Exploiting __VIEWSTATE knowing the secrets

The content of this post was extracted from https://soroush.secproject.com/blog/2019/04/exploiting-deserialisation-in-asp-net-via-viewstate/****

Introduction

ASP.NET web applications use ViewState in order to maintain a page state and persist data in a web form.

It is normally possible to run code on a web server where a valid ViewState can be forged. This can be done when the MAC validation feature has been disabled or by knowing the:

  • Validation key and its algorithm prior to .NET Framework version 4.5

  • Validation key, validation algorithm, decryption key, and decryption algorithm in .NET Framework version 4.5 or above

In order to prevent manipulation attacks, .NET Framework can sign and encrypt the ViewState that has been serialised using the LosFormatter class. It then verifies the signature using the message authentication code (MAC) validation mechanism. The ObjectStateFormatter class performs the signing, encryption, and verification tasks. The keys required to perform the signing and/or encryption mechanism can be stored in the machineKey section of the web.config (application level) or machine.config (machine level) files. This is normally the case when multiple web servers are used to serve the same application often behind a load balancer in a Web Farm or cluster. The following shows the machineKey section’s format in a configuration file of an ASP.NET application that uses .NET Framework version 2.0 or above:

<machineKey validationKey="[String]"  decryptionKey="[String]" validation="[SHA1 | MD5 | 3DES | AES | HMACSHA256 | HMACSHA384 | HMACSHA512 | alg:algorithm_name]"  decryption="[Auto | DES | 3DES | AES | alg:algorithm_name]" />
<machineKey validationKey="70DBADBFF4B7A13BE67DD0B11B177936F8F3C98BCE2E0A4F222F7A769804D451ACDB196572FFF76106F33DCEA1571D061336E68B12CF0AF62D56829D2A48F1B0" decryptionKey="34C69D15ADD80DA4788E6E3D02694230CF8E9ADFDA2708EF43CAEF4C5BC73887" validation="SHA1" decryption="AES"  />

It should be noted that when a machineKey section has not been defined within the configuration files or when the validationKey and decryptionKey attributes have been set to AutoGenerate, the application generates the required values dynamically based on a cryptographically random secret. The algorithms can also be selected automatically. Currently in the latest version of .NET Framework, the default validation algorithm is HMACSHA256 and the default decryption algorithm is AES. See [13] for more details.

RCE with disabled ViewState MAC Validation

In the past, it was possible to disable the MAC validation simply by setting the enableViewStateMac property to False. Microsoft released a patch in September 2014 [3] to enforce the MAC validation by ignoring this property in all versions of .NET Framework. Although some of us might believe that “the ViewState MAC can no longer be disabled” [4], it is still possible to disable the MAC validation feature by setting the AspNetEnforceViewStateMac registry key to zero in:

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

Alternatively, adding the following dangerous setting to the application level web.config file can disable the MAC validation as well:

<configuration>
…
    <appSettings>
      <add key="aspnet:AllowInsecureDeserialization" value="true" />
    </appSettings>
</configuration>

When ViewState MAC validation has been disabled, the YSoSerial.Net project can be used to generate LosFormatter payloads as the ViewState in order to run arbitrary code on the server.

Prior to the .NET Framework version 4.5, the __VIEWSTATE parameter could be encrypted whilst the MAC validation feature was disabled. It should be noted that most scanners do not attempt to send an unencrypted ViewState parameter to identify this vulnerability. As a result, manual testing is required to check whether the MAC validation is disabled when the __VIEWSTATE parameter has been encrypted. This can be checked by sending a short random base64 string in the __VIEWSTATE parameter. The following URL shows an example:

https://victim.com/path/page.aspx?__VIEWSTATE=AAAA

If the target page responds with an error, the MAC validation feature has been disabled otherwise it would have suppressed the MAC validation error message. However, in scenarios where you cannot see the error message this trick won't work.

Automated scanners should use a payload that causes a short delay on the server-side. This can be achieved by executing the following ASP.NET code as an example to create a 10-second delay:

System.Threading.Thread.Sleep(10000);
string xaml_payload = @"<ResourceDictionary
  xmlns=""http://schemas.microsoft.com/winfx/2006/xaml/presentation""
  xmlns:x=""http://schemas.microsoft.com/winfx/2006/xaml""
  xmlns:System=""clr-namespace:System;assembly=mscorlib""
  xmlns:Thr=""clr-namespace:System.Threading;assembly=mscorlib"">
     <ObjectDataProvider x:Key=""x"" ObjectType = ""{ x:Type Thr:Thread}"" MethodName = ""Sleep"" >
     <ObjectDataProvider.MethodParameters>
        <System:Int32>10000</System:Int32>
     </ObjectDataProvider.MethodParameters>
    </ObjectDataProvider>
</ResourceDictionary>";

RCE with enabled ViewState MAC Validation

In older versions (prior to 4.5), .NET Framework uses the TemplateSourceDirectory property [15] when signing a serialised object. Since version 4.5 however, it uses the Purpose strings in order to create the hash. Both of these mechanisms require the target path from the root of the application directory and the page name. These parameters can be extracted from the URL.

Applications that use an older framework and enforce ViewState encryption can still accept a signed ViewState without encryption. This means that knowing the validation key and its algorithm is enough to exploit a website. It seems ViewState is encrypted by default since version 4.5 even when the viewStateEncryptionMode property has been set to Never. This means that in the latest .NET Framework versions the decryption key and its algorithm are also required in order to create a payload.

The ASP.NET ViewState contains a property called ViewStateUserKey [16] that can be used to mitigate risks of cross-site request forgery (CSRF) attacks [4]. Value of the ViewStateUserKey property (when it is not null) is also used during the ViewState signing process. Although not knowing the value of this parameter can stop our attack, its value can often be found in the cookies or in a hidden input parameter ([17] shows an implemented example).

ViewState YSoSerial.Net plugins

In YSoSerial.Net master and YSoSerial.Netv2 you can find a plugin (this and this) to exploit this technique when all the information is known.

For .NET Framework >= 4.5:

.\ysoserial.exe -p ViewState -g TextFormattingRunProperties -c "echo 123 > c:\windows\temp\test.txt" --path="/somepath/testaspx/test.aspx" --apppath="/testaspx/" --decryptionalg="AES" --decryptionkey="34C69D15ADD80DA4788E6E3D02694230CF8E9ADFDA2708EF43CAEF4C5BC73887" --validationalg="HMACSHA256" --validationkey="70DBADBFF4B7A13BE67DD0B11B177936F8F3C98BCE2E0A4F222F7A769804D451ACDB196572FFF76106F33DCEA1571D061336E68B12CF0AF62D56829D2A48F1B0"

For .NET Framework <= 4.0 (legacy):

The decryptionKey and its algorithm are not required here:

.\ysoserial.exe -p ViewState -g TypeConfuseDelegate -c "echo 123 > c:\windows\temp\test.txt" --apppath="/testaspx/" --islegacy --validationalg="SHA1" --validationkey="70DBADBFF4B7A13BE67DD0B11B177936F8F3C98BCE2E0A4F222F7A769804D451ACDB196572FFF76106F33DCEA1571D061336E68B12CF0AF62D56829D2A48F1B0" --isdebug

Apart from using different gadgets, it is possible to use the __VIEWSTATEGENERATOR parameter instead of providing the paths:

.\ysoserial.exe -p ViewState -g TextFormattingRunProperties -c "echo 123 > c:\windows\temp\test.txt" --generator=93D20A1B --validationalg="SHA1" --validationkey="70DBADBFF4B7A13BE67DD0B11B177936F8F3C98BCE2E0A4F222F7A769804D451ACDB196572FFF76106F33DCEA1571D061336E68B12CF0AF62D56829D2A48F1B0"

It uses the ActivitySurrogateSelector gadget by default that requires compiling the ExploitClass.cs class in YSoSerial.Net project. The ViewState payload can also be encrypted to avoid WAFs when the decryptionKey value is known:

.\ysoserial.exe -p ViewState -c "foo to use ActivitySurrogateSelector" --path="/somepath/testaspx/test.aspx" --apppath="/testaspx/" --islegacy --decryptionalg="AES" --decryptionkey="34C69D15ADD80DA4788E6E3D02694230CF8E9ADFDA2708EF43CAEF4C5BC73887" --isencrypted --validationalg="SHA1" --validationkey="70DBADBFF4B7A13BE67DD0B11B177936F8F3C98BCE2E0A4F222F7A769804D451ACDB196572FFF76106F33DCEA1571D061336E68B12CF0AF62D56829D2A48F1B0"

Note: Due to the nature of used gadgets in YSoSerial.Net, the target ASP.NET page always responds with an error even when an exploit has been executed successfully on the server-side.

Application path

it is important to find the root of the application path in order to create a valid ViewState unless:

  • The application uses .NET Framework version 4.0 or below; and

  • The __VIEWSTATEGENERATOR parameter is known.

The following screenshot shows the path tree in IIS:

You can check [20] if you are not familiar with virtual directory and application terms in IIS.

In order to generate a ViewState for the above URL, the --path and --apppath arguments should be as follows:

--path=/dir1/vDir1/dir2/app1/dir3/app2/vDir2/dir4
--apppath=/app2/

If we did not know that “app2” was an application name, we could use trial and error to test all the directory names in the URL one by one until finding a ViewState that can execute code on the server (perhaps by getting a DNS request or causing a delay).

Generator

In this case, the --generator argument can be used. The --isdebug argument can be used to check whether the plugin also calculates the same __VIEWSTATEGENERATOR parameter when the --path and --apppath arguments have been provided.

Exploiting Older Versions

No gadget was identified to exploit .NET Framework v1.1 at the time of writing this blog post.

In order to exploit applications that use .NET Framework v4.0 or below, the YSoSerial.Net v2.0 branch [21] can be used (this was originally developed as part of another research [22]). However, this project only supports a limited number of gadgets, and also requires the target box to have .NET Framework 3.5 or above installed.

Other tools

It seems Immunity Canvas supports creating the ViewState parameter when the validation and encryption keys are known [29]. The following tools were also released coincidentally at the same time as I was about to publish my work which was quite surprising:

I think these tools currently do not differentiate between different versions of .NET Framework and target the legacy cryptography. Additionally, they do not use the ViewStateUserKey parameter that might be in use to stop CSRF attacks.

Additional Tips

Using GET requests

It is also possible to send the __VIEWSTATE parameter in the URL via a GET request. The only limiting factor is the URL length that limits the type of gadgets that can be used here.

Encryption in .NET Framework prior to version 4.5

As mentioned previously, the __VIEWSTATE parameter does not need to be encrypted when exploiting .NET Framework 4.0 and below (tested on v2.0 through v4.0) even when the ViewStateEncryptionMode property has been set to Always. ASP.NET decides whether or not the ViewState has been encrypted by finding the __VIEWSTATEENCRYPTED parameter in the request (it does not need to have any value). Therefore, it is possible to send an unencrypted ViewStated by removing the __VIEWSTATEENCRYPTED parameter from the request.

This also means that changing the decryption key or its algorithm cannot stop the attacks when the validation key and its algorithm have been stolen.

The __VIEWSTATE parameter can be encrypted in order to bypass any WAFs though.

Bypassing anti-CSRF (anti-XSRF) mechanism

An ASP.NET page produces an error when an invalid __VIEWSTATE parameter is used. However, the page can still receive its inputs when Request.Form is used directly in the code for example by using Request.Form["txtMyInput"] rather than txtMyInput.Text. The CSRF attack can be achieved by removing the __VIEWSTATE parameter from the request or by adding the __PREVIOUSPAGE parameter with an invalid value. As the __PREVIOUSPAGE parameter is encrypted and base64 formatted by default, even providing a single character as its value should cause an error.

This might result in bypassing the anti-CSRF protection mechanism that has been implemented by setting the Page.ViewStateUserKey parameter.

Usage of the ViewStateGenerator parameter

When the __VIEWSTATEGENERATOR parameter is known, it can be used for the ASP.NET applications that use .NET Framework version 4.0 or below in order to sign a serialised object without knowing the application path.

ViewState chunking to bypass WAFs

It is possible to break the __VIEWSTATE parameter into multiple parts when the MaxPageStateFieldLength property has been set to a positive value. Its default value is negative and it means that the __VIEWSTATE parameter cannot be broken into multiple parts.

This might be useful to bypass some WAFs when ViewState chunking is allowed.

Exploiting the EventValidation parameter

The __EVENTVALIDATION parameter and a few other parameters are also serialised similar to the __VIEWSTATE parameter and can be targeted similarly. Exploiting a deserialisation issue via __EVENTVALIDATION is more restricted and requires:

  • A POST request

  • An ASP.NET page that accepts input parameters

  • A valid input parameter name. For example, the myinput parameter in the POST request when we have the following code on the server-side:

<asp:TextBox runat="server" ID="myinput" />

Value of the __VIEWSTATE parameter can be empty in the request when exploiting the __EVENTVALIDATION parameter but it needs to exist.

The Purpose string that is used by .NET Framework 4.5 and above to create a valid signature is different based on the used parameter. The following table shows the defined Purpose strings in .NET Framework:

The table above shows all input parameters that could be targeted.

Beware of the PreviousPage parameter

When the __PREVIOUSPAGE parameter exists in the request with invalid data, the application does not deserialise the __VIEWSTATE parameter. Providing the __CALLBACKID parameter prevents this behaviour.

Errors reliability

As explained previously, we sometimes use errors to check whether a generated ViewState is valid. ASP.NET does not show the MAC validation error by default when an invalid __VIEWSTATEGENERATOR parameter is used. This behaviour changes when the ViewStateUserKey property is used, as ASP.NET will not suppress the MAC validation errors anymore.

In addition to this, ASP.NET web applications can ignore the MAC validation errors with the following setting even when the ViewStateUserKey property is used:

<appSettings>
      <add key="aspnet:AlwaysIgnoreViewStateValidationErrors" value="true" />
</appSettings>

Web.config as a backdoor

If attackers can change the web.config within the root of an application, they can easily run code on the server. However, embedding a stealthy backdoor on the application might be a good choice for an attacker. This can be done by disabling the MAC validation and setting the viewStateEncryptionMode property to Always. This means that all ASP.NET pages that do not set the ViewStateEncryptionMode property to Auto or Never always use encrypted ViewState parameters. However, as the ViewState do not use the MAC validation feature, they are now vulnerable to remote code execution via deserialising untrusted data. The following shows an example:

<configuration>
…
    <system.web>
…
        <pages enableViewStateMac="false" viewStateEncryptionMode="Always" />
    </system.web>
    <appSettings>
        <add key="aspnet:AllowInsecureDeserialization" value="false" />
    </appSettings>
</configuration>

Another option for a stand-alone website would be to set the machineKey section with arbitrary keys and algorithms to stop other attackers!

It should be noted that setting the EnableViewState property to False does not stop this attack as the ViewState will still be parsed by ASP.NET.

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