How to check viewstate tampering?

To check for ViewState tampering in an ASP.NET application, you can compare the ViewState between the client and the server during each postback. Here's an example of how you can perform ViewState tampering checks:

1. Enable ViewState MAC: First, ensure that 'ViewState' MAC (Message Authentication Code) is enabled in your application. This can be done by setting the 'EnableViewStateMac' attribute to 'true' in the '<pages>' element of the web.config file:

<pages enableViewStateMac="true" />

Enabling ViewState MAC adds a digital signature to the ViewState, allowing ASP.NET to verify its integrity during each postback.

2. Validate ViewState Integrity: In your server-side code, you can validate the integrity of the 'ViewState' by comparing it with the value received from the client. The ViewState on the server can be accessed through the ViewState property of the page or the specific controls.

protected void Page_Load(object sender, EventArgs e)
{
    if (IsPostBack)
    {
        if (!IsValidViewState())
        {
            // ViewState has been tampered with
            // Take appropriate action (e.g., logging, redirecting, showing an error message)
        }
    }
}

private bool IsValidViewState()
{
    string clientViewState = Request.Form["__VIEWSTATE"];
    string serverViewState = ViewState.ToString();

    // Compare the client and server ViewState
    return string.Equals(clientViewState, serverViewState);
}

In this example, during a postback, the 'IsValidViewState' method compares the client's ViewState received in the request ('Request.Form["__VIEWSTATE"]') with the server's ViewState ('ViewState.ToString()'). If the two ViewState values do not match, it indicates potential tampering.

If the comparison fails, you can take appropriate action based on your application's requirements, such as logging the tampering attempt, redirecting the user to an error page, or showing an error message indicating that the ViewState has been tampered with.

It's worth noting that ViewState MAC (enabled through 'EnableViewStateMac') helps detect tampering attempts automatically. However, performing an explicit comparison of the ViewState values provides an additional layer of verification.

Remember that while these measures help detect ViewState tampering, they do not prevent other security vulnerabilities or attacks. Proper application security practices should be followed to mitigate other risks, such as XSS (Cross-Site Scripting) or server-side vulnerabilities.