Split Viewstate in Use

Viewstate is a feature in ASP.NET that allows the state of a web page to be persisted across postbacks. The viewstate is encrypted and sent to the client in a hidden field. When the page is posted back to the server, the server decrypts the viewstate and retrieves the values stored in it.

The Split Viewstate in Use vulnerability occurs when the viewstate is split across multiple hidden fields on the page. This can happen when the size of the viewstate exceeds the maximum allowed by ASP.NET. The vulnerability can be exploited by an attacker who can modify one of the hidden fields to change the state of the page.

In this article, we will provide a detailed step-by-step guide on how to fix the Split Viewstate in Use vulnerability.

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Step 1: Determine if your application is vulnerable

The first step is to determine if your application is vulnerable to the Split Viewstate in Use vulnerability. This can be done by using a vulnerability scanner or by examining the source code of your application.

If you are using a vulnerability scanner, it should be able to detect the Split Viewstate in Use vulnerability. The scanner will generate a report that will identify the vulnerable pages and provide recommendations on how to fix the issue.

If you are examining the source code of your application, you should look for code that splits the viewstate across multiple hidden fields. The following code is an example of how the viewstate can be split across multiple hidden fields:

<input type="hidden" name="__VIEWSTATE_1" value="..." /> <input type="hidden" name="__VIEWSTATE_2" value="..." /> <input type="hidden" name="__VIEWSTATE_3" value="..." />

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Step 2: Increase the maximum allowed viewstate size

The Split Viewstate in Use vulnerability occurs when the viewstate size exceeds the maximum allowed by ASP.NET. To fix the vulnerability, you need to increase the maximum allowed viewstate size.

This can be done by adding the following code to the web.config file:

<system.web> <pages maxViewStateSize="1000000" /> </system.web>

The value of maxViewStateSize should be set to a value that is appropriate for your application. It should be large enough to accommodate the largest viewstate size that your application can generate.

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Step 3: Use a single hidden field for the viewstate

To fix the Split Viewstate in Use vulnerability, you need to ensure that the viewstate is stored in a single hidden field on the page. This can be done by adding the following code to the web.config file:

<system.web> <pages enableViewStateMac="true" viewStateEncryptionMode="Always" /> </system.web>

The enableViewStateMac attribute ensures that the viewstate is protected by a message authentication code (MAC). The viewStateEncryptionMode attribute ensures that the viewstate is encrypted.

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Step 4: Test your application

After making the changes to your application, you should test it to ensure that the Split Viewstate in Use vulnerability has been fixed. You can do this by using a vulnerability scanner or by manually testing the application.

If you are using a vulnerability scanner, it should no longer detect the Split Viewstate in Use vulnerability. The scanner should generate a report that indicates that the vulnerability has been fixed.

If you are manually testing the application, you should verify that the viewstate is stored in a single hidden field on the page. You can do this by examining the HTML source code of the page. The following code is an example of how the viewstate should be stored in a single hidden field:

<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="..." />

You should also verify that the viewstate is protected by a message authentication code (MAC) and encrypted. You can do this by examining the HTTP response headers for the page. The headers should include the following:

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Cache-Control: no-cache Pragma: no-cache Content-Type: text/html; charset=utf-8 Content-Encoding: gzip Vary: Accept-Encoding Server: Microsoft-IIS/10.0 X-AspNet-Version: 4.0.30319 X-Powered-By: ASP.NET Date: [current date and time] Content-Length: [length of the HTTP response body] X-Content-Type-Options: nosniff X-Frame-Options: SAMEORIGIN Strict-Transport-Security: max-age=31536000; includeSubDomains X-XSS-Protection: 1; mode=block

The X-Content-Type-Options header should be set to nosniff to prevent content sniffing attacks. The X-Frame-Options header should be set to SAMEORIGIN to prevent clickjacking attacks. The Strict-Transport-Security header should be set to a value that is appropriate for your application to enforce the use of HTTPS. The X-XSS-Protection header should be set to 1 to enable the XSS filter in modern browsers.

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Step 5: Educate developers and testers

The final step is to educate developers and testers on how to prevent the Split Viewstate in Use vulnerability from occurring in the future. This can be done by providing training and guidance on secure coding practices.

Developers should be encouraged to use a single hidden field for the viewstate and to ensure that the viewstate is protected by a message authentication code (MAC) and encrypted. Testers should be trained to test for the Split Viewstate in Use vulnerability and to verify that the viewstate is stored in a single hidden field on the page.

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Conclusion:

The Split Viewstate in Use vulnerability can be exploited by an attacker who can modify one of the hidden fields to change the state of the page. To fix the vulnerability, you need to increase the maximum allowed viewstate size, use a single hidden field for the viewstate, and ensure that the viewstate is protected by a message authentication code (MAC) and encrypted. Developers and testers should be educated on how to prevent the Split Viewstate in Use vulnerability from occurring in the future.

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