In Web.config - Machinekey Validationkey

Because the keys on Server A and Server B do not match, Server B cannot validate the HMAC generated by Server A. The result is a runtime error, usually the dreaded "Invalid Viewstate" exception, or a forced log-out if the authentication ticket fails validation.

By mastering the validationKey in Web.config , you ensure that your ASP.NET application remains secure, scalable, and free from the dreaded "MAC validation failed" errors. Treat this knowledge as a core part of your deployment and security toolkit. machinekey validationkey in web.config

– In rare cases with legacy ASP.NET 2.0/3.5, you may need compatibilityMode="Framework20SP1" . Avoid unless required. Because the keys on Server A and Server