Never treat action or controller as unpermitted params

actionpack/lib/action_controller/metal/strong_parameters.rb

@@ -95,6 +95,10 @@ class Parameters < ActiveSupport::HashWithIndifferentAccess
     cattr_accessor :permit_all_parameters, instance_accessor: false
     cattr_accessor :raise_on_unpermitted_parameters, instance_accessor: false
 
+    # Never raise an UnpermittedParameters exception because of these params
+    # are present. They are added by Rails and it's of no concern.
+    NEVER_UNPERMITTED_PARAMS = %w( controller action )
+
     # Returns a new instance of <tt>ActionController::Parameters</tt>.
     # Also, sets the +permitted+ attribute to the default value of
     # <tt>ActionController::Parameters.permit_all_parameters</tt>.
@@ -251,12 +255,7 @@ def permit(*filters)
         end
       end
 
-      if Parameters.raise_on_unpermitted_parameters
-        unpermitted_keys = self.keys - params.keys
-        if unpermitted_keys.any?
-          raise ActionController::UnpermittedParameters.new(unpermitted_keys)
-        end
-      end
+      raise_on_unpermitted_parameters!(params)
 
       params.permit!
     end
@@ -336,6 +335,16 @@ def each_element(object)
           yield object
         end
       end
+
+      def raise_on_unpermitted_parameters!(params)
+        if self.class.raise_on_unpermitted_parameters && unpermitted_keys(params).any?
+          raise ActionController::UnpermittedParameters.new(unpermitted_keys(params))
+        end
+      end
+
+      def unpermitted_keys(params)
+        self.keys - params.keys - NEVER_UNPERMITTED_PARAMS
+      end
   end
 
   # == Strong \Parameters

actionpack/test/controller/parameters/raise_on_unpermitted_parameters_test.rb

@@ -30,4 +30,14 @@ def teardown
       params.permit(book: [:pages])
     end
   end
+
+  test "action and controller keys are safe to ignore" do
+    params = ActionController::Parameters.new({
+      action: 'index', controller: 'stuff', book: { pages: 65 }
+    })
+
+    assert_nothing_raised do
+      params.permit(book: [:pages])
+    end
+  end
 end