Azure B2C App Always Redirects to <domain>/MicrosoftIdentity/Account/Error

I have a simple .NET 8 razor pages test application where I'm trying to leverage an Azure B2C tenant for identity. I have Azure AD B2C configured in my program.cs file, and I have added authorization to all pages so that an unauthenticated user is always directed to log in, but rather than getting the azure b2c login page, I keep getting directed to https://localhost:7027/MicrosoftIdentity/Account/Error, which results in a 404 error. Im doing this in debug mode in visual studio. Im not sure what I may have configured incorrectly, as I don't have any real experience in B2C prior to this.

Program.cs

using Microsoft.Extensions.Configuration;
using Microsoft.Identity.Web;

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
builder.Services.AddRazorPages();

// Add Azure AD B2C authentication
builder.Services.AddAuthentication(OpenIdConnectDefaults.AuthenticationScheme)
    .AddMicrosoftIdentityWebApp(builder.Configuration.GetSection("AzureAdB2C"));

builder.Services.AddAuthorization();

// Apply [Authorize] attribute globally
builder.Services.AddRazorPages(options =>
{
    options.Conventions.AuthorizePage("/");
    options.Conventions.AuthorizeFolder("/");
});

var app = builder.Build();

// Configure the HTTP request pipeline.
if (!app.Environment.IsDevelopment())
{
    app.UseExceptionHandler("/Error");
    // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
    app.UseHsts();
}

app.UseHttpsRedirection();
app.UseStaticFiles();

app.UseRouting();

app.UseAuthentication();
app.UseAuthorization();

app.MapRazorPages();

app.Run();

appsettings.json

{
  "AzureAdB2C": {
    "Instance": "https://<my-tenant-name>.b2clogin.com",
    "Domain": "<my-tenant-name>.onmicrosoft.com",
    "TenantId": "<my-tenant-id>",
    "ClientId": "<my-client-id>",
    "ClientSecret": "<my-client-secret>",
    "CallbackPath": "/signin-oidc",
    "SignUpSignInPolicyId": "<my-signinsignup-policy>",
    "ResetPasswordPolicyId": "<my-passwordreset-policy>",
    "EditProfilePolicyId": "<my-profileediting-policy>"
  },
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Warning",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  },
  "AllowedHosts": "*"
}

Solution

I got the same error while authenticating to Azure AD B2C, the error message is related to Azure AD B2C cannot find the 'b2c-extension-app` in the tenant.

enter image description here

To resolve the issue, you can download the extension app in your Azure AD B2C, please refer this doc to download the extension.
You can also create a new Azure AD B2C tenant to avoid the error.

I've created a sample application and successfully configured Azure B2C authentication.

Program.cs :

using Microsoft.AspNetCore.Authentication;
using Microsoft.AspNetCore.Authentication.OpenIdConnect;
using Microsoft.AspNetCore.Authorization;
using Microsoft.AspNetCore.Mvc.Authorization;
using Microsoft.Identity.Web;
using Microsoft.Identity.Web.UI;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddAuthentication(OpenIdConnectDefaults.AuthenticationScheme)
    .AddMicrosoftIdentityWebApp(builder.Configuration.GetSection("AzureAd"));

builder.Services.AddAuthorization(options =>
{
        options.FallbackPolicy = options.DefaultPolicy;
});
builder.Services.AddRazorPages()
    .AddMicrosoftIdentityUI();

var app = builder.Build();


if (!app.Environment.IsDevelopment())
{
    app.UseExceptionHandler("/Error");
    app.UseHsts();
}
app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseRouting();
app.UseAuthentication();
app.UseAuthorization();
app.MapRazorPages();
app.MapControllers();
app.Run();

appsettings.json:

{
  "AzureAd": {
    "Instance": "https://{ Azure B2C Domain Name}.b2clogin.com/",
    "Domain": "{ Azure B2C Domain Name}.onmicrosoft.com",
    "TenantId": "<Tenant-id>",
    "ClientId": "<Client-id>",
    "CallbackPath": "/signin-oidc",
    "SignUpSignInPolicyId": "B2C_1_signupsignindemo",
    "SignedOutCallbackPath": "/signout/B2C_1_susi",
    "ResetPasswordPolicyId": "b2c_1_reset",
    "EditProfilePolicyId": "b2c_1_edit_profile",
    "EnablePiiLogging": true
  },
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  },
  "AllowedHosts": "*"
}

I've successfully authenticated to Azure B2C.

Output:

enter image description here