diff --git a/actionpack/lib/action_controller/test_case.rb b/actionpack/lib/action_controller/test_case.rb
index 16ddd3b304..09f2a79d85 100644
--- a/actionpack/lib/action_controller/test_case.rb
+++ b/actionpack/lib/action_controller/test_case.rb
@@ -51,7 +51,7 @@ module ActionController
       super(env)
 
       self.session = session
-      self.session_options = TestSession::DEFAULT_OPTIONS
+      self.session_options = TestSession::DEFAULT_OPTIONS.dup
       @controller_class = controller_class
       @custom_param_parsers = {
         xml: lambda { |raw_post| Hash.from_xml(raw_post)["hash"] }
diff --git a/actionpack/test/controller/request/test_request_test.rb b/actionpack/test/controller/request/test_request_test.rb
index 6e0ca957c3..fa049fbc9f 100644
--- a/actionpack/test/controller/request/test_request_test.rb
+++ b/actionpack/test/controller/request/test_request_test.rb
@@ -6,6 +6,11 @@ class ActionController::TestRequestTest < ActionController::TestCase
     assert @request.session_options
   end
 
+  def test_mutating_session_options_does_not_affect_default_options
+    @request.session_options[:myparam] = 123
+    assert_equal nil, ActionController::TestSession::DEFAULT_OPTIONS[:myparam]
+  end
+
   ActionDispatch::Session::AbstractStore::DEFAULT_OPTIONS.each_key do |option|
     test "rack default session options #{option} exists in session options and is default" do
       assert_equal(ActionDispatch::Session::AbstractStore::DEFAULT_OPTIONS[option],