Azure Web App + Node.js + Azure AD = Error 431
This is a known issue since there is limitation with more recent versions of node that utilize a hard cap of 8KB for headers. EasyAuth adds some very large headers to the request, which can cause the node container to reject the request made by the middleware container with a 400. Read more about it here: This can be mitigated by setting the app setting WEBSITE_AUTH_DISABLE_IDENTITY_FLOW to true. This will remove one of the largest headers we add to the request. This header is generally only used by .NET Framework and Azure Functions apps, and so this setting should be safe.
If the issue still occurs, you can try with NodeJS 12 versions or increase the header size parameter: node server.js --max-http-header-size 81000