Unable to connect to Azure Key Vault from Azure Web App - azure

Question

I am trying to access Azure Key Vault from my Azure App Service. I followed the steps outlined on this documentation: https://learn.microsoft.com/en-us/azure/key-vault/managed-identity (turned on system assigned identity for the app service, updated the access policy of the key vault to include the app with Get,List secret permissions).
However, when I run the web application, it is not able to get the secret from the key vault and my web service hits the following error:
502 - Web server received an invalid response while acting as a gateway or proxy server.
There is a problem with the page you are looking for, and it cannot be displayed. When the Web server (while acting as a gateway or proxy) contacted the upstream content server, it received an invalid response from the content server.
This is what my code looks like:
AzureServiceTokenProvider azureServiceTokenProvider = new AzureServiceTokenProvider();
KeyVaultClient keyVaultClient = new KeyVaultClient(new KeyVaultClient.AuthenticationCallback(azureServiceTokenProvider.KeyVaultTokenCallback));
var secret = keyVaultClient.GetSecretAsync(KeyVaultUrl);
authenticationKey = secret.Result.Value;
The service gets stuck on the secret.Result.Value line. Is there something else I need to do?

Answer 1

This is much easier with the new package, like Azure.Security.KeyVault.Secrets. Together with Azure.Identity, you can just pass a DefaultAzureCredential like in our samples.
var client = new SecretClient(
new Uri("https://myvault.vault.azure.net"),
new DefaultAzureCredential());
KeyVaultSecret secret = await client.GetSecretAsync("secret-name");
string authenticationKey = secret.Value;
The DefaultAzureCredential is optimized to work for managed identity, service principals from the environment, and interactive logins to support the same code running both in production and on developer machines. The new libraries are also faster with fewer allocations, and have much better diagnostics - defaulted to on when using Azure Application Monitor.
They target netstandard2.0 so should be compatible with the older packages these replace. Would you be able to upgrade? We're only making critical fixes for the older packages, and recommending people upgrade to Azure.* packages intead of the older Microosft.Azure.* packages.
As for the problem itself, it's hard to say without knowing when you're calling this in your application. During startup? What version of .NET? What are you using for your ASP.NET application framework?
While it's probably not the cause of the problem, it's hard to ignore that you're calling an async method synchronously, which can also cause problems. If you're in an async method, you should write your code like so:
var secret = await keyVaultClient.GetSecretAsync(KeyVaultUrl);
authenticationKey = secret.Value;
If not, call:
var secret = keyVaultClient.GetSecretAsnc(KeyVaultUrl).GetAwaiter().GetResult();
This is not recommended, though. In the new packages I mentioned above, we have both sync and async versions that are either sync or async all the way through the call stack and safer to use. Generally, though, you should use async calls - especially for network traffic like accessing Key Vault because, depending on what thread you call it, it can hang your UI.

Answer 2

When you test in local:
Add your vs signed account into azure keyvault. Go to keyvault> Access policy> add your account with get secret permmission.
When you publish to azure:
1.Enable webapp MSI.
2.Go to keyvault> Access policy> add your webapp's service principal with get secret permission.
The code:
AzureServiceTokenProvider azureServiceTokenProvider = new AzureServiceTokenProvider();
KeyVaultClient keyVaultClient = new KeyVaultClient(new Microsoft.Azure.KeyVault.KeyVaultClient.AuthenticationCallback(azureServiceTokenProvider.KeyVaultTokenCallback));
var secret = keyVaultClient.GetSecretAsync("https://yourkevaultname.vault.azure.net/secrets/secretname/437d301daxxxxxx");
var authenticationKey = secret.Result.Value;
ViewBag.Message = authenticationKey.ToString();