Refs #31274 -- Removed support for the pre-Django 3.1 encoding format of sessions.

Per deprecation timeline.

3 files changed, 15 insertions, 70 deletions

diff --git a/django/contrib/sessions/backends/base.py b/django/contrib/sessions/backends/base.py
index 143c08fbef..62ee1f0d89 100644
--- a/django/contrib/sessions/backends/base.py
+++ b/django/contrib/sessions/backends/base.py
@@ -1,16 +1,11 @@
-import base64
 import logging
 import string
 from datetime import datetime, timedelta
 
 from django.conf import settings
-from django.contrib.sessions.exceptions import SuspiciousSession
 from django.core import signing
-from django.core.exceptions import SuspiciousOperation
 from django.utils import timezone
-from django.utils.crypto import (
-    constant_time_compare, get_random_string, salted_hmac,
-)
+from django.utils.crypto import get_random_string
 from django.utils.module_loading import import_string
 
 # session_key should not be case sensitive because some backends can store it
@@ -91,16 +86,8 @@ class SessionBase:
     def delete_test_cookie(self):
         del self[self.TEST_COOKIE_NAME]
 
-    def _hash(self, value):
-        # RemovedInDjango40Warning: pre-Django 3.1 format will be invalid.
-        key_salt = "django.contrib.sessions" + self.__class__.__name__
-        return salted_hmac(key_salt, value).hexdigest()
-
     def encode(self, session_dict):
         "Return the given session dictionary serialized and encoded as a string."
-        # RemovedInDjango40Warning: DEFAULT_HASHING_ALGORITHM will be removed.
-        if settings.DEFAULT_HASHING_ALGORITHM == 'sha1':
-            return self._legacy_encode(session_dict)
         return signing.dumps(
             session_dict, salt=self.key_salt, serializer=self.serializer,
             compress=True,
@@ -109,44 +96,14 @@ class SessionBase:
     def decode(self, session_data):
         try:
             return signing.loads(session_data, salt=self.key_salt, serializer=self.serializer)
-        # RemovedInDjango40Warning: when the deprecation ends, handle here
-        # exceptions similar to what _legacy_decode() does now.
         except signing.BadSignature:
-            try:
-                # Return an empty session if data is not in the pre-Django 3.1
-                # format.
-                return self._legacy_decode(session_data)
-            except Exception:
-                logger = logging.getLogger('django.security.SuspiciousSession')
-                logger.warning('Session data corrupted')
-                return {}
+            logger = logging.getLogger('django.security.SuspiciousSession')
+            logger.warning('Session data corrupted')
         except Exception:
-            return self._legacy_decode(session_data)
-
-    def _legacy_encode(self, session_dict):
-        # RemovedInDjango40Warning.
-        serialized = self.serializer().dumps(session_dict)
-        hash = self._hash(serialized)
-        return base64.b64encode(hash.encode() + b':' + serialized).decode('ascii')
-
-    def _legacy_decode(self, session_data):
-        # RemovedInDjango40Warning: pre-Django 3.1 format will be invalid.
-        encoded_data = base64.b64decode(session_data.encode('ascii'))
-        try:
-            # could produce ValueError if there is no ':'
-            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 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).
-            if isinstance(e, SuspiciousOperation):
-                logger = logging.getLogger('django.security.%s' % e.__class__.__name__)
-                logger.warning(str(e))
-            return {}
+            # ValueError, unpickling exceptions. If any of these happen, just
+            # return an empty dictionary (an empty session).
+            pass
+        return {}
 
     def update(self, dict_):
         self._session.update(dict_)
diff --git a/docs/releases/4.0.txt b/docs/releases/4.0.txt
index a2f04227fd..a6f5bdb12e 100644
--- a/docs/releases/4.0.txt
+++ b/docs/releases/4.0.txt
@@ -283,3 +283,5 @@ to remove usage of these features.
 
 * Support for the pre-Django 3.1 password reset tokens in the admin site (that
   use the SHA-1 hashing algorithm) is removed.
+
+* Support for the pre-Django 3.1 encoding format of sessions is removed.
diff --git a/tests/sessions_tests/tests.py b/tests/sessions_tests/tests.py
index 73d2a13a9f..c2743ef48b 100644
--- a/tests/sessions_tests/tests.py
+++ b/tests/sessions_tests/tests.py
@@ -31,13 +31,13 @@ from django.core import management
 from django.core.cache import caches
 from django.core.cache.backends.base import InvalidCacheBackendError
 from django.core.exceptions import ImproperlyConfigured
+from django.core.signing import TimestampSigner
 from django.http import HttpResponse
 from django.test import (
     RequestFactory, SimpleTestCase, TestCase, ignore_warnings,
     override_settings,
 )
 from django.utils import timezone
-from django.utils.deprecation import RemovedInDjango40Warning
 
 from .models import SessionStore as CustomDatabaseSession
 
@@ -315,25 +315,6 @@ class SessionTestsMixin:
         encoded = self.session.encode(data)
         self.assertEqual(self.session.decode(encoded), data)
 
-    @override_settings(SECRET_KEY='django_tests_secret_key')
-    def test_decode_legacy(self):
-        # RemovedInDjango40Warning: pre-Django 3.1 sessions will be invalid.
-        legacy_encoded = (
-            'OWUzNTNmNWQxNTBjOWExZmM4MmQ3NzNhMDRmMjU4NmYwNDUyNGI2NDp7ImEgdGVzd'
-            'CBrZXkiOiJhIHRlc3QgdmFsdWUifQ=='
-        )
-        self.assertEqual(
-            self.session.decode(legacy_encoded),
-            {'a test key': 'a test value'},
-        )
-
-    @ignore_warnings(category=RemovedInDjango40Warning)
-    def test_default_hashing_algorith_legacy_decode(self):
-        with self.settings(DEFAULT_HASHING_ALGORITHM='sha1'):
-            data = {'a test key': 'a test value'}
-            encoded = self.session.encode(data)
-            self.assertEqual(self.session._legacy_decode(encoded), data)
-
     def test_decode_failure_logged_to_security(self):
         tests = [
             base64.b64encode(b'flaskdj:alkdjf').decode('ascii'),
@@ -346,6 +327,11 @@ class SessionTestsMixin:
                 # The failed decode is logged.
                 self.assertIn('Session data corrupted', cm.output[0])
 
+    def test_decode_serializer_exception(self):
+        signer = TimestampSigner(salt=self.session.key_salt)
+        encoded = signer.sign(b'invalid data')
+        self.assertEqual(self.session.decode(encoded), {})
+
     def test_actual_expiry(self):
         # this doesn't work with JSONSerializer (serializing timedelta)
         with override_settings(SESSION_SERIALIZER='django.contrib.sessions.serializers.PickleSerializer'):