Exception using Azure Managed Service Identity across tenants - azure-managed-identity

Question

I'm building an Azure web app for a client that will be provisioned into many other directories for their customers. This app will call a web API in my client's directory, which will then call back to another web API in the customer's directory. Something like this:
Other Customer AAD1 --------- My client AAD2
App --------------------------------> Web API 2
Web API 1 <-------------------------- Web API 2
We have been able to get the first call to work. This requires a corresponding App Registation for Web API 2 in AAD1. We figure that we could get the callback to work by following the same pattern, with a registration for Web API1 in AAD2. However, that might be a LOT of these 'proxy' registration in my client's AAD, so we're looking at alternatives.
We are exploring using Managed Service Identity, which we think will allow us to get tokens that are valid for resources in other tenants. If there's a better way, I'm certainly interested in knowing about it.
I've followed the code example from here using the Microsoft.Azure.Services.AppAuthentication library: https://learn.microsoft.com/en-us/azure/app-service/app-service-managed-service-identity#obtaining-tokens-for-azure-resources
// In Web API 2
using Microsoft.Azure.Services.AppAuthentication;
// ...
var azureServiceTokenProvider = new AzureServiceTokenProvider();
string accessToken = await azureServiceTokenProvider.GetAccessTokenAsync(
"https://<App ID URI for Web API1>");
Web API2 is configured to have a Managed Service Identity.
I'm currently running this on my local machine, and I've installed Azure CLI and I'm logged in. I've tried 'az account get-access-token', and I get a valid token.
When Web API2 tries to get the token to be able to call Web API1, I get an exception:
Parameters: Connectionstring: [No connection string specified], Resource: , Authority: . Exception Message: Tried the following 2 methods to get an access token, but none of them worked.
Parameters: Connectionstring: [No connection string specified], Resource: , Authority: . Exception Message: Tried to get token using Managed Service Identity. Unable to connect to the Managed Service Identity (MSI) endpoint. Please check that you are running on an Azure resource that has MSI setup.
Parameters: Connectionstring: [No connection string specified], Resource: , Authority: . Exception Message: Tried to get token using Azure CLI. Access token could not be acquired. ERROR: Get Token request returned http error: 400 and server response: {"error":"invalid_grant","error_description":"AADSTS65001: The user or administrator has not consented to use the application with ID '04b07795-8ddb-461a-bbee-02f9e1bf7b46' named 'Web API 1'. Send an interactive authorization request for this user and resource.\r\nTrace ID: f5bb0d4d-6f92-4fdd-81b7-e82a78720a00\r\nCorrelation ID: 04f92114-8d9d-40c6-b292-965168d6a919\r\nTimestamp: 2017-10-19 16:39:22Z","error_codes":[65001],"timestamp":"2017-10-19 16:39:22Z","trace_id":"f5bb0d4d-6f92-4fdd-81b7-e82a78720a00","correlation_id":"04f92114-8d9d-40c6-b292-965168d6a919"}
What's interesting is that there's no application with ID '04b07795-8ddb-461a-bbee-02f9e1bf7b46' in either AAD1 or AAD2. Is this a known Azure app? I thought that it might be the Service Management API, but I'm not sure.
In any case, I'm not sure of the proper way to grant permission. I've tried building different content URLs like this into my browser, but none of them seem to have done the trick:
https://login.microsoftonline.com/(AAD1 ID)/adminconsent
?client_id=(App ID)
&redirect_uri=https://localhost:44341
&resource=(App ID URI for Web API1)
&prompt=admin_consent
https://login.microsoftonline.com/(AAD1 ID)/adminconsent
?client_id=04b07795-8ddb-461a-bbee-02f9e1bf7b46
&redirect_uri=https://localhost:44341
&resource=(App ID URI for Web API1)
&prompt=admin_consent
(This last one tells me that the reply URL is incorrect; since it's not one of my apps, I can't find the reply URL)
Note that the tenant is AAD1.
Am I missing something, or am I not using this feature correctly?
Thanks in advance.

Answer 1

AzureServiceTokenProvider uses Azure CLI (among other options) for local development. For a scenario where a service calls an Azure Service, this works using the developer identity from Azure CLI, since Azure services allow access to both users and applications.
For a scenario where a service calls another custom service (like your scenario), you need to use a service principal for local development. For this, you have two options:
Login to Azure CLI using a service principal.
First, create a service principal for local development
https://learn.microsoft.com/en-us/cli/azure/create-an-azure-service-principal-azure-cli?view=azure-cli-latest
Then login to Azure CLI using it.
az login --service-principal -u 25922285-eab9-4262-ba61-8083533a929b --password <<pwd>> --tenant 72f988bf-86f1-41af-91ab-2d7cd011db47 --allow-no-subscriptions
Use the --allow-no-subscriptions argument since this service principal may not have access to any subscription.
Now, AzureServiceTokenProvider will get a token using this service principal for local development.
Specify service principal details in an environment variable. AzureServiceTokenProvider will use the specified service principal for local development. Please see the section Running the application using a service principal in local development environment in this sample on how to do that. https://github.com/Azure-Samples/app-service-msi-keyvault-dotnet
Note: Ths is only for local development. AzureServiceTokenProvider will use MSI when deployed to App Service.