Manage user from Azure AD B2C using AD Graph API -

We are building a Xamarin Native mobile apps and using Azure AD B2C for authenticating users using their social logins.

We decided use MSAL native library (Xamarin) for authenticating using B2C. And our mobile app required to manage(full access) the signed-in user profile. Since this feature isn't available in MSAL we have decided to go with ADAL for the time being. Followed the instruction provided in the link below and the sample works. But I started experimenting by deleting the API access provided in the application (created in b2c tenant) and the ran the application with "Get-user" parameter. And the application is still able to get the users from AD. Not sure how secure is this thing?

Then deleted the application key from the B2c tenant application and ran the console application sample. And received an error AADSTS70002: Error validating credentials. AADSTS50012: Invalid client secret is provided. Trace ID: cef09957-06bf-462e-a0c3-4ed6bae11e00 Correlation ID: afab126d-8694-479a-8a21-c12eb7cb176c

https://docs.microsoft.com/en-us/azure/active-directory-b2c/active-directory-b2c-devquickstarts-graph-dotnet

Any Idea why this is happening. I would like to implement this on a xamarin.ios app and any guidance much appreciated.

The answer to this is very similar to the answer to your other question: Exception access Azure AD B2C using ADAL library for user management, which can be summarized as:

Azure AD B2C does not yet support delegated permissions to the Azure AD or Microsoft Graph. The correct way to work around this limitation at this time is to have your native client application call a web API (using MSAL) which would in turn call the Graph API (using ADAL). This web API is an API you build which has authorization logic to scope the user management operations.

Once user management in Azure AD B2C is supported via the Microsoft Graph, you won't need this API and will be able to use delegated permissions (vs application permissions using client credentials) to have your native client application talk directly to the Microsoft Graph. In the interim, you'll have to stand up your own Web API as per the guidance above.

UPDATE: the Azure AD v2.0 endpoint and Microsoft Graph API now support client credentials flow, so you can also use MSAL for your Microsoft Graph API calls. However if you need to call the Azure AD Graph, then you will still need to use ADAL.