Dotnet Blazor Auth

dotnet-blazor-auth

Authentication and authorization across all Blazor hosting models. Covers AuthorizeView, CascadingAuthenticationState, Identity UI scaffolding, role/policy-based authorization, per-hosting-model auth flow differences (cookie vs token), and external identity providers.

Scope

• Auth flow per Blazor hosting model (Server, WASM, Auto, SSR, Hybrid)
• AuthorizeView and CascadingAuthenticationState patterns
• Identity UI scaffolding and customization
• Role/policy-based authorization in Blazor
• Client-side token handling and external identity providers
• Explicit login/logout/auth UI implementation tasks for Blazor apps

Out of scope

• JWT token generation and validation -- see [skill:dotnet-api-security]
• OWASP security principles -- see [skill:dotnet-security-owasp]
• CSRF/XSS/CSP/rate-limiting hardening without auth-flow work -- see [skill:dotnet-security-owasp]
• Hardening-only reviews of existing login pages without auth-flow implementation changes -- see [skill:dotnet-security-owasp]
• bUnit testing of auth components -- see [skill:dotnet-blazor-testing]
• E2E auth testing -- see [skill:dotnet-playwright]
• UI framework selection -- see [skill:dotnet-ui-chooser]

Cross-references: [skill:dotnet-api-security] for API-level auth, [skill:dotnet-security-owasp] for OWASP principles, [skill:dotnet-blazor-patterns] for hosting models, [skill:dotnet-blazor-components] for component architecture, [skill:dotnet-blazor-testing] for bUnit testing, [skill:dotnet-playwright] for E2E testing, [skill:dotnet-ui-chooser] for framework selection.

Routing note: do not load this skill for OWASP hardening reviews unless the task explicitly includes Blazor auth flow/UI implementation.

Auth Flow per Hosting Model

Authentication patterns differ significantly across Blazor hosting models:

InteractiveServer Auth

Server-side Blazor uses cookie authentication. The user authenticates via a standard ASP.NET Core login flow, and the cookie is sent with the initial HTTP request that establishes the SignalR circuit.

// Program.cs
builder.Services.AddAuthentication(CookieAuthenticationDefaults.AuthenticationScheme)
    .AddCookie(options =>
    {
        options.LoginPath = "/Account/Login";
        options.AccessDeniedPath = "/Account/AccessDenied";
    });

builder.Services.AddCascadingAuthenticationState();
builder.Services.AddAuthorization();

Gotcha: HttpContext is available during the initial HTTP request but is null inside interactive components after the SignalR circuit is established. Do not access HttpContext in interactive component lifecycle methods. Use AuthenticationStateProvider instead.

InteractiveWebAssembly Auth

WASM runs in the browser. Cookie auth works for same-origin APIs (and Backend-for-Frontend / BFF patterns), but token-based auth (OIDC/JWT) is the standard approach for cross-origin APIs and delegated access scenarios:

// Client Program.cs (WASM)
builder.Services.AddOidcAuthentication(options =>
{
    options.ProviderOptions.Authority = "https://login.example.com";
    options.ProviderOptions.ClientId = "blazor-wasm-client";
    options.ProviderOptions.ResponseType = "code";
    options.ProviderOptions.DefaultScopes.Add("api");
});

// Attach tokens to API calls using BaseAddressAuthorizationMessageHandler
// (auto-attaches tokens for requests to the app's base address)
builder.Services.AddHttpClient("API", client =>
    client.BaseAddress = new Uri("https://api.example.com"))
    .AddHttpMessageHandler(sp =>
        sp.GetRequiredService<AuthorizationMessageHandler>()
            .ConfigureHandler(
                authorizedUrls: ["https://api.example.com"],
                scopes: ["api"]));

builder.Services.AddScoped(sp =>
    sp.GetRequiredService<IHttpClientFactory>().CreateClient("API"));

InteractiveAuto Auth

Auto mode starts as InteractiveServer (cookie auth), then transitions to WASM (token auth). Handle both:

// Server Program.cs
builder.Services.AddAuthentication()
    .AddCookie()
    .AddJwtBearer(); // For WASM API calls after transition

builder.Services.AddCascadingAuthenticationState();

Hybrid (MAUI) Auth

// Register platform-specific auth
builder.Services.AddAuthorizationCore();
builder.Services.AddScoped<AuthenticationStateProvider, MauiAuthStateProvider>();

// Custom provider using secure storage
public class MauiAuthStateProvider : AuthenticationStateProvider
{
    public override async Task<AuthenticationState> GetAuthenticationStateAsync()
    {
        var token = await SecureStorage.Default.GetAsync("auth_token");
        if (string.IsNullOrEmpty(token))
        {
            return new AuthenticationState(new ClaimsPrincipal(new ClaimsIdentity()));
        }

        var claims = ParseClaimsFromJwt(token);
        var identity = new ClaimsIdentity(claims, "jwt");
        return new AuthenticationState(new ClaimsPrincipal(identity));
    }
}

AuthorizeView

AuthorizeView conditionally renders content based on the user's authentication and authorization state.

Basic Usage

<AuthorizeView>
    <Authorized>
        <p>Welcome, @context.User.Identity?.Name!</p>
        <a href="/Account/Logout">Log out</a>
    </Authorized>
    <NotAuthorized>
        <a href="/Account/Login">Log in</a>
    </NotAuthorized>
    <Authorizing>
        <p>Checking authentication...</p>
    </Authorizing>
</AuthorizeView>

Role-Based

<AuthorizeView Roles="Admin,Manager">
    <Authorized>
        <AdminDashboard />
    </Authorized>
    <NotAuthorized>
        <p>You do not have access to the admin dashboard.</p>
    </NotAuthorized>
</AuthorizeView>

Policy-Based

<AuthorizeView Policy="CanEditProducts">
    <Authorized>
        <button @onclick="EditProduct">Edit</button>
    </Authorized>
</AuthorizeView>

// Register policy in Program.cs
builder.Services.AddAuthorizationBuilder()
    .AddPolicy("CanEditProducts", policy =>
        policy.RequireClaim("permission", "products.edit"));

CascadingAuthenticationState

CascadingAuthenticationState provides the current AuthenticationState as a cascading parameter to all descendant components.

Setup

// Program.cs -- register cascading auth state
builder.Services.AddCascadingAuthenticationState();

This replaces wrapping the entire app in <CascadingAuthenticationState> (the older pattern). The service-based registration (.NET 8+) is preferred.

Consuming Auth State in Components

@code {
    [CascadingParameter]
    private Task<AuthenticationState>? AuthState { get; set; }

    private string? userName;

    protected override async Task OnInitializedAsync()
    {
        if (AuthState is not null)
        {
            var state = await AuthState;
            userName = state.User.Identity?.Name;
        }
    }
}

Accessing Claims

var state = await AuthState;
var user = state.User;

// Check authentication
if (user.Identity?.IsAuthenticated == true)
{
    var email = user.FindFirst(ClaimTypes.Email)?.Value;
    var roles = user.FindAll(ClaimTypes.Role).Select(c => c.Value);
    var isAdmin = user.IsInRole("Admin");
}

Identity UI Scaffolding

ASP.NET Core Identity provides a complete authentication system with registration, login, email confirmation, password reset, and two-factor authentication.

Adding Identity to a Blazor Web App

# Add Identity scaffolding
dotnet add package Microsoft.AspNetCore.Identity.EntityFrameworkCore
dotnet add package Microsoft.AspNetCore.Identity.UI

// Program.cs
builder.Services.AddDbContext<ApplicationDbContext>(options =>
    options.UseSqlServer(builder.Configuration.GetConnectionString("Default")));

builder.Services.AddIdentity<ApplicationUser, IdentityRole>(options =>
{
    options.Password.RequireDigit = true;
    options.Password.RequiredLength = 8;
    options.Password.RequireNonAlphanumeric = true;
    options.SignIn.RequireConfirmedAccount = true;
})
.AddEntityFrameworkStores<ApplicationDbContext>()
.AddDefaultTokenProviders();

Scaffolding Identity Pages

# Scaffold individual Identity pages for customization
dotnet aspnet-codegenerator identity -dc ApplicationDbContext --files "Account.Login;Account.Register;Account.Logout"

Custom Identity UI with Blazor Components

For a fully Blazor-native auth experience, create Blazor components that call Identity APIs:

@page "/Account/Login"
@inject SignInManager<ApplicationUser> SignInManager
@inject NavigationManager Navigation

<EditForm Model="loginModel" FormName="login" Enhance>
    <DataAnnotationsValidator />
    <ValidationSummary />

    <div>
        <InputText @bind-Value="loginModel.Email" placeholder="Email" />
    </div>
    <div>
        <InputText @bind-Value="loginModel.Password" type="password" placeholder="Password" />
    </div>
    <div>
        <InputCheckbox @bind-Value="loginModel.RememberMe" /> Remember me
    </div>

    <button type="submit">Log in</button>
</EditForm>

@if (!string.IsNullOrEmpty(errorMessage))
{
    <p class="text-danger">@errorMessage</p>
}

@code {
    [SupplyParameterFromForm]
    private LoginModel loginModel { get; set; } = new();

    private string? errorMessage;

    private async Task HandleLogin()
    {
        var result = await SignInManager.PasswordSignInAsync(
            loginModel.Email, loginModel.Password,
            loginModel.RememberMe, lockoutOnFailure: true);

        if (result.Succeeded)
        {
            Navigation.NavigateTo("/", forceLoad: true);
        }
        else if (result.RequiresTwoFactor)
        {
            Navigation.NavigateTo("/Account/LoginWith2fa");
        }
        else if (result.IsLockedOut)
        {
            errorMessage = "Account is locked. Try again later.";
        }
        else
        {
            errorMessage = "Invalid login attempt.";
        }
    }
}

Gotcha: SignInManager uses HttpContext to set cookies. In Interactive render modes, HttpContext is not available after the circuit is established. Login/logout pages must use Static SSR (no @rendermode) so they have access to HttpContext for cookie operations.

Role and Policy-Based Authorization

Page-Level Authorization

@page "/admin"
@attribute [Authorize(Roles = "Admin")]

<h1>Admin Panel</h1>

@page "/products/manage"
@attribute [Authorize(Policy = "ProductManager")]

<h1>Manage Products</h1>

Defining Policies

builder.Services.AddAuthorizationBuilder()
    .AddPolicy("ProductManager", policy =>
        policy.RequireRole("Admin", "ProductManager"))
    .AddPolicy("CanDeleteOrders", policy =>
        policy.RequireClaim("permission", "orders.delete")
              .RequireAuthenticatedUser())
    .AddPolicy("MinimumAge", policy =>
        policy.AddRequirements(new MinimumAgeRequirement(18)));

Custom Authorization Handler

public sealed class MinimumAgeRequirement(int minimumAge) : IAuthorizationRequirement
{
    public int MinimumAge { get; } = minimumAge;
}

public sealed class MinimumAgeHandler : AuthorizationHandler<MinimumAgeRequirement>
{
    protected override Task HandleRequirementAsync(
        AuthorizationHandlerContext context,
        MinimumAgeRequirement requirement)
    {
        var dateOfBirthClaim = context.User.FindFirst("date_of_birth");
        if (dateOfBirthClaim is not null
            && DateOnly.TryParse(dateOfBirthClaim.Value, out var dob))
        {
            var age = DateOnly.FromDateTime(DateTime.UtcNow).Year - dob.Year;
            if (age >= requirement.MinimumAge)
            {
                context.Succeed(requirement);
            }
        }
        return Task.CompletedTask;
    }
}

// Register
builder.Services.AddSingleton<IAuthorizationHandler, MinimumAgeHandler>();

Procedural Authorization in Components

@inject IAuthorizationService AuthorizationService

@code {
    [CascadingParameter]
    private Task<AuthenticationState>? AuthState { get; set; }

    private bool canEdit;

    protected override async Task OnInitializedAsync()
    {
        if (AuthState is not null)
        {
            var state = await AuthState;
            var result = await AuthorizationService.AuthorizeAsync(
                state.User, "CanEditProducts");
            canEdit = result.Succeeded;
        }
    }
}

External Identity Providers

Adding External Providers

builder.Services.AddAuthentication()
    .AddMicrosoftAccount(options =>
    {
        options.ClientId = builder.Configuration["Auth:Microsoft:ClientId"]!;
        options.ClientSecret = builder.Configuration["Auth:Microsoft:ClientSecret"]!;
    })
    .AddGoogle(options =>
    {
        options.ClientId = builder.Configuration["Auth:Google:ClientId"]!;
        options.ClientSecret = builder.Configuration["Auth:Google:ClientSecret"]!;
    });

External Login Flow per Hosting Model

For WASM, configure the OIDC provider in the client project:

// Client Program.cs
builder.Services.AddOidcAuthentication(options =>
{
    options.ProviderOptions.Authority = "https://login.microsoftonline.com/{tenant}";
    options.ProviderOptions.ClientId = "{client-id}";
    options.ProviderOptions.ResponseType = "code";
});

For MAUI Hybrid:

var result = await WebAuthenticator.Default.AuthenticateAsync(
    new Uri("https://login.example.com/authorize"),
    new Uri("myapp://callback"));
var token = result.AccessToken;

Agent Gotchas

1. Do not access HttpContext in interactive components. HttpContext is only available during the initial HTTP request. After the SignalR circuit is established (InteractiveServer) or the WASM runtime loads, it is null. Use AuthenticationStateProvider or CascadingAuthenticationState instead.
2. Do not rely on cookies for cross-origin or delegated API access in WASM. Use OIDC/JWT with AuthorizationMessageHandler for cross-origin APIs. Same-origin and Backend-for-Frontend (BFF) cookie auth remains valid for WASM apps.
3. Do not render login/logout pages in Interactive mode. SignInManager requires HttpContext to set/clear cookies. Login and logout pages must use Static SSR render mode.
4. Do not store tokens in localStorage without considering XSS. If the app is vulnerable to XSS, tokens in localStorage can be stolen. Use sessionStorage (cleared on tab close) or the OIDC library's built-in storage mechanisms with PKCE.
5. Do not forget AddCascadingAuthenticationState(). Without it, [CascadingParameter] Task<AuthenticationState> is always null in components, silently breaking auth checks.
6. Do not use AddIdentity and AddDefaultIdentity together. AddDefaultIdentity includes UI scaffolding; AddIdentity does not. Choose one based on whether you want the default Identity UI pages.

Prerequisites

• .NET 8.0+ (Blazor Web App with render modes, AddCascadingAuthenticationState service registration)
• Microsoft.AspNetCore.Identity.EntityFrameworkCore for Identity with EF Core
• Microsoft.AspNetCore.Identity.UI for default Identity UI scaffolding
• Microsoft.AspNetCore.Authentication.MicrosoftAccount / .Google for external providers
• Microsoft.Authentication.WebAssembly.Msal for WASM with Microsoft Identity (Azure AD/Entra)

References

• Blazor Authentication and Authorization
• Blazor Server Auth
• Blazor WebAssembly Auth
• ASP.NET Core Identity
• External Login Providers
• Role/Policy-Based Authorization