Configure Windows Authentication in ASP.NET Core

By Rick Anderson and Kirk Larkin

Windows Authentication (also known as Negotiate, Kerberos, or NTLM authentication) can be configured for ASP.NET Core apps hosted with IIS, Kestrel, or HTTP.sys.

Windows Authentication relies on the operating system to authenticate users of ASP.NET Core apps. Windows Authentication is used for servers that run 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.

Note

Windows Authentication isn't supported with HTTP/2. Authentication challenges can be sent on HTTP/2 responses, but the client must downgrade to HTTP/1.1 before authenticating.

Proxy and load balancer scenarios

Windows Authentication is a stateful scenario primarily used in an intranet, where a proxy or load balancer doesn't usually handle traffic between clients and servers. If a proxy or load balancer is used, Windows Authentication only works if the proxy or load balancer:

• Handles the authentication.
• Passes the user authentication information to the app (for example, in a request header), which acts on the authentication information.

An alternative to Windows Authentication in environments where proxies and load balancers are used is Active Directory Federated Services (ADFS) with OpenID Connect (OIDC).

IIS/IIS Express

Add the NuGet package Microsoft.AspNetCore.Authentication.Negotiate and authentication services by calling AddAuthentication in Program.cs:

using Microsoft.AspNetCore.Authentication.Negotiate;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddAuthentication(NegotiateDefaults.AuthenticationScheme)
   .AddNegotiate();

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

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.Run();

The preceding code was generated by the ASP.NET Core Razor Pages template with Windows Authentication specified.

Launch settings (debugger)

Configuration for launch settings only affects the Properties/launchSettings.json file for IIS Express and doesn't configure IIS for Windows Authentication. Server configuration is explained in the IIS section.

The Web Application templates available via Visual Studio or the .NET CLI can be configured to support Windows Authentication, which updates the Properties/launchSettings.json file automatically.

Visual Studio

New project

Create a new Razor Pages or MVC app. In the Additional information dialog, set the Authentication type to Windows.

Run the app. The username appears in the rendered app's user interface.

Existing project

The project's properties enable Windows Authentication and disable Anonymous Authentication. Open the launch profiles dialog:

1. In Solution Explorer, right click the project and select Properties.
2. Select the Debug > General tab and select Open debug launch profiles UI.
3. Clear the checkbox for Enable Anonymous Authentication.
4. Select the checkbox for Enable Windows Authentication.

Alternatively, the properties can be configured in the iisSettings node of the launchSettings.json file:

"iisSettings": {
    "windowsAuthentication": true,
    "anonymousAuthentication": false,
    "iisExpress": {
        "applicationUrl": "http://localhost:52171/",
        "sslPort": 44308
    }
}

.NET CLI

New project

Execute the dotnet new command with the webapp argument (ASP.NET Core Web App) and --auth Windows switch:

dotnet new webapp --auth Windows

Existing project

Update the iisSettings node of the launchSettings.json file:

"iisSettings": {
    "windowsAuthentication": true,
    "anonymousAuthentication": false,
    "iisExpress": {
        "applicationUrl": "http://localhost:52171/",
        "sslPort": 44308
    }
}

IIS

IIS uses the ASP.NET 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.

If you haven't already done so, enable IIS to host ASP.NET Core apps. For more information, see Host ASP.NET Core on Windows with IIS.

Enable the IIS Role Service for Windows Authentication. For more information, see Enable Windows Authentication in IIS Role Services (see Step 2).

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).

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.

Use either of the following approaches:

• Before publishing and deploying the project, add the following web.config file to the project root:

  <?xml version="1.0" encoding="utf-8"?>
  <configuration>
    <location path="." inheritInChildApplications="false">
      <system.webServer>
        <security>
          <authentication>
            <anonymousAuthentication enabled="false" />
            <windowsAuthentication enabled="true" />
          </authentication>
        </security>
      </system.webServer>
    </location>
  </configuration>
  

  When the project is published by the .NET Core 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 Host ASP.NET Core on Windows with IIS.

• After publishing and deploying the project, perform server-side configuration with the IIS Manager:

  1. In IIS Manager, select the IIS site under the Sites node of the Connections sidebar.
  2. Double-click Authentication in the IIS area.
  3. Select Anonymous Authentication. Select Disable in the Actions sidebar.
  4. 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:

  <system.webServer>
    <security>
      <authentication>
        <anonymousAuthentication enabled="false" />
        <windowsAuthentication enabled="true" />
      </authentication>
    </security>
  </system.webServer>
  

  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 to the current app. To prevent inheritance, move the added <security> section inside of the <location><system.webServer> section that the .NET Core 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.

Kestrel

The Microsoft.AspNetCore.Authentication.Negotiate NuGet package can be used with Kestrel to support Windows Authentication using Negotiate and Kerberos on Windows, Linux, and macOS.

Warning

Credentials can be persisted across requests on a connection. Negotiate authentication must not be used with proxies unless the proxy maintains a 1:1 connection affinity (a persistent connection) with Kestrel.

Note

The Negotiate handler detects if the underlying server supports Windows Authentication natively and if it is enabled. If the server supports Windows Authentication but it is disabled, an error is thrown asking you to enable the server implementation. When Windows Authentication is enabled in the server, the Negotiate handler transparently forwards authentication requests to it.

Authentication is enabled by the following highlighted code to Program.cs:

using Microsoft.AspNetCore.Authentication.Negotiate;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddAuthentication(NegotiateDefaults.AuthenticationScheme)
   .AddNegotiate();

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

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.Run();

The preceding code was generated by the ASP.NET Core Razor Pages template with Windows Authentication specified. The following APIs are used in the preceding code:

• AddAuthentication
• AddNegotiate
• UseAuthentication

Kerberos authentication and role-based access control (RBAC)

Kerberos authentication on Linux or macOS doesn't provide any role information for an authenticated user. To add role and group information to a Kerberos user, the authentication handler must be configured to retrieve the roles from an LDAP domain. The most basic configuration only specifies an LDAP domain to query against and uses the authenticated user's context to query the LDAP domain:

using Microsoft.AspNetCore.Authentication.Negotiate;
using System.Runtime.InteropServices;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddAuthentication(NegotiateDefaults.AuthenticationScheme)
    .AddNegotiate(options =>
    {
        if (RuntimeInformation.IsOSPlatform(OSPlatform.Linux))
        {
            options.EnableLdap("contoso.com");
        }
    });

Some configurations may require specific credentials to query the LDAP domain. The credentials can be specified in the following highlighted options:

using Microsoft.AspNetCore.Authentication.Negotiate;
using System.Runtime.InteropServices;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddAuthentication(NegotiateDefaults.AuthenticationScheme)
        .AddNegotiate(options =>
        {
            if (RuntimeInformation.IsOSPlatform(OSPlatform.Linux))
            {
                options.EnableLdap(settings =>
                {
                    settings.Domain = "contoso.com";
                    settings.MachineAccountName = "machineName";
                    settings.MachineAccountPassword =
                                      builder.Configuration["Password"];
                });
            }
        });

builder.Services.AddRazorPages();

By default, the negotiate authentication handler resolves nested domains. In a large or complicated LDAP environment, resolving nested domains may result in a slow lookup or a lot of memory being used for each user. Nested domain resolution can be disabled using the IgnoreNestedGroups option.

Anonymous requests are allowed. Use ASP.NET Core Authorization to challenge anonymous requests for authentication.

Windows environment configuration

The Microsoft.AspNetCore.Authentication.Negotiate component performs User Mode authentication. Service Principal Names (SPNs) must be added to the user account running the service, not the machine account. Execute setspn -S HTTP/myservername.mydomain.com myuser in an administrative command shell.

Kerberos vs NTLM

The Negotiate package on Kestrel for ASP.NET Core attempts to use Kerberos, which is a more secure and peformant authentication scheme than NTLM:

using Microsoft.AspNetCore.Authentication.Negotiate;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddAuthentication(NegotiateDefaults.AuthenticationScheme)
   .AddNegotiate();

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

var app = builder.Build();

NegotiateDefaults.AuthenticationScheme specifies Kerberos because it's the default.

IIS, IISExpress, and Kestrel support both Kerberos and NTLM.

Examining the WWW-Authenticate: header using IIS or IISExpress with a tool like Fiddler shows either Negotiate or NTLM.

Kestrel only shows WWW-Authenticate: Negotiate

The WWW-Authenticate: Negotiate header means that the server can use NTLM or Kerberos. Kestrel requires the Negotiate header prefix, it doesn’t support directly specifying NTLM in the request or response auth headers. NTLM is supported in Kestrel, but it must be sent as Negotiate.

On Kestrel, to see if NTLM or Kerberos is used, Base64 decode the header and it shows either NTLM or HTTP. HTTP indicates Kerberos was used.

Linux and macOS environment configuration

Instructions for joining a Linux or macOS machine to a Windows domain are available in the Connect Azure Data Studio to your SQL Server using Windows authentication - Kerberos article. The instructions create a machine account for the Linux machine on the domain. SPNs must be added to that machine account.

Note

When following the guidance in the Connect Azure Data Studio to your SQL Server using Windows authentication - Kerberos article, replace python-software-properties with python3-software-properties if needed.

Once the Linux or macOS machine is joined to the domain, additional steps are required to provide a keytab file with the SPNs:

• On the domain controller, add new web service SPNs to the machine account:
  • setspn -S HTTP/mywebservice.mydomain.com mymachine
  • setspn -S HTTP/mywebservice@MYDOMAIN.COM mymachine
• Use ktpass to generate a keytab file:
  • ktpass -princ HTTP/mywebservice.mydomain.com@MYDOMAIN.COM -pass myKeyTabFilePassword -mapuser MYDOMAIN\mymachine$ -pType KRB5_NT_PRINCIPAL -out c:\temp\mymachine.HTTP.keytab -crypto AES256-SHA1
  • Some fields must be specified in uppercase as indicated.
• Copy the keytab file to the Linux or macOS machine.
• Select the keytab file via an environment variable: export KRB5_KTNAME=/tmp/mymachine.HTTP.keytab
• Invoke klist to show the SPNs currently available for use.

Note

A keytab file contains domain access credentials and must be protected accordingly.

HTTP.sys

HTTP.sys supports Kernel Mode Windows Authentication using Negotiate, NTLM, or Basic authentication.

The following code adds authentication and configures the app's web host to use HTTP.sys with Windows Authentication:

using Microsoft.AspNetCore.Server.HttpSys;
using System.Runtime.InteropServices;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddAuthentication(HttpSysDefaults.AuthenticationScheme);

if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
{
    builder.WebHost.UseHttpSys(options =>
        {
            options.Authentication.Schemes =
                AuthenticationSchemes.NTLM |
                AuthenticationSchemes.Negotiate;
            options.Authentication.AllowAnonymous = false;
        });
}

Note

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.

Note

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 (see https://hub.docker.com/_/microsoft-windows-servercore). For more information on Server Core, see What is the Server Core installation option in Windows Server?.

Authorize users

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.

Disallow anonymous access

When Windows Authentication is enabled and anonymous access is disabled, the [Authorize] and [AllowAnonymous] attributes have no effect. If an IIS site is configured to disallow anonymous access, the request never reaches the app. For this reason, the [AllowAnonymous] attribute isn't applicable.

Allow anonymous access

When both Windows Authentication and anonymous access are enabled, use the [Authorize] and [AllowAnonymous] attributes. The [Authorize] attribute allows you to secure endpoints of the app which require authentication. The [AllowAnonymous] attribute overrides the [Authorize] attribute in apps that allow anonymous access. For attribute usage details, see Simple authorization in ASP.NET Core.

Note

By default, users who lack authorization to access a page are presented with an empty HTTP 403 response. The StatusCodePages Middleware can be configured to provide users with a better "Access Denied" experience.

Impersonation

ASP.NET Core doesn't implement impersonation. Apps run with the app's identity for all requests, using app pool or process identity. If the app should perform an action on behalf of a user, use WindowsIdentity.RunImpersonated or RunImpersonatedAsync in a terminal inline middleware in Program.cs. Run a single action in this context and then close the context.

app.Run(async (context) =>
{
    try
    {
        var user = (WindowsIdentity)context.User.Identity!;

        await context.Response
            .WriteAsync($"User: {user.Name}\tState: {user.ImpersonationLevel}\n");

        await WindowsIdentity.RunImpersonatedAsync(user.AccessToken, async () =>
        {
            var impersonatedUser = WindowsIdentity.GetCurrent();
            var message =
                $"User: {impersonatedUser.Name}\t" +
                $"State: {impersonatedUser.ImpersonationLevel}";

            var bytes = Encoding.UTF8.GetBytes(message);
            await context.Response.Body.WriteAsync(bytes, 0, bytes.Length);
        });
    }
    catch (Exception e)
    {
        await context.Response.WriteAsync(e.ToString());
    }
});

While the Microsoft.AspNetCore.Authentication.Negotiate package enables authentication on Windows, Linux, and macOS, impersonation is only supported on Windows.

Claims transformations

When hosting with IIS, AuthenticateAsync isn't called internally to initialize a user. Therefore, an 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, see Differences between in-process and out-of-process hosting.

Additional resources

• dotnet publish
• Host ASP.NET Core on Windows with IIS
• ASP.NET Core Module (ANCM) for IIS
• Visual Studio publish profiles (.pubxml) for ASP.NET Core app deployment