I am using a custom Api Token implementation using WCF Web API on Azure. This uses FormsAuthentication.Decrypt in order to obtain a FormsAuthenticationTicket. To make sure that the decrpyt process works across multiple instances, I have provided a MachineKey in my web.config. However, I've noticed that the MachineKey doesn't seem to be working on Azure because it looks like Azure is using a random machinekey and overwriting the one I specificed in the web.config I'm using the latest Azure SDK 1.5 (or 1.6?)
I am well aware of this issue with Azure SDK 1.3 and I believe this was rectified in 1.4. Is there a chance that this issue has since re-appeared on Azure SDK1.5/1.6?
I was having the same problem where my FormsAuthentication tickets were not validating across sub domains after the recent Microsoft .Net 4.0 Security upgrade KB2656351.
My FormsAuth tickets are generated from my dedicated servers and read on sub domains on Windows Azure.
In order to get all sub domains to decrypt the tickets I made sure all my dedicated servers were patched with the latest .Net updates via Windows Update. Then I upgraded my Azure project to version 1.6 and selected the latest Azure OS after deploying. This seemed to do the trick.
Here are some articles about the issue:
http://technet.microsoft.com/en-us/security/bulletin/ms11-100.mspx
cheers
Francesco