Work with user identities in Azure App Service authentication

This article shows you how to work with user identities when using the built-in authentication and authorization in App Service.

Access user claims in app code

For all language frameworks, App Service makes the claims in the incoming token (whether from an authenticated end user or a client application) available to your code by injecting them into the request headers. External requests aren't allowed to set these headers, so they're present only if set by App Service. Some example headers include:

Provider tokens are also exposed through similar headers. For example, the Microsoft Identity Provider also sets X-MS-TOKEN-AAD-ACCESS-TOKEN and X-MS-TOKEN-AAD-ID-TOKEN as appropriate.

Code that is written in any language or framework can get the information that it needs from these headers. Decoding the client principal header covers this process. For some frameworks, the platform also provides extra options that may be more convenient.

Decoding the client principal header

X-MS-CLIENT-PRINCIPAL contains the full set of available claims as Base64 encoded JSON. These claims go through a default claims-mapping process, so some may have different names than you would see if processing the token directly. The decoded payload is structured as follows:

{
    "auth_typ": "",
    "claims": [
        {
            "typ": "",
            "val": ""
        }
    ],
    "name_typ": "",
    "role_typ": ""
}

To process this header, your app will need to decode the payload and iterate through the claims array to find the claims of interest. It may be convenient to convert these into a representation used by the app's language framework. Here's an example of this process in C# that constructs a ClaimsPrincipal type for the app to use:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Security.Claims;
using System.Text;
using System.Text.Json;
using System.Text.Json.Serialization;
using Microsoft.AspNetCore.Http;

public static class ClaimsPrincipalParser
{
    private class ClientPrincipalClaim
    {
        [JsonPropertyName("typ")]
        public string Type { get; set; }
        [JsonPropertyName("val")]
        public string Value { get; set; }
    }

    private class ClientPrincipal
    {
        [JsonPropertyName("auth_typ")]
        public string IdentityProvider { get; set; }
        [JsonPropertyName("name_typ")]
        public string NameClaimType { get; set; }
        [JsonPropertyName("role_typ")]
        public string RoleClaimType { get; set; }
        [JsonPropertyName("claims")]
        public IEnumerable<ClientPrincipalClaim> Claims { get; set; }
    }

    public static ClaimsPrincipal Parse(HttpRequest req)
    {
        var principal = new ClientPrincipal();

        if (req.Headers.TryGetValue("x-ms-client-principal", out var header))
        {
            var data = header[0];
            var decoded = Convert.FromBase64String(data);
            var json = Encoding.UTF8.GetString(decoded);
            principal = JsonSerializer.Deserialize<ClientPrincipal>(json, new JsonSerializerOptions { PropertyNameCaseInsensitive = true });
        }

        /** 
         *  At this point, the code can iterate through `principal.Claims` to
         *  check claims as part of validation. Alternatively, we can convert
         *  it into a standard object with which to perform those checks later
         *  in the request pipeline. That object can also be leveraged for 
         *  associating user data, etc. The rest of this function performs such
         *  a conversion to create a `ClaimsPrincipal` as might be used in 
         *  other .NET code.
         */

        var identity = new ClaimsIdentity(principal.IdentityProvider, principal.NameClaimType, principal.RoleClaimType);
        identity.AddClaims(principal.Claims.Select(c => new Claim(c.Type, c.Value)));
        
        return new ClaimsPrincipal(identity);
    }
}

Framework-specific alternatives

For ASP.NET 4.6 apps, App Service populates ClaimsPrincipal.Current with the authenticated user's claims, so you can follow the standard .NET code pattern, including the [Authorize] attribute. Similarly, for PHP apps, App Service populates the _SERVER['REMOTE_USER'] variable. For Java apps, the claims are accessible from the Tomcat servlet.

For Azure Functions, ClaimsPrincipal.Current isn't populated for .NET code, but you can still find the user claims in the request headers, or get the ClaimsPrincipal object from the request context or even through a binding parameter. For more information, see Working with client identities in Azure Functions.

For .NET Core, Microsoft.Identity.Web supports populating the current user with App Service authentication. To learn more, you can read about it on the Microsoft.Identity.Web wiki, or see it demonstrated in this tutorial for a web app accessing Microsoft Graph.

Access user claims using the API

If the token store is enabled for your app, you can also obtain other details on the authenticated user by calling /.auth/me.