[Windows Authentication](/iis/configuration/system.webServer/security/authentication/windowsAuthentication/) can be configured for ASP.NET Core apps hosted with [IIS](xref:host-and-deploy/iis/index) or [HTTP.sys](xref:fundamentals/servers/httpsys).
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 Windows accounts to identify users. Windows Authentication is best suited to intranet environments where users, client apps, and web servers belong to the same Windows domain.
When modifying an existing project, confirm that the project file includes a package reference for the [Microsoft.AspNetCore.App metapackage](xref:fundamentals/metapackage-app) **or** the [Microsoft.AspNetCore.Authentication](https://www.nuget.org/packages/Microsoft.AspNetCore.Authentication/) NuGet package.
IIS uses the [ASP.NET Core Module](xref:host-and-deploy/aspnet-core-module) to host ASP.NET Core apps. Windows Authentication is configured for IIS via the *web.config* file. The following sections show how to:
* Provide a local *web.config* file that activates Windows Authentication on the server when the app is deployed.
* Use the IIS Manager to configure the *web.config* file of an ASP.NET Core app that has already been deployed to the server.
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).
When the project is published by the SDK (without the `<IsTransformWebConfigDisabled>` property set to `true` in the project file), the published *web.config* file includes the `<location><system.webServer><security><authentication>` section. For more information on the `<IsTransformWebConfigDisabled>` property, see <xref:host-and-deploy/iis/index#webconfig-file>.
#### Server-side configuration with the IIS Manager
Perform the following steps **after** you [publish and deploy your project](#publish-and-deploy-your-project-to-the-iis-site-folder).
1. In IIS Manager, select the IIS site under the **Sites** node of the **Connections** sidebar.
1. Double-click **Authentication** in the **IIS** area.
1. Select **Anonymous Authentication**. Select **Disable** in the **Actions** sidebar.
1. Select **Windows Authentication**. Select **Enable** in the **Actions** sidebar.
When these actions are taken, IIS Manager modifies the app's *web.config* file. A `<system.webServer><security><authentication>` node is added with updated settings for `anonymousAuthentication` and `windowsAuthentication`:
The `<system.webServer>` section added to the *web.config* file by IIS Manager is outside of the app's `<location>` section added by the .NET Core SDK when the app is published. Because the section is added outside of the `<location>` node, the settings are inherited by any [sub-apps](xref:host-and-deploy/iis/index#sub-applications) to the current app. To prevent inheritance, move the added `<security>` section inside of the `<location><system.webServer>` section that the SDK provided.
When IIS Manager is used to add the IIS configuration, it only affects the app's *web.config* file on the server. A subsequent deployment of the app may overwrite the settings on the server if the server's copy of *web.config* is replaced by the project's *web.config* file. Use **either** of the following approaches to manage the settings:
* Use IIS Manager to reset the settings in the *web.config* file after the file is overwritten on deployment.
* Add a *web.config file* to the app locally with the settings. For more information, see the [Development-side configuration](#development-side-configuration-with-a-local-webconfig-file) section.
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).
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) 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 app's 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](xref:System.Security.Principal.WindowsIdentity.RunImpersonated*) in a [terminal inline middleware](xref:fundamentals/middleware/index#create-a-middleware-pipeline-with-iapplicationbuilder) in `Startup.Configure`. Run a single action in this context and then close the context.
`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.
When hosting with IIS in-process mode, <xref:Microsoft.AspNetCore.Authentication.AuthenticationService.AuthenticateAsync*> isn't called internally to initialize a user. Therefore, an <xref:Microsoft.AspNetCore.Authentication.IClaimsTransformation> implementation used to transform claims after every authentication isn't activated by default. For more information and a code example that activates claims transformations when hosting in-process, see <xref:host-and-deploy/aspnet-core-module#in-process-hosting-model>.
