fix serialization vulnerability

activerecord/lib/active_record/attribute_methods.rb

@@ -80,7 +80,9 @@ def define_attribute_methods
           end
 
           unless instance_method_already_implemented?("#{name}=")
-            if create_time_zone_conversion_attribute?(name, column)
+            if self.serialized_attributes[name]
+              define_write_method_for_serialized_attribute(name)
+            elsif create_time_zone_conversion_attribute?(name, column)
               define_write_method_for_time_zone_conversion(name)
             else  
               define_write_method(name.to_sym)
@@ -184,6 +186,19 @@ def define_question_method(attr_name)
         def define_write_method(attr_name)
           evaluate_attribute_method attr_name, "def #{attr_name}=(new_value);write_attribute('#{attr_name}', new_value);end", "#{attr_name}="
         end
+
+        # Defined for all serialized attributes. Disallows assigning already serialized YAML.
+        def define_write_method_for_serialized_attribute(attr_name)
+          method_body = <<-EOV
+            def #{attr_name}=(value)
+              if value.is_a?(String) and value =~ /^---/
+                raise ActiveRecordError, "You tried to assign already serialized content to #{attr_name}. This is disabled due to security issues."
+              end
+              write_attribute(:#{attr_name}, value)
+            end
+          EOV
+          evaluate_attribute_method attr_name, method_body, "#{attr_name}="
+        end
         
         # Defined for all +datetime+ and +timestamp+ attributes when +time_zone_aware_attributes+ are enabled.
         # This enhanced write method will automatically convert the time passed to it to the zone stored in Time.zone.

activerecord/test/cases/base_test.rb

@@ -1499,6 +1499,12 @@ def test_nil_serialized_attribute_with_class_constraint
     assert_nil topic.content
   end
 
+  def test_should_raise_exception_on_assigning_already_serialized_content
+    topic = Topic.new
+    serialized_content = %w[foo bar].to_yaml
+    assert_raise(ActiveRecord::ActiveRecordError) { topic.content = serialized_content }
+  end
+
   def test_should_raise_exception_on_serialized_attribute_with_type_mismatch
     myobj = MyObject.new('value1', 'value2')
     topic = Topic.new(:content => myobj)