External json vulnerable because of Json.Net TypeNameHandling auto?

I'm operating a small website where users can upload custom "objects" defined in JSON. Recently I've learned about possible threats using JSON with automatic type deserialization: JSON problem. I think I understand the problematics but I have to ask to be sure. If I only deserialize the incoming JSON with a given specific type (here MyObject) JsonConvert.DeserializeObject<MyObject>(json, settings); and no type inside MyObject and no subtype of any member of MyObject has the type System.Object or dynamic there is nothing that can go bad, right?

TypeNameHandling of settings is set to TypeNameHandling.Auto (let's not question this decision it probably could work with None, but I want to understand the question with it set to Auto.)

Edit: More information: I have tested the JSON from the previously mentioned website:

{
    "obj": {
        "$type": "System.IO.FileInfo, System.IO.FileSystem",
        "fileName": "rce-test.txt",
        "IsReadOnly": true
    }
}

If MyObject has a System.Object or dynamic typed field obj I can reproduce the threat. But what I want to know: I'm on the safe side with bad prepared user-json even if MyObject is a very complex Object with lots of (derived) sub objects but NONE of them is or has a System.Object or a dynamic field (also not something like List<Object>)? E.g. I could imagine that Json.NET does something like creating objects because of the $type information even if no according field in MyObject can be found.

TL/DR: In the absence of any obvious object or dynamic members, you may well be safe, but you are not guaranteed to be safe. To further decrease your risk you should follow the recommendations from the Newtonsoft documentation:

TypeNameHandling should be used with caution when your application deserializes JSON from an external source. Incoming types should be validated with a custom SerializationBinder when deserializing with a value other than None.

Full Answer

The attacks described in How to configure Json.NET to create a vulnerable web API, TypeNameHandling caution in Newtonsoft Json and Alvaro Muñoz & Oleksandr Mirosh's blackhat paper all depend on using the TypeNameHandling setting of Json.NET to trick the receiver into constructing an attack gadget - a instance of a type that when constructed, populated or disposed effects an attack on the receiving system.

Json.NET does two things that help protect against such attacks. Firstly, it ignores unknown properties. Thus simply adding an additional, unknown property to a JSON payload whose value contains a "$type" property should do no harm. Secondly, during deserialization of a polymorphic value, when resolving the "$type" property, it checks to see whether the resolved type is compatible with the expected type in JsonSerializerInternalReader.ResolveTypeName():

    if (objectType != null
#if HAVE_DYNAMIC
        && objectType != typeof(IDynamicMetaObjectProvider)
#endif
        && !objectType.IsAssignableFrom(specifiedType))
    {
        throw JsonSerializationException.Create(reader, "Type specified in JSON '{0}' is not compatible with '{1}'.".FormatWith(CultureInfo.InvariantCulture, specifiedType.AssemblyQualifiedName, objectType.AssemblyQualifiedName));
    }

If the expected type of the polymorphic value is not compatible with any attack gadget type, the attack will fail. Provided you have no serializable members of type object, dynamic or IDynamicMetaObjectProvider, this is likely to be true. But not certain!

Cases in which an attack gadget might get constructed even without any obvious untyped members in your data model include:

• Deserialization of untyped collections. If you are deserializing any sort of untyped collection or dictionary such as ArrayList, List<object>, Dictionary<string, dynamic> or HashTable, then your system is vulnerable to attack gadgets contained in the collection's items.

• Deserialization of any of the dozens of collections inheriting from CollectionBase. This type predates the introduction of generics in .Net and represents a "semi-typed" collection, in which the types of the items are validated in runtime as they are added. Since the validation occurs after construction, there is a window in which an attack gadget might get constructed.

  Sample fiddle showing just this.

• Deserialization of values that share a common base type or interface with an attack gadget other than just object. TempFileCollection implements ICollection and IDisposable. ObjectDataProvider implements INotifyPropertyChanged and ISupportInitialize. If you have any polymorphic members or values that are declared to be any of these interfaces, you are vulnerable.

• Deserialization of types that implement ISerializable. Json.NET supports this interface by default, and it is possible that a seemingly-harmless type in some external library is deserializing untyped members inside its streaming constructor without your knowledge.

  One obvious example is Sytem.Exception (or any of its subtypes) which deserializes an untyped dictionary "Data" inside its streaming constructor which corresponds to the untyped dictionary Exception.Data. If you are deserializing an Exception (contained in a log file for example, which is very common), the following JSON should effect an attack:

  {
    "$type": "System.Exception, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089",
    "ClassName": "System.Exception",
    "Message": "naughty exception",
    "Data": {
      "$type": "System.Collections.ListDictionaryInternal, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089",
      "data": {
        "$type": "System.IO.FileInfo, System.IO.FileSystem",
        "fileName": "rce-test.txt",
        "IsReadOnly": true    
      }
    },
  }
  

  The attack can be mitigated without creation of a custom serialization binder by setting DefaultContractResolver.IgnoreSerializableInterface = true. Of course, this may cause problems with serialization of certain .Net class library types.

• Deserializing types marked with [Serializable] can have a similar problem if you set DefaultContractResolver.IgnoreSerializableAttribute = false. However, the default is true, so you should be OK if you don't change this setting.

• Deserializing types with members that you think are not serialized -- but will be deserialized if present. E.g. consider the following type:

  public MyType
  {
      public object tempData;
      public bool ShouldSerializeTempData() { return false; }
  }
  

  Thanks to Json.NET's conditional serialization functionality, the tempData member will never be serialized, so you might think you're in the clear. But it will be deserialized if present! An attacker who decompiles your code and notices such a member will be able to craft an attack gadget payload for MyType.

And that's just what I was able to think of off the top of my head. As you can see, verifying that, in a large object graph, there is never an attempt to deserialize a polymorphic type that is compatible with some attack gadget is substantially nontrivial. Thus I'd strongly recommend the additional protection of a custom SerializationBinder that ensures that no unexpected types are deserialized.