Work with user identities in Azure App Service authentication

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07/22/2024

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:

Header	Description
`X-MS-CLIENT-PRINCIPAL`	A Base64 encoded JSON representation of available claims. For more information, see Decoding the client principal header.
`X-MS-CLIENT-PRINCIPAL-ID`	An identifier for the caller set by the identity provider.
`X-MS-CLIENT-PRINCIPAL-NAME`	A human-readable name for the caller set by the identity provider, such as email address or user principal name.
`X-MS-CLIENT-PRINCIPAL-IDP`	The name of the identity provider used by App Service Authentication.

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

Note

Different language frameworks might present these headers to the app code in different formats, such as lowercase or title case.

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 might 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 might 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": ""
}

Property	Type	Description
`auth_typ`	string	The name of the identity provider used by App Service Authentication.
`claims`	array of objects	An array of objects representing the available claims. Each object contains `typ` and `val` properties.
`typ`	string	The name of the claim. It might be subject to default claims mapping and could be different from the corresponding claim contained in a token.
`val`	string	The value of the claim.
`name_typ`	string	The name claim type, which is typically a URI providing scheme information about the `name` claim if one is defined.
`role_typ`	string	The role claim type, which is typically a URI providing scheme information about the `role` claim if one is defined.

To process this header, your app needs to decode the payload and iterate through the claims array to find the claims of interest. It might be convenient to convert them 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.

Note

For claims mapping to work, you must enable the Token store.

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.

Next steps

Tutorial: Authenticate and authorize users end-to-end

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