Solved: “Validation of viewstate MAC failed”

You may come across this error message when you get around in pages of your ASP.NET website:

Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.

Background

ASP.NET uses view-state variable to rebuild pages after post-backs. The text of your buttons and the value of the form fields are the examples that this variable stores.

In order to prevent tempering attacks that try to play around with view-state data to force your webpage behave unexpectedly, web server validates the view-state data between page redirections. If the data doesn’t match, you receive the error message above.

Solution

The issue of unmatched view-state data could be related to server configuration or session cookie. Here are the most common root causes:

  • Web server and application pool configuration related issues. Read details in this Microsoft Support article
  • If you are using ViewStateUserKey to prevent Cross-site Request Forgery (CSRF) attacks, make sure the value you assign to this variable is the same in all pages. The most common usage is that assigning session ID or username to ViewStateUserKey. Your website might be losing the session between page redirections. Check these two StackOverflow topics for details: Link 1link 2
  • Redirecting the page right after setting session variables may be the issue. You should avoid using Response.Redirect in this case. Details
  • Antivirus software might be causing the issue. Add scanning exceptions for IIS and your application’s folders. Details