We are in the process of moving an application from in house to an Azure VM and Application Gateway v2 was selected as the proxy for the system. The application uses NTLM Authentication to identify the users.
During the setup up of the application it appears that NTLM is working correctly. It prompts the user for credentials and we can valid them against the Local Active Directory.
However once we moved to user testing. We started to notice that people were at times seeing the wrong credentials in the application. After further testing, it turns out with more than one person accessing the application at the same time, that Application Gateway will switch back and forth between the credentials that are sent to the application.
So while it may appear that NTLM authentication is working, it is not working correctly. It is like it is failing silently, would be better if it through an error if NTLM was present in the request.
Those that think it is working for them, might need to test it further and check they are always getting the correct credentials.