Fixed #29728 -- Prevented session resaving if CSRF cookie is unchanged.

django/middleware/csrf.py

@@ -181,7 +181,8 @@ class CsrfViewMiddleware(MiddlewareMixin):
 
     def _set_token(self, request, response):
         if settings.CSRF_USE_SESSIONS:
-            request.session[CSRF_SESSION_KEY] = request.META['CSRF_COOKIE']
+            if request.session.get(CSRF_SESSION_KEY) != request.META['CSRF_COOKIE']:
+                request.session[CSRF_SESSION_KEY] = request.META['CSRF_COOKIE']
         else:
             response.set_cookie(
                 settings.CSRF_COOKIE_NAME,

tests/csrf_tests/tests.py

 import re
 
 from django.conf import settings
+from django.contrib.sessions.backends.cache import SessionStore
 from django.core.exceptions import ImproperlyConfigured
 from django.http import HttpRequest
 from django.middleware.csrf import (
@@ -24,8 +25,7 @@ class TestingHttpRequest(HttpRequest):
     """
     def __init__(self):
         super().__init__()
-        # A real session backend isn't needed.
-        self.session = {}
+        self.session = SessionStore()
 
     def is_secure(self):
         return getattr(self, '_is_secure_override', False)
@@ -693,6 +693,19 @@ class CsrfViewMiddlewareUseSessionsTests(CsrfViewMiddlewareTestMixin, SimpleTest
         ensure_csrf_cookie_view(req)
         self.assertTrue(req.session.get(CSRF_SESSION_KEY, False))
 
+    def test_session_modify(self):
+        """The session isn't saved if the CSRF cookie is unchanged."""
+        req = self._get_GET_no_csrf_cookie_request()
+        self.mw.process_view(req, ensure_csrf_cookie_view, (), {})
+        resp = ensure_csrf_cookie_view(req)
+        self.mw.process_response(req, resp)
+        self.assertIsNotNone(req.session.get(CSRF_SESSION_KEY))
+        req.session.modified = False
+        self.mw.process_view(req, ensure_csrf_cookie_view, (), {})
+        resp = ensure_csrf_cookie_view(req)
+        self.mw.process_response(req, resp)
+        self.assertFalse(req.session.modified)
+
     def test_ensures_csrf_cookie_with_middleware(self):
         """
         The ensure_csrf_cookie() decorator works with the CsrfViewMiddleware