Add extra claims to an Azure B2C user flow using API connectors and ASP.NET Core

This post shows how to implement an ASP.NET Core Razor Page application which authenticates using Azure B2C and uses custom claims implemented using the Azure B2C API connector. The claims provider is implemented using an ASP.NET Core API application and the Azure API connector requests the data from this API. The Azure API connector adds the claims after an Azure B2C sign in flow or whatever settings you configured in the Azure B2C user flow.


Setup the Azure B2C App Registration

An Azure App registration is setup for the ASP.NET Core Razor page application. A client secret is used to authenticate the client. The redirect URI is added for the app. This is a standard implementation.

Setup the API connector

The API connector is setup to add the extra claims after a sign in. This defines the API endpoint and the authentication method. Only Basic or certificate authentication is possible for this API service. Both of these are not ideal for implementing and using this service to add extra claims to the identity. I started ngrok using the cmd and used the URL from this to configure Azure B2C API connector. Maybe two separate connectors could be setup for a solution, one like this for development and a second one with the Azure App service host address and certificate authentication used.

Azure B2C user attribute

The custom claims are added to the Azure B2C user attributes. The custom claims can be add as required.

Setup to Azure B2C user flow

The Azure B2C user flow is configured to used the API connector. This flow adds the application claims to the token which it receives from the API call used in the API connector.

The custom claims are added then using the application claims blade. This is required if the custom claims are to be added.

I also added the custom claims to the Azure B2C user flow user attributes.

Azure B2C is now setup to use the custom claims and the data for these claims will be set used the API connector service.

ASP.NET Core Razor Page

The ASP.NET Core Razor Page uses Microsoft.Identity.Web to authenticate using Azure B2C. This is a standard setup for a B2C user flow.


builder.Services.AddAuthorization(options =>
options.FallbackPolicy = options.DefaultPolicy;

var app = builder.Build();


The main difference between an Azure B2C user flow and an Azure AD authentication is the configuration. The SignUpSignInPolicyId is set to match the configured Azure B2C user flow and the Instance uses the b2clogin from the domain unlike the AAD configuration definition.

“AzureAdB2C”: {
“Instance”: “”,
“ClientId”: “ab393e93-e762-4108-a3f5-326cf8e3874b”,
“Domain”: “”,
“SignUpSignInPolicyId”: “B2C_1_ExtraClaims”,
“TenantId”: “f611d805-cf72-446f-9a7f-68f2746e4724”,
“CallbackPath”: “/signin-oidc”,
“SignedOutCallbackPath”: “/signout-callback-oidc”
//”ClientSecret”: “–in-user-settings–”

The index Razor page returns the claims and displays the values in the UI.

public class IndexModel : PageModel
public IEnumerable<Claim> Claims { get; set; } = Enumerable.Empty<Claim>();

public void OnGet()
Claims = User.Claims;

This is all the end user application requires, there is no special setup here.

ASP.NET Core API connector implementation

The API implemented for the Azure API connector uses a HTTP Post. Basic authentication is used to validate the request as well as the client ID which needs to match the configured App registration. This is weak authentication and should not be used in production especially since the API provides sensitive PII data. If the request provides the correct credentials and the correct client ID, the data is returned for the email. In this demo, the email is returned in the custom claim. Normal the data would be returned using some data store or whatever.

public async Task<IActionResult> PostAsync()
// Check HTTP basic authorization
if (!IsAuthorized(Request))
_logger.LogWarning(“HTTP basic authentication validation failed.”);
return Unauthorized();

string content = await new System.IO.StreamReader(Request.Body).ReadToEndAsync();
var requestConnector = JsonSerializer.Deserialize<RequestConnector>(content);

// If input data is null, show block page
if (requestConnector == null)
return BadRequest(new ResponseContent(“ShowBlockPage”, “There was a problem with your request.”));

string clientId = _configuration[“AzureAdB2C:ClientId”];
if (!clientId.Equals(requestConnector.ClientId))
_logger.LogWarning(“HTTP clientId is not authorized.”);
return Unauthorized();

// If email claim not found, show block page. Email is required and sent by default.
if (requestConnector.Email == null || requestConnector.Email == “” || requestConnector.Email.Contains(“@”) == false)
return BadRequest(new ResponseContent(“ShowBlockPage”, “Email name is mandatory.”));

var result = new ResponseContent
// use the objectId of the email to get the user specfic claims
MyCustomClaim = $”everything awesome {requestConnector.Email}”

return Ok(result);

private bool IsAuthorized(HttpRequest req)
string username = _configuration[“BasicAuthUsername”];
string password = _configuration[“BasicAuthPassword”];

// Check if the HTTP Authorization header exist
if (!req.Headers.ContainsKey(“Authorization”))
_logger.LogWarning(“Missing HTTP basic authentication header.”);
return false;

// Read the authorization header
var auth = req.Headers[“Authorization”].ToString();

// Ensure the type of the authorization header id `Basic`
if (!auth.StartsWith(“Basic “))
_logger.LogWarning(“HTTP basic authentication header must start with ‘Basic ‘.”);
return false;

// Get the the HTTP basinc authorization credentials
var cred = System.Text.Encoding.UTF8.GetString(Convert.FromBase64String(auth.Substring(6))).Split(‘:’);

// Evaluate the credentials and return the result
return (cred[0] == username && cred[1] == password);

The ResponseContent class is used to return the data for the identity. All custom claims must be prefixed with the extension_ The data is then added to the profile data.

public class ResponseContent
public const string ApiVersion = “1.0.0”;

public ResponseContent()
Version = ApiVersion;
Action = “Continue”;

public ResponseContent(string action, string userMessage)
Version = ApiVersion;
Action = action;
UserMessage = userMessage;
if (action == “ValidationError”)
Status = “400”;

public string Version { get; }

public string Action { get; set; }

public string? UserMessage { get; set; }

public string? Status { get; set; }

public string MyCustomClaim { get; set; } = string.Empty;

With this, custom claims can be added to Azure B2C identities. This can be really useful when for example implementing verifiable credentials using id_tokens. This is much more complicated to implement compared to other IDPs but at least it is possible and can be solved. The technical solution to secure the API has room for improvements.


The applications can be started and the API connector needs to be mapped to a public IP. After starting the apps, start ngrok with a matching configuration for the HTTP address of the API connector API.

ngrok http https://localhost:5002

This URL in the API connector configured on Azure needs to match this ngrok URL. all good, the applications will run and the custom claim will be displayed in the UI.


The profile data in this API is very sensitive and you should use maximal security protections which are possible. Using Basic authentication alone for this type of API is not a good idea. It would be great to see managed identities supported or something like this. I used basic authentication so that I could use ngrok to demo the feature, we need a public endpoint for testing. I would not use this in a productive deployment. I would use certificate authentication with an Azure App service deployment and the certificate created and deployed using Azure Key Vault. Certificate rotation would have to be setup. I am not sure how good API connector infrastructure automation can be implemented, I have not tried this yet. A separate security solution would need to be implemented for local development. This is all a bit messy as all these extra steps end up in costs or developers taking short cuts and deploying with less security.


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