•The existing implementation on EPA Plugin is dependent on browser to loopback communication. The browser should be able to communicate with 127.0.0.1:3198 for the EPA process to work
•The article: https://developer.microsoft.com/en-us/office/blogs/microsoft-edge-webview-for-office-add-ins/ states to need to allow loopback communication for WebView need to execute the following command: CheckNetIsolation LoopbackExempt -a -n=”microsoft.win32webviewhost_cw5n1h2txyewy” where “microsoft.win32webviewhost_cw5n1h2txyewy” is the APPID for the WebView, this was done but it did not resolve the issue.
• Further looking through the netstat O/P we noted the process ID of the application trying to communicate with 127.0.0.1:3198, that process ID belonged to the application “Microsoft.AAD.BrokerPlugin.exe” having APPID “Microsoft.AAD.BrokerPlugin_cw5n1h2txyewy” Although the epaplugin component (nglauncher.exe) was listening on 127.0.0.1:3198 the above process id was unable to communicate with it and was stuck in SYN_SENT
• So, at that point it was identified the webview itself does not handle the network communication during O365 authentication, instead uses the Microsoft.AAD.BrokerPlugin.exe application, therefore it’s this application which has to be allowed loopback communication.