Asp.net Validation of viewstate MAC failed

Question

I am receiving the following error at certain times on asp.net website.

 Sys.WebForms.PageRequestManagerServerErrorException: 
 Validation of viewstate MAC f         


        

Answer 1

This solution worked for me in ASP.NET 4.5 using a Web Forms site.

1. Use the following site to generate a Machine Key: http://www.blackbeltcoder.com/Resources/MachineKey.aspx
2. Copy Full Machine Key Code.
3. Go To your Web.Config File.
4. Paste the Machine Key in the following code section:

You should not see the viewstate Mac failed error anymore. Each website in the same app pool should have a separate machine key otherwise this error will continue.