Viewstate without MAC Signature (Sure)

Viewstate is a hidden form field that stores the state of a web page, such as the values of form controls, the user's selections, and other page-related data. Viewstate is used by ASP.NET and other web technologies to maintain the state of a web page between postbacks, which is when the page is submitted to the server.

What is the Viewstate without MAC Signature (Sure) vulnerability?

The Viewstate without MAC Signature (Sure) vulnerability occurs when a web page uses Viewstate but does not include a Message Authentication Code (MAC) to protect it. Without a MAC, an attacker can modify the Viewstate value, which could allow them to perform session hijacking, CSRF attacks, or access sensitive information.

How does the Viewstate without MAC Signature (Sure) vulnerability work?

When a web page is submitted to the server, the server generates a new Viewstate value and sends it back to the client. The client then includes the Viewstate value in the form submission to the server. The server checks the Viewstate value to ensure that it has not been modified since it was generated.

If the Viewstate value has been modified, the server rejects the submission, as it is likely that an attacker is attempting to manipulate the form data. However, without a MAC, an attacker can modify the Viewstate value and submit the form to the server without being detected.

How to fix the Viewstate without MAC Signature (Sure) vulnerability

Fixing the Viewstate without MAC Signature (Sure) vulnerability involves adding a Message Authentication Code (MAC) to the Viewstate value. A MAC is a cryptographic checksum that ensures that the Viewstate value has not been modified since it was generated. Here are the steps to fix the vulnerability:

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Step 1: Enable MAC validation in your web application

The first step is to ensure that your web application is configured to validate the MAC of the Viewstate value. In an ASP.NET web application, this can be done by adding the following code to the web.config file:

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

This enables MAC validation for all pages in the application. If you want to enable MAC validation for a specific page, you can add the following directive to the top of the page:

<%@ Page ViewStateEncryptionMode="Always" %>

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Step 2: Use SSL to encrypt Viewstate values

Viewstate values should always be encrypted using SSL to protect them in transit. SSL encrypts the data between the client and server, making it difficult for an attacker to intercept and modify the Viewstate value.

Step 3: Limit the size of Viewstate values

Viewstate values can become quite large, which can increase the risk of them being intercepted and modified by an attacker. To reduce this risk, limit the size of Viewstate values by removing unnecessary data and compressing them.

<system.web> <pages maxPageStateFieldLength="4096" /> </system.web>

This limits the size of Viewstate values to 4KB. If you need to store more data in the Viewstate, consider using session state or other techniques.

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Step 4: Use a custom MAC key

By default, ASP.NET uses a machine key to generate MACs for Viewstate values. This can be a security risk if an attacker gains access to the machine key, as they could use it to generate their own MAC. To mitigate this risk, it is recommended to use a custom MAC key. This can be done by adding the following code to the web.config file:

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<system.web> <machineKey validationKey="..." decryptionKey="..." validation="SHA1" decryption="AES" /> </system.web>

Replace the "..." with a random string of characters that is at least 64 characters long. This will be used as the validation key and decryption key for the MAC. The validation algorithm is set to SHA1 and the decryption algorithm to AES, which are recommended values.

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Step 5: Verify that the vulnerability has been fixed

After implementing these changes, it is important to verify that the vulnerability has been fixed. You can do this by scanning your web application again with the external vulnerability scanner that originally identified the vulnerability. If the vulnerability is no longer detected, then you have successfully fixed the issue.

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Conclusion

The Viewstate without MAC Signature (Sure) vulnerability is a serious security issue that can lead to sensitive information leakage, session hijacking, and other attacks. By enabling MAC validation, using SSL to encrypt Viewstate values, limiting their size, using a custom MAC key, and verifying that the vulnerability has been fixed, you can effectively mitigate this risk and protect your web application from attacks.

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