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**ViewState** is the method that the ASP.NET framework uses by default to p**reserve page and control values between web pages**. When the HTML for the page is rendered, the current state of the page and values that need to be retained during postback are serialized into base64-encoded strings and output in the ViewState hidden field or fields.\
One may assume that if **ViewState** is **not present**, their implementation is **secure** from any potential vulnerabilities arising with ViewState deserialization.\
However, that is not the case. If we **add ViewState parameter** to the request body and send our serialized payload created using ysoserial, we will still be able to achieve **code execution** as shown in **Case 1**.
As the parameter is MAC protected this time to successfully execute the attack we first need the key used. In this case, BurpSuite will let us know that the parameter is MAC protected:
You can try to use [**Blacklist3r(AspDotNetWrapper.exe)** ](https://github.com/NotSoSecure/Blacklist3r/tree/master/MachineKey/AspDotNetWrapper)to find the key used.
In cases where `_VIEWSTATEGENERATOR` parameter **isn't sent** by the server you **don't** need to **provide** the `--generator` parameter **but these ones**:
In this case Burp doesn't find if the parameter is protected with MAC because it doesn't recognise the values. Then, the value is probably encrypted and you will **need the Machine Key to encrypt your payload** to exploit the vulnerability.
**In this case the** [**Blacklist3r**](https://github.com/NotSoSecure/Blacklist3r/tree/master/MachineKey/AspDotNetWrapper) **module is under development...**
**Prior to .NET 4.5**, ASP.NET can **accept** an **unencrypted** \_`__VIEWSTATE`\_parameter from the users **even** if **`ViewStateEncryptionMode`** has been set to _**Always**_. ASP.NET **only checks** the **presence** of the **`__VIEWSTATEENCRYPTED`** parameter in the request. **If one removes this parameter, and sends the unencrypted payload, it will still be processed.**
Threfore, if the Machinekey is known (e.g. via a directory traversal issue), [**YSoSerial.Net**](https://github.com/pwntester/ysoserial.net) command used in the **Case 2**, can be used to perform RCE using ViewState deserialization vulnerability.
* Remove `__VIEWSTATEENCRYPTED` parameter from the request in order to exploit the ViewState deserialization vulnerability, else it will return a Viewstate MAC validation error and exploit will fail as shown in Figure:
We can force the usage of ASP.NET framework by specifying the below parameter inside the web.config file as shown below.
```markup
<httpRuntimetargetFramework="4.5"/>
```
Alternatively, this can be done by specifying the below option inside the `machineKey` paramter of web.config file.
```bash
compatibilityMode="Framework45"
```
As in the previous case Burp doesn't identify if the request is MAC protected because the **value is encrypted.** Then, to send a **valid payload the attacker need the key**.
You can try to use [**Blacklist3r(AspDotNetWrapper.exe)** ](https://github.com/NotSoSecure/Blacklist3r/tree/master/MachineKey/AspDotNetWrapper)to find the key being used:
--TargetPagePath = {Target page path in application}
```
For a more detailed description for IISDirPath and TargetPagePath [refer here](https://soroush.secproject.com/blog/2019/04/exploiting-deserialisation-in-asp-net-via-viewstate/)
Once a valid Machine key is identified, **the next step is to generate a serialized payload using** [**YSoSerial.Net**](https://github.com/pwntester/ysoserial.net)
If you have the value of `__VIEWSTATEGENERATOR` you can try to **use** the `--generator` parameter with that value and **omit** the parameters `--path` and `--apppath`
If the ViewState deserialization vulnerability is successfully exploited, an attacker-controlled server will receive an out of band request containing the username. [PoC of Successful Exploitation](https://www.notsosecure.com/exploiting-viewstate-deserialization-using-blacklist3r-and-ysoserial-net/#PoC)
The **ViewStateUserKey** property can be used to **defend** against a **CSRF attack**. If such a key has been defined in the application and we try to generate the **ViewState** payload with the methods discussed till now, the **payload won’t be processed by the application**.\
For all the test cases, if the ViewState YSoSerial.Net payload works **successfully** then the server responds with “**500 Internal server error**” having response content “**The state information is invalid for this page and might be corrupted**” and we get the OOB request as shown in Figures below:
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