Windows Authentication can be configured for ASP.NET Core apps hosted with IIS, [HTTP.sys](xref:fundamentals/servers/httpsys), or [WebListener](xref:fundamentals/servers/weblistener).
Windows Authentication relies on the operating system to authenticate users of ASP.NET Core apps. You can use Windows Authentication when your server runs on a corporate network using Active Directory domain identities or other Windows accounts to identify users. Windows Authentication is best suited to intranet environments in which users, client applications, and web servers belong to the same Windows domain.
[Learn more about Windows Authentication and installing it for IIS](/iis/configuration/system.webServer/security/authentication/windowsAuthentication/).
For development work using IIS Express, the template provides all the configuration necessary to use Windows Authentication. The following section shows how to manually configure an ASP.NET Core app for Windows Authentication.
IIS uses the [ASP.NET Core Module](xref:fundamentals/servers/aspnet-core-module) to host ASP.NET Core apps. Windows Authentication is configured in IIS, not the app. The following sections show how to use IIS Manager to configure an ASP.NET Core app to use Windows Authentication.
### IIS configuration
Enable the IIS Role Service for Windows Authentication. For more information, see [Enable Windows Authentication in IIS Role Services (see Step 2)](xref:host-and-deploy/iis/index#iis-configuration).
IIS Integration Middleware is configured to automatically authenticate requests by default. For more information, see [Host ASP.NET Core on Windows with IIS: IIS options (AutomaticAuthentication)](xref:host-and-deploy/iis/index#iis-options).
The ASP.NET Core Module is configured to forward the Windows Authentication token to the app by default. For more information, see [ASP.NET Core Module configuration reference: Attributes of the aspNetCore element](xref:host-and-deploy/aspnet-core-module#attributes-of-the-aspnetcore-element).
Although Kestrel doesn't support Windows Authentication, you can use [HTTP.sys](xref:fundamentals/servers/httpsys) to support self-hosted scenarios on Windows. The following example configures the app's web host to use HTTP.sys with Windows Authentication:
> HTTP.sys delegates to kernel mode authentication with the Kerberos authentication protocol. User mode authentication isn't supported with Kerberos and HTTP.sys. The machine account must be used to decrypt the Kerberos token/ticket that's obtained from Active Directory and forwarded by the client to the server to authenticate the user. Register the Service Principal Name (SPN) for the host, not the user of the app.
> HTTP.sys isn't supported on Nano Server version 1709 or later. To use Windows Authentication and HTTP.sys with Nano Server, use a [Server Core (microsoft/windowsservercore) container](https://hub.docker.com/r/microsoft/windowsservercore/). For more information on Server Core, see [What is the Server Core installation option in Windows Server?](/windows-server/administration/server-core/what-is-server-core).
Although Kestrel doesn't support Windows Authentication, you can use [WebListener](xref:fundamentals/servers/weblistener) to support self-hosted scenarios on Windows. The following example configures the app's web host to use WebListener with Windows Authentication:
> WebListener delegates to kernel mode authentication with the Kerberos authentication protocol. User mode authentication isn't supported with Kerberos and WebListener. The machine account must be used to decrypt the Kerberos token/ticket that's obtained from Active Directory and forwarded by the client to the server to authenticate the user. Register the Service Principal Name (SPN) for the host, not the user of the app.
The configuration state of anonymous access determines the way in which the `[Authorize]` and `[AllowAnonymous]` attributes are used in the app. The following two sections explain how to handle the disallowed and allowed configuration states of anonymous access.
When Windows Authentication is enabled and anonymous access is disabled, the `[Authorize]` and `[AllowAnonymous]` attributes have no effect. If the IIS site (or HTTP.sys or WebListener server) is configured to disallow anonymous access, the request never reaches your app. For this reason, the `[AllowAnonymous]` attribute isn't applicable.
When both Windows Authentication and anonymous access are enabled, use the `[Authorize]` and `[AllowAnonymous]` attributes. The `[Authorize]` attribute allows you to secure pieces of the app which truly do require Windows Authentication. The `[AllowAnonymous]` attribute overrides `[Authorize]` attribute usage within apps which allow anonymous access. See [Simple Authorization](xref:security/authorization/simple) for attribute usage details.
In ASP.NET Core 2.x, the `[Authorize]` attribute requires additional configuration in *Startup.cs* to challenge anonymous requests for Windows Authentication. The recommended configuration varies slightly based on the web server being used.
> By default, users who lack authorization to access a page are presented with an empty HTTP 403 response. The [StatusCodePages middleware](xref:fundamentals/error-handling#configure-status-code-pages) can be configured to provide users with a better "Access Denied" experience.
ASP.NET Core doesn't implement impersonation. Apps run with the application identity for all requests, using app pool or process identity. If you need to explicitly perform an action on behalf of a user, use `WindowsIdentity.RunImpersonated`. Run a single action in this context and then close the context.
Note that `RunImpersonated` doesn't support asynchronous operations and shouldn't be used for complex scenarios. For example, wrapping entire requests or middleware chains isn't supported or recommended.
