Adding additional logic to Bearer authorization

Question

I am attempting to implement OWIN bearer token authorization, and based on this article. However, there's one additional piece of information I need in bearer token that I don't know how to implement.

In my application, I need to deduce from the bearer token user information (say userid). This is important because I don't want an authorized user from being able to act as another user. Is this doable? Is it even the correct approach? If the userid is a guid, then this would be simple. It's an integer in this case. An authorized user can potentially impersonate another just by guessing / brute force, which is unacceptable.

Looking at this code:

public void ConfigureOAuth(IAppBuilder app)
{
    OAuthAuthorizationServerOptions OAuthServerOptions = new OAuthAuthorizationServerOptions()
    {
        AllowInsecureHttp = true,
        TokenEndpointPath = new PathString("/token"),
        AccessTokenExpireTimeSpan = TimeSpan.FromDays(1),
        Provider = new SimpleAuthorizationServerProvider()
    };

    // Token Generation
    app.UseOAuthAuthorizationServer(OAuthServerOptions);
    app.UseOAuthBearerAuthentication(new OAuthBearerAuthenticationOptions());
}

public class SimpleAuthorizationServerProvider : OAuthAuthorizationServerProvider
{
    public override async Task ValidateClientAuthentication(OAuthValidateClientAuthenticationContext context)
    {
        context.Validated();
    }

    public override async Task GrantResourceOwnerCredentials(OAuthGrantResourceOwnerCredentialsContext context)
    {
        context.OwinContext.Response.Headers.Add("Access-Control-Allow-Origin", new[] { "*" });

        using (AuthRepository _repo = new AuthRepository())
        {
            IdentityUser user = await _repo.FindUser(context.UserName, context.Password);

            if (user == null)
            {
                context.SetError("invalid_grant", "The user name or password is incorrect.");
                return;
            }
        }

        var identity = new ClaimsIdentity(context.Options.AuthenticationType);
        identity.AddClaim(new Claim("sub", context.UserName));
        identity.AddClaim(new Claim("role", "user"));

        context.Validated(identity);
    }
}

I would think that it is possible to override the authorization / authentication to accommodate what I need?

Answer 1

On a side note, if you want to set a custom error message you'll have to swap the order of the context.Rejected and context.SetError.

    // Summary:
    //     Marks this context as not validated by the application. IsValidated and HasError
    //     become false as a result of calling.
    public virtual void Rejected();

If you place context.Rejected after context.SetError then the property context.HasError will be reset to false therefore the correct way to use it is:

    // Client could not be validated.
    context.Rejected();
    context.SetError("invalid_client", "Client credentials are invalid.");

Answer 2

It seems there's something missing in your code.
You're not validating your client.

You should implement ValidateClientAuthentication and check your client's credentials there.

This is what I do:

public override async Task ValidateClientAuthentication(OAuthValidateClientAuthenticationContext context)
{
        string clientId = string.Empty;
        string clientSecret = string.Empty;

        if (!context.TryGetBasicCredentials(out clientId, out clientSecret)) 
        {
            context.SetError("invalid_client", "Client credentials could not be retrieved through the Authorization header.");
            context.Rejected();
            return;
        }

        ApplicationDatabaseContext dbContext = context.OwinContext.Get<ApplicationDatabaseContext>();
        ApplicationUserManager userManager = context.OwinContext.GetUserManager<ApplicationUserManager>();

        if (dbContext == null)
        {
            context.SetError("server_error");
            context.Rejected();
            return;
        }

        try
        {
            AppClient client = await dbContext
                .Clients
                .FirstOrDefaultAsync(clientEntity => clientEntity.Id == clientId);

            if (client != null && userManager.PasswordHasher.VerifyHashedPassword(client.ClientSecretHash, clientSecret) == PasswordVerificationResult.Success)
            {
                // Client has been verified.
                context.OwinContext.Set<AppClient>("oauth:client", client);
                context.Validated(clientId);
            }
            else
            {
                // Client could not be validated.
                context.SetError("invalid_client", "Client credentials are invalid.");
                context.Rejected();
            }
        }
        catch (Exception ex)
        {
            string errorMessage = ex.Message;
            context.SetError("server_error");
            context.Rejected();
        }
  }

A good article full of details can be found here.
A even better explanation can be found in this blog series.

UPDATE:

I did some digging and webstuff is right.

In order to pass errorDescription to the client we need to Rejected before we set the error with SetError:

context.Rejected();
context.SetError("invalid_client", "The information provided are not valid !");
return;

or we can extend it passing a serialized json object in the description:

context.Rejected();
context.SetError("invalid_client", Newtonsoft.Json.JsonConvert.SerializeObject(new { result = false, message = "The information provided are not valid !" }));
return;

With a javascript/jQuery client we could deserialize the text response and read the extended message:

$.ajax({
    type: 'POST',
    url: '<myAuthorizationServer>',
    data: { username: 'John', password: 'Smith', grant_type: 'password' },
    dataType: "json",
    contentType: 'application/x-www-form-urlencoded; charset=utf-8',
    xhrFields: {
        withCredentials: true
    },
    headers: {
        'Authorization': 'Basic ' + authorizationBasic
    },  
    error: function (req, status, error) {
            if (req.responseJSON && req.responseJSON.error_description)
            {
               var error = $.parseJSON(req.responseJSON.error_description);
                    alert(error.message);
            }
    }
});

Answer 3

Just to add on to LeftyX's answer, here's how you can completely control the response being sent to the client once the context is rejected. Pay attention to the code comments.

Based on Greg P's original answer, with some modifications

Step1: Create a class which will act as your middleware

using AppFunc = System.Func<System.Collections.Generic.IDictionary<string, System.Object>,
System.Threading.Tasks.Task>;

namespace SignOnAPI.Middleware.ResponseMiddleware {

public class ResponseMiddleware 
{
    AppFunc _next;
    ResponseMiddlewareOptions _options;

    public ResponseMiddleware(AppFunc nex, ResponseMiddlewareOptions options)
    {
        _next = next;
    }

    public async Task Invoke(IDictionary<string, object> environment)
    {
        var context = new OwinContext(environment);

        await _next(environment);

        if (context.Response.StatusCode == 400 && context.Response.Headers.ContainsKey("Change_Status_Code"))
        {
            //read the status code sent in the response
            var headerValues = context.Response.Headers.GetValues("Change_Status_Code");

            //replace the original status code with the new one
            context.Response.StatusCode = Convert.ToInt16(headerValues.FirstOrDefault());

            //remove the unnecessary header flag
            context.Response.Headers.Remove("Change_Status_Code");
        }
    }
}

Step2 : Create the extensions class (Can be omitted).

This step is optional, can be modified to accept options that can be passed to the middleware.

public static class ResponseMiddlewareExtensions
{
    //method name that will be used in the startup class, add additional parameter to accept middleware options if necessary
    public static void UseResponseMiddleware(this IAppBuilder app)
    {
        app.Use<ResponseMiddleware>();
    }
}

Step3: Modify GrantResourceOwnerCredentials method in your OAuthAuthorizationServerProvider implementation

public override async Task GrantResourceOwnerCredentials(OAuthGrantResourceOwnerCredentialsContext context)
    {

        context.OwinContext.Response.Headers.Add("Access-Control-Allow-Origin", new[] { "*" });

        if (<logic to validate username and password>)
        {
            //first reject the context, to signify that the client is not valid
            context.Rejected();

            //set the error message
            context.SetError("invalid_username_or_password", "Invalid userName or password" );

            //add a new key in the header along with the statusCode you'd like to return
            context.Response.Headers.Add("Change_Status_Code", new[] { ((int)HttpStatusCode.Unauthorized).ToString() }); 
            return;
        }
    }

Step4: Use this middleware in the startup class

public void Configuration(IAppBuilder app)
{
    app.UseResponseMiddleware();

    //configure the authentication server provider
    ConfigureOAuth(app);

    //rest of your code goes here....
}