https://learn.microsoft.com/en-us/exchange/client-developer/exchange-web-services/how-to-authenticate-an-ews-application-by-using-oauth

Authenticate an EWS application by using OAuth

Learn how to use OAuth authentication with your EWS Managed API applications.

You can use the OAuth authentication service provided by Microsoft Entra to enable your EWS Managed API applications to access Exchange Online in Office 365. To use OAuth with your application you will need to:

1. Register your application with Microsoft Entra.
2. Add code to get an authentication token to get an authentication token from a token server.
3. Add an authentication token to EWS requests that you send.

To use the code in this article, you will need to have access to the following:

• A Microsoft 365 account with an Exchange Online mailbox. If you do not have a Microsoft 365 account, you can sign up for the Microsoft 365 Developer Program to get a free Microsoft 365 subscription.
• The Microsoft Authentication Library for .NET.
• The EWS Managed API.

There are two types of OAuth permissions that can be used to access EWS APIs in Exchange Online. Before you proceed with the tutorial, you will need to choose the specific permission type to use.

• Delegated permissions are used by apps that have a signed-in user present. For these apps, either the user or an administrator consents to the permissions that the app requests and the app can act as the signed-in user when making API calls.
• Application permissions are used by apps that run without a signed-in user present; for example, apps that run as background services or daemons and can access multiple mailboxes.

Register your application

To use OAuth, an application must have an application ID issued by Microsoft Entra. In this tutorial, it is assumed that the application is a console application, so you need to register your application as a public client with Microsoft Entra. You can register an application in the Microsoft Entra admin center or by using Microsoft Graph.

1. Open a browser and navigate to the Microsoft Entra admin center and login using a Work or School Account.

2. Select Identity in the left-hand navigation, then select App registrations under Applications.

3. Select New registration. On the Register an application page, set the values as follows.

   • Set Name to a friendly name for your app.
   • Set Supported account types to the choice that makes sense for your scenario.
   • For Redirect URI, change the dropdown to Public client (mobile & desktop) and set the value to https://login.microsoftonline.com/common/oauth2/nativeclient.

4. Choose Register. On the next page, copy the values of the Application (client) ID and Directory (tenant) ID and save them, you will need them later.

Configure for delegated authentication

If your application uses delegated authentication, no further configuration is required. The Microsoft identity platform allows apps to request permissions dynamically, so you do not have to pre-configure permissions on the app registration. However, in some scenarios (like the on-behalf-of flow) pre-configuring permissions is required. Use the following steps to pre-configure EWS permissions.

1. Select Manifest in the left-hand navigation under Manage.

2. Locate the requiredResourceAccess property in the manifest, and add the following inside the square brackets ([]):

   {
       "resourceAppId": "00000002-0000-0ff1-ce00-000000000000",
       "resourceAccess": [
           {
               "id": "3b5f3d61-589b-4a3c-a359-5dd4b5ee5bd5",
               "type": "Scope"
           }
       ]
   }

3. Select Save.

4. Select API permissions under Manage. Confirm that the EWS.AccessAsUser.All permission is listed.

Configure for app-only authentication

To use application permissions, follow these additional steps.

1. Select Manifest in the left-hand navigation under Manage.

2. Locate the requiredResourceAccess property in the manifest, and add the following inside the square brackets ([]):

   {
       "resourceAppId": "00000002-0000-0ff1-ce00-000000000000",
       "resourceAccess": [
           {
               "id": "dc890d15-9560-4a4c-9b7f-a736ec74ec40",
               "type": "Role"
           }
       ]
   }

3. Select Save.

4. Select API permissions under Manage. Confirm that the full_access_as_app permission is listed.

5. Select Grant admin consent for org and accept the consent dialog.

6. Select Certificates & Secrets in the left-hand navigation under Manage.

7. Select New client secret, enter a short description and select Add.

8. Copy the Value of the newly added client secret and save it, you will need it later.

Add code to get an authentication token

The following code snippets show how to use the Microsoft Authentication Library to get authentication tokens for delegated permissions and application permissions. These snippets assume that the information required to make the authentication request is stored in the application's App.config file. These examples do not include error checking, see the Code samples for the complete code.

Get a token with delegated auth

// Using Microsoft.Identity.Client 4.22.0

// Configure the MSAL client to get tokens
var pcaOptions = new PublicClientApplicationOptions
{
    ClientId = ConfigurationManager.AppSettings["appId"],
    TenantId = ConfigurationManager.AppSettings["tenantId"]
};

var pca = PublicClientApplicationBuilder
    .CreateWithApplicationOptions(pcaOptions).Build();

// The permission scope required for EWS access
var ewsScopes = new string[] { "https://outlook.office365.com/EWS.AccessAsUser.All" };

// Make the interactive token request
var authResult = await pca.AcquireTokenInteractive(ewsScopes).ExecuteAsync();

Get a token with app-only auth

// Using Microsoft.Identity.Client 4.22.0
var cca = ConfidentialClientApplicationBuilder
    .Create(ConfigurationManager.AppSettings["appId"])
    .WithClientSecret(ConfigurationManager.AppSettings["clientSecret"])
    .WithTenantId(ConfigurationManager.AppSettings["tenantId"])
    .Build();

// The permission scope required for EWS access
var ewsScopes = new string[] { "https://outlook.office365.com/.default" };

//Make the token request
var authResult = await cca.AcquireTokenForClient(ewsScopes).ExecuteAsync();

Add an authentication token to EWS requests

After you've received the AuthenticationResult object you can use the AccessToken property to get the token issued by the token service.

// Configure the ExchangeService with the access token
var ewsClient = new ExchangeService();
ewsClient.Url = new Uri("https://outlook.office365.com/EWS/Exchange.asmx");
ewsClient.Credentials = new OAuthCredentials(authResult.AccessToken);

To use application permissions, you will also need to explicitly impersonate a mailbox that you would like to access.

//Impersonate the mailbox you'd like to access.
ewsClient.ImpersonatedUserId = new ImpersonatedUserId(ConnectingIdType.SmtpAddress, "test@demotenant.onmicrosoft.com");

Code samples

Delegated authentication

The following is the complete code sample that demonstrates making an OAuth-authenticated EWS request using delegated authentication.

using Microsoft.Exchange.WebServices.Data;
using Microsoft.Identity.Client;
using System;
using System.Configuration;

namespace EwsOAuth
{
    class Program
    {
        static async System.Threading.Tasks.Task Main(string[] args)
        {
            // Using Microsoft.Identity.Client 4.22.0

            // Configure the MSAL client to get tokens
            var pcaOptions = new PublicClientApplicationOptions
            {
                ClientId = ConfigurationManager.AppSettings["appId"],
                TenantId = ConfigurationManager.AppSettings["tenantId"]
            };

            var pca = PublicClientApplicationBuilder
                .CreateWithApplicationOptions(pcaOptions).Build();

            // The permission scope required for EWS access
            var ewsScopes = new string[] { "https://outlook.office365.com/EWS.AccessAsUser.All" };

            try
            {
                // Make the interactive token request
                var authResult = await pca.AcquireTokenInteractive(ewsScopes).ExecuteAsync();

                // Configure the ExchangeService with the access token
                var ewsClient = new ExchangeService();
                ewsClient.Url = new Uri("https://outlook.office365.com/EWS/Exchange.asmx");
                ewsClient.Credentials = new OAuthCredentials(authResult.AccessToken);

                // Make an EWS call
                var folders = ewsClient.FindFolders(WellKnownFolderName.MsgFolderRoot, new FolderView(10));
                foreach(var folder in folders)
                {
                    Console.WriteLine($"Folder: {folder.DisplayName}");
                }
            }
            catch (MsalException ex)
            {
                Console.WriteLine($"Error acquiring access token: {ex}");
            }
            catch (Exception ex)
            {
                Console.WriteLine($"Error: {ex}");
            }

            if (System.Diagnostics.Debugger.IsAttached)
            {
                Console.WriteLine("Hit any key to exit...");
                Console.ReadKey();
            }
        }
    }
}

App-only authentication

The following is the complete code sample that demonstrates making an OAuth-authenticated EWS request using app-only authentication.

using Microsoft.Exchange.WebServices.Data;
using Microsoft.Identity.Client;
using System;
using System.Configuration;

namespace EwsOAuth
{
    class Program
    {
        static async System.Threading.Tasks.Task Main(string[] args)
        {
            // Using Microsoft.Identity.Client 4.22.0
            var cca = ConfidentialClientApplicationBuilder
                .Create(ConfigurationManager.AppSettings["appId"])
                .WithClientSecret(ConfigurationManager.AppSettings["clientSecret"])
                .WithTenantId(ConfigurationManager.AppSettings["tenantId"])
                .Build();

            var ewsScopes = new string[] { "https://outlook.office365.com/.default" };

            try
            {
                var authResult = await cca.AcquireTokenForClient(ewsScopes)
                    .ExecuteAsync();

                // Configure the ExchangeService with the access token
                var ewsClient = new ExchangeService();
                ewsClient.Url = new Uri("https://outlook.office365.com/EWS/Exchange.asmx");
                ewsClient.Credentials = new OAuthCredentials(authResult.AccessToken);
                ewsClient.ImpersonatedUserId =
                    new ImpersonatedUserId(ConnectingIdType.SmtpAddress, "meganb@contoso.onmicrosoft.com");

                //Include x-anchormailbox header
                ewsClient.HttpHeaders.Add("X-AnchorMailbox", "meganb@contoso.onmicrosoft.com");

                // Make an EWS call
                var folders = ewsClient.FindFolders(WellKnownFolderName.MsgFolderRoot, new FolderView(10));
                foreach(var folder in folders)
                {
                    Console.WriteLine($"Folder: {folder.DisplayName}");
                }
            }
            catch (MsalException ex)
            {
                Console.WriteLine($"Error acquiring access token: {ex}");
            }
            catch (Exception ex)
            {
                Console.WriteLine($"Error: {ex}");
            }

            if (System.Diagnostics.Debugger.IsAttached)
            {
                Console.WriteLine("Hit any key to exit...");
                Console.ReadKey();
            }
        }
    }
}

The sample code in both cases requires an App.config file with the following entries:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <startup>
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.7.2" />
  </startup>
  <appSettings>
    <!-- The application ID from your app registration -->
    <add key="appId" value="YOUR_APP_ID_HERE" />
    <!-- The tenant ID copied from your app registration -->
    <add key="tenantId" value="YOUR_TENANT_ID_HERE"/>
    <!-- The application's client secret from your app registration. Needed for application permission access -->
    <add key="clientSecret" value="YOUR_CLIENT_SECRET_HERE"/>
  </appSettings>
</configuration>

See also