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How does the OAuth 2.0 Authorization Code flow work, and why is it preferred over the deprecated Implicit flow for web applications? How do you implement policy-based authorization for more complex, custom access rules beyond simple roles? How does token revocation work with stateless JWTs, given they can't simply be 'deleted' server-side once issued? How does ASP.NET Core support multiple authentication schemes simultaneously (e.g., cookies for a web UI and JWT for an API in the same application)? How does token validation work under the hood when the JwtBearer middleware receives a request, and what checks does it perform? How do authentication and authorization work together in a .NET application, and how would you implement both role-based and policy-based access control?
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How do you integrate external identity providers (Google, Microsoft Entra ID, Facebook) using OAuth/OpenID Connect in ASP.NET Core?

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Answer

ASP.NET Core provides dedicated authentication handler packages (Microsoft.AspNetCore.Authentication.Google, .MicrosoftAccount, etc., or the generic OpenID Connect handler for any compliant provider) that implement the OAuth/OIDC redirect flow, handle token exchange, and map the provider's returned claims into a local ClaimsPrincipal -- letting users 'Sign in with Google' without the application ever handling their Google password directly.
builder.Services.AddAuthentication()
    .AddGoogle(options => {
        options.ClientId = builder.Configuration["Google:ClientId"];
        options.ClientSecret = builder.Configuration["Google:ClientSecret"];
    });

// Triggering the flow:
// GET /Account/ExternalLogin?provider=Google
Real-world example A B2B SaaS platform lets enterprise customers sign in with Microsoft Entra ID (formerly Azure AD) using the OpenID Connect handler, integrating with each customer's existing corporate identity system instead of maintaining separate app-specific passwords.

Common follow-ups: What's the difference between OAuth 2.0 and OpenID Connect?;How do you link an external login to an existing local account?

Authentication & Authorization (Identity JWT OAuth);Configuration & Options
All 16 Authentication & Authorization (Identity, JWT, OAuth) questions