Automating SharePoint scripts/code with LegacyAuthProtocolsEnabled set to false

We use the Microsoft.SharePoint.Client library to automate SharePoint work from our workflow engine but yesterday, one of our client informed us they wanted to disable the Legacy Authentication (LegacyAuthProtocolsEnabled to false).

Once I tried it on our end, I ended up getting an Unauthorised exception.

All in good wanting to disable the Legacy Authentication for obvious security reason, but the problem with the Modern Authentication is that it requires user interaction which is clearly not a solution since we are running tasks in the background.

I've been googling this for quite some time but I haven't found a solution as of yet on how to handle automatic authentication for background work.

Is there a way to "authenticate" to SharePoint without any user interaction while LegacyAuthProtocolsEnabled is set to false?

I found an article that suggested using the App Authentication but after reading more about it, I believe this is considered an old method to authenticate and is likely to be deprecated as well over time, but I thought I'd still give it a go just in case but it did not work. When I got to

https://tenant.sharepoint.com/_layouts/15/appregnew.aspx

Where tenant is our company domain name, and I click on the "Create" button after filling in all the relevant fields, I get the following error, which is completely useless:

Sorry, something went wrong An unexpected error has occurred. TECHNICAL DETAILS

According to this article HOW TO HARDEN YOUR SHAREPOINT ONLINE ENVIRONMENT BY DISABLING LEGACY AUTHENTICATION, Legacy Authentication was no longer be an option as of the 13/10/2020, yet here we are, and the option is still available in SharePoint 365 and while the article is interesting explain why Legacy Authentication should be switched off, etc... it does not get into any details as to how automated solutions should be handled.

Also found an old thread "LegacyAuthProtocolsEnabled" and Scripted Logons to SharePoint Online? where @DeanWang suggests leaving it turned on as:

All custom CSOM, PowerShell code will stop working

This may also prevent third-party apps from accessing SharePoint Online resources.

I'm going to stop here as I could keep going and the question is already too long for my liking and bottom line is, does anyone know if there is a way, and what is the best way, to authenticate to SharePoint while running automated "scripts/code" from a background task without requiring any user interaction while the Legacy Authentication is switch off?

Thanks

Update-1

After reading articles after articles, I've yet to connect to SharePoint 365.

I also spend more time on the PnP Framework as recommended by numerous articles. I created a dummy app with the following sample code which is used again in various articles, including this one:

Secure Authentication of SharePoint with PnP Framework with C#(Code)

My code is identical as you can see:

var clientContext = new AuthenticationManager().GetACSAppOnlyContext(
    "https://mycompany.sharepoint.com/sites",
    "MyClientid",
    "MySecretId");

using (clientContext)
{
    //Get Lists
    var web = clientContext.Web;
    var lists = web.Lists;

    clientContext.Load(lists);
    clientContext.ExecuteQuery();

    foreach (var list in lists)
    { 
    }
}

And even though I've granted full control in Azure for the specific test app that's using the specific ClientId and SecretId

I'm still getting the following error (401 - unauthorized):

System.Exception
  HResult=0x80131500
  Message=Token request failed.
  Source=PnP.Framework
  StackTrace:
   at SharePointPnP.IdentityModel.Extensions.S2S.Protocols.OAuth2.OAuth2S2SClient.Issue(String securityTokenServiceUrl, OAuth2AccessTokenRequest oauth2Request) in /_/src/lib/PnP.Framework/Utilities/OAuth/OAuth2S2SClient.cs:line 18
   at PnP.Framework.Utilities.TokenHelper.GetAppOnlyAccessToken(String targetPrincipalName, String targetHost, String targetRealm) in /_/src/lib/PnP.Framework/Utilities/TokenHelper.cs:line 116
   at PnP.Framework.Utilities.ACSTokenGenerator.GetToken(Uri siteUrl) in /_/src/lib/PnP.Framework/Utilities/ACSTokenGenerator.cs:line 37
   at PnP.Framework.AuthenticationManager.<GetContextAsync>b__59_0(String site) in /_/src/lib/PnP.Framework/AuthenticationManager.cs:line 971
   at PnP.Framework.AuthenticationManager.<>c__DisplayClass75_0.<GetAccessTokenContext>b__0(Object sender, WebRequestEventArgs args) in /_/src/lib/PnP.Framework/AuthenticationManager.cs:line 1336
   at Microsoft.SharePoint.Client.ClientRuntimeContext.OnExecutingWebRequest(WebRequestEventArgs args)
   at Microsoft.SharePoint.Client.ClientContext.FireExecutingWebRequestEventInternal(WebRequestEventArgs args)
   at Microsoft.SharePoint.Client.ClientContext.GetWebRequestExecutor()
   at Microsoft.SharePoint.Client.ClientContext.GetFormDigestInfoPrivate()
   at Microsoft.SharePoint.Client.ClientContext.EnsureFormDigest()
   at Microsoft.SharePoint.Client.ClientContext.ExecuteQuery()
   at ConsoleApp5.Program.Main(String[] args) in C:\Users\myuser\source\repos\ConsoleApp5\ConsoleApp5\Program.cs:line 23

  This exception was originally thrown at this call stack:
    [External Code]
    SharePointPnP.IdentityModel.Extensions.S2S.Protocols.OAuth2.OAuth2WebRequest.GetResponse() in OAuth2WebRequest.cs
    SharePointPnP.IdentityModel.Extensions.S2S.Protocols.OAuth2.OAuth2S2SClient.Issue(string, SharePointPnP.IdentityModel.Extensions.S2S.Protocols.OAuth2.OAuth2AccessTokenRequest) in OAuth2S2SClient.cs

Inner Exception 1:
WebException: The remote server returned an error: (401) Unauthorized.

Is there another section I should be looking at (and change) in the App Registration in Azure

Since it's the SharePoint Online that we are talking about, one easy way to connect to different SharePoint Sites is by using the Azure AD App-Only approach and since you are talking about a Deamon Service you can easily use Application Permissions when registering the App Registration.

You can, and you should, read more about it from the linked Microsoft Docs article.

You can also loggin via certificate or app registration secret as it is discribed in the Log in to Microsoft 365 in order to create automated CI CD SPFx pipelines, for example.

Hope the above helps, if not feel free to ask :)

Update: Please read below in order to have a better understanding.

Firstly, in your code segment you are using a wrong method from the PnP.Framework package.

AuthenticationManager().GetACSAppOnlyContext()

The above method refers to a completely different method of obtaining an authentication token, more specifically the Sharepoint App-Only model, which... well.... more or less is not being used nowadays quite so ofte. I think I read somewhere that MS is thinking of retiring this kind of Authentication and going onwards on the path of Azure Active Directory authentication, but, unfotunately, I cannot seem to find the link.

Furthermore, I have collected three projects and uploaded them to github for you to see. You can simply clone the repo and run the projects as-is from HERE.

As you will be able to see for yourself, there are three projects in the solution, which you can run each one individually from VSCode or Vs.

More in detail:

ConsoleApp1

(sorry for the name but forgot to switch it :) )

This is a Deamon Console Project that references the PnP.Framework namespace and tries to utilize all of the goodies that the good folks form the PnP Community have contributed. The procedure is straight forward and is the same for all three projects ->

1. Read the AppConfiguration
2. Request the Access Token with appropriate scopes (Depending the service that i am referencing)
3. Declare the Token to be used by our Client Context.

In the PnP.Framework-related project the above cycle can be seen as below

1. AuthenticationConfiguration config = AuthenticationConfiguration.ReadFromJsonFile("appsettings.json");
2. var authManager = new PnP.Framework.AuthenticationManager(config.ClientId, config.Certificate.CertificateDiskPath, config.Certificate.CertificatePassword, config.Tenant);
3. using (var cc = authManager.GetAccessTokenContext("https://<REPLACE:name of tenant>.sharepoint.com/sites/testsite2", (string siteURL) => authManager.GetAccessToken(siteURL)))

ConsoleAppMSGraph

As the name suggests this Deamon Console App utilizes GraphServiceClient graphClient in order to get all the information that you request through the graph endpoint. Subsequntly, you will notice that for this porject the scope name changes to

string[] scopes = new string[] { "https://graph.microsoft.com/.default" };

In addition, we request a collection of all the lists that currently reside in our SharePoint Root Site with the below segment:

var lists = await graphClient.Sites["root"].Lists
                            .Request()
                            .GetAsync();

ConsoleAppSPClient

This app is the default and most simple way of accessing data on Sharepoint.

The projects utilizes MSAL.Net and Microsoft.Sharepoint.Client namespaces in order to fetch an access token and, subsequently, embed that token in all our next requests.

In order to keep the answer a bit short, please refer to here in order to see how we initiate a Confidential App Client, request for a token and, later on, embedd it in our ClientContext object.

Notes

1. I have listed in the Readme.md of the repo, which permissions you should give to your app registration. You can view them Here.
2. I am using the Sites.FullControl.All but you can narrow down the list of sites that the app registration will have access by using the Sites.Selected.
3. All of the above projects, reference a common class library that serves as a strongly typed configuration object.
4. IMPORTANT you should always use a certificate to authenticate the client app as it is mentioned here. The previous link also describes the way you can create a certificate and upload it to the store of the app registration.