrade
+and switch from pickle to JSON, sessions created before the upgrade will be
+lost. While JSON serialization does not support all Python objects like
+:mod:`pickle` does, we highly recommend using JSON-serialized sessions. See the
+:ref:`session_serialization` documentation for more details.
+
 Miscellaneous
 ~~~~~~~~~~~~~
 

docs/topics/http/sessions.txt

@@ -128,8 +128,9 @@ and the :setting:`SECRET_KEY` setting.
 
 .. warning::
 
-    **If the SECRET_KEY is not kept secret, this can lead to arbitrary remote
-    code execution.**
+    **If the SECRET_KEY is not kept secret and you are using the**
+    :class:`~django.contrib.sessions.serializers.PickleSerializer`, **this can
+    lead to arbitrary remote code execution.**
 
     An attacker in possession of the :setting:`SECRET_KEY` can not only
     generate falsified session data, which your site will trust, but also
@@ -256,7 +257,9 @@ You can edit it multiple times.
         in 5 minutes.
 
       * If ``value`` is a ``datetime`` or ``timedelta`` object, the
-        session will expire at that specific date/time.
+        session will expire at that specific date/time. Note that ``datetime``
+        and ``timedelta`` values are only serializable if you are using the
+        :class:`~django.contrib.sessions.serializers.PickleSerializer`.
 
       * If ``value`` is ``0``, the user's session cookie will expire
         when the user's Web browser is closed.
@@ -301,6 +304,72 @@ You can edit it multiple times.
       Removes expired sessions from the session store. This class method is
       called by :djadmin:`clearsessions`.
 
+.. _session_serialization:
+
+Session serialization
+---------------------
+
+.. versionchanged:: 1.6
+
+Before version 1.6, Django defaulted to using :mod:`pickle` to serialize
+session data before storing it in the backend. If you're using the :ref:`signed
+cookie session backend<cookie-session-backend>` and :setting:`SECRET_KEY` is
+known by an attacker, the attacker could insert a string into his session
+which, when unpickled, executes arbitrary code on the server. The technique for
+doing so is simple and easily available on the internet. Although the cookie
+session storage signs the cookie-stored data to prevent tampering, a
+:setting:`SECRET_KEY` leak immediately escalates to a remote code execution
+vulnerability.
+
+This attack can be mitigated by serializing session data using JSON rather
+than :mod:`pickle`. To facilitate this, Django 1.5.3 introduced a new setting,
+:setting:`SESSION_SERIALIZER`, to customize the session serialization format.
+For backwards compatibility, this setting defaults to
+using :class:`django.contrib.sessions.serializers.PickleSerializer` in
+Django 1.5.x, but, for security hardening, defaults to
+:class:`django.contrib.sessions.serializers.JSONSerializer` in Django 1.6.
+Even with the caveats described in :ref:`custom-serializers`, we highly
+recommend sticking with JSON serialization *especially if you are using the
+cookie backend*.
+
+Bundled Serializers
+^^^^^^^^^^^^^^^^^^^
+
+.. class:: serializers.JSONSerializer
+
+    A wrapper around the JSON serializer from :mod:`django.core.signing`. Can
+    only serialize basic data types. See the :ref:`custom-serializers` section
+    for more details.
+
+.. class:: serializers.PickleSerializer
+
+    Supports arbitrary Python objects, but, as described above, can lead to a
+    remote code execution vulnerability if :setting:`SECRET_KEY` becomes known
+    by an attacker.
+
+.. _custom-serializers:
+
+Write Your Own Serializer
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Note that unlike :class:`~django.contrib.sessions.serializers.PickleSerializer`,
+the :class:`~django.contrib.sessions.serializers.JSONSerializer` cannot handle
+arbitrary Python data types. As is often the case, there is a trade-off between
+convenience and security. If you wish to store more advanced data types
+including ``datetime`` and ``Decimal`` in JSON backed sessions, you will need
+to write a custom serializer (or convert such values to a JSON serializable
+object before storing them in ``request.session``). While serializing these
+values is fairly straightforward
+(``django.core.serializers.json.DateTimeAwareJSONEncoder`` may be helpful),
+writing a decoder that can reliably get back the same thing that you put in is
+more fragile. For example, you run the risk of returning a ``datetime`` that
+was actually a string that just happened to be in the same format chosen for
+``datetime``\s).
+
+Your serializer class must implement two methods,
+``dumps(self, obj)`` and ``loads(self, data)``, to serialize and deserialize
+the dictionary of session data, respectively.
+
 Session object guidelines
 -------------------------
 
@@ -390,14 +459,15 @@ An API is available to manipulate session data outside of a view::
     >>> from django.contrib.sessions.backends.db import SessionStore
     >>> import datetime
     >>> s = SessionStore()
-    >>> s['last_login'] = datetime.datetime(2005, 8, 20, 13, 35, 10)
+    >>> # stored as seconds since epoch since datetimes are not serializable in JSON.
+    >>> s['last_login'] = 1376587691
     >>> s.save()
     >>> s.session_key
     '2b1189a188b44ad18c35e113ac6ceead'
 
     >>> s = SessionStore(session_key='2b1189a188b44ad18c35e113ac6ceead')
     >>> s['last_login']
-    datetime.datetime(2005, 8, 20, 13, 35, 0)
+    1376587691
 
 In order to prevent session fixation attacks, sessions keys that don't exist
 are regenerated::
@@ -543,8 +613,11 @@ behavior:
 Technical details
 =================
 
-* The session dictionary should accept any pickleable Python object. See
-  the :mod:`pickle` module for more information.
+* The session dictionary accepts any :mod:`json` serializable value when using
+  :class:`~django.contrib.sessions.serializers.JSONSerializer` or any
+  pickleable Python object when using
+  :class:`~django.contrib.sessions.serializers.PickleSerializer`. See the
+  :mod:`pickle` module for more information.
 
 * Session data is stored in a database table named ``django_session`` .
 

tests/defer_regress/tests.py

@@ -7,6 +7,7 @@ from django.contrib.sessions.backends.db import SessionStore
 from django.db.models import Count
 from django.db.models.loading import cache
 from django.test import TestCase
+from django.test.utils import override_settings
 
 from .models import (
     ResolveThis, Item, RelatedItem, Child, Leaf, Proxy, SimpleItem, Feature,
@@ -83,24 +84,6 @@ class DeferRegressionTest(TestCase):
         self.assertEqual(results[0].child.name, "c1")
         self.assertEqual(results[0].second_child.name, "c2")
 
-        # Test for #12163 - Pickling error saving session with unsaved model
-        # instances.
-        SESSION_KEY = '2b1189a188b44ad18c35e1baac6ceead'
-
-        item = Item()
-        item._deferred = False
-        s = SessionStore(SESSION_KEY)
-        s.clear()
-        s["item"] = item
-        s.save()
-
-        s = SessionStore(SESSION_KEY)
-        s.modified = True
-        s.save()
-
-        i2 = s["item"]
-        self.assertFalse(i2._deferred)
-
         # Regression for #16409 - make sure defer() and only() work with annotate()
         self.assertIsInstance(
             list(SimpleItem.objects.annotate(Count('feature')).defer('name')),
@@ -147,6 +130,27 @@ class DeferRegressionTest(TestCase):
                 cache.get_app("defer_regress"), include_deferred=True))
         )
 
+    @override_settings(SESSION_SERIALIZER='django.contrib.sessions.serializers.PickleSerializer')
+    def test_ticket_12163(self):
+        # Test for #12163 - Pickling error saving session with unsaved model
+        # instances.
+        SESSION_KEY = '2b1189a188b44ad18c35e1baac6ceead'
+
+        item = Item()
+        item._deferred = False
+        s = SessionStore(SESSION_KEY)
+        s.clear()
+        s["item"] = item
+        s.save()
+
+        s = SessionStore(SESSION_KEY)
+        s.modified = True
+        s.save()
+
+        i2 = s["item"]
+        self.assertFalse(i2._deferred)
+
+    def test_ticket_16409(self):
         # Regression for #16409 - make sure defer() and only() work with annotate()
         self.assertIsInstance(
             list(SimpleItem.objects.annotate(Count('feature')).defer('name')),