Access Control in ASP.NET MVC depending on input parameters / service layer?

Question

Preamble: this is a bit of a philosophical question. I'm looking more for the "right" way to do this rather than "a" way to do this.

Let's imagine I have some products, and an ASP.NET MVC application performing CRUD on those products:-

mysite.example/products/1
mysite.example/products/1/edit

I'm using the repository pattern, so it doesn't matter where these products come from:-

public interface IProductRepository
{
  IEnumberable<Product> GetProducts();
  ....
}

Also my Repository describes a list of Users, and which products they are managers for (many-many between Users and Products). Elsewhere in the application, a Super-Admin is performing CRUD on Users and managing the relationship between Users and the Products they are permitted to manage.

Anyone is allowed to view any product, but only users who are designated as "admins" for a particular product are allowed to invoke e.g. the Edit action.

How should I go about implementing that in ASP.NET MVC? Unless I've missed something, I can't use the built-in ASP.NET Authorize attribute as first I'd need a different role for every product, and second I won't know which role to check for until I've retrieved my Product from the Repository.

Obviously you can generalise from this scenario to most content-management scenarios - e.g. Users are only allowed to edit their own Forum Posts. StackOverflow users are only allowed to edit their own questions - unless they've got 2000 or more rep...

The simplest solution, as an example, would be something like:-

public class ProductsController
{
  public ActionResult Edit(int id)
  {
    Product p = ProductRepository.GetProductById(id);
    User u = UserService.GetUser(); // Gets the currently logged in user
    if (ProductAdminService.UserIsAdminForProduct(u, p))
    {
      return View(p);
    }
    else
    {
      return RedirectToAction("AccessDenied");
    }
  }
}

My issues:

• Some of this code will need to be repeated - imagine there are several operations (Update, Delete, SetStock, Order, CreateOffer) depending on the User-Products relationship. You'd have to copy-paste several times.
• It's not very testable - you've got to mock up by my count four objects for every test.
• It doesn't really seem like the controller's "job" to be checking whether the user is allowed to perform the action. I'd much rather a more pluggable (e.g. AOP via attributes) solution. However, would that necessarily mean you'd have to SELECT the product twice (once in the AuthorizationFilter, and again in the Controller)?
• Would it be better to return a 403 if the user isn't allowed to make this request? If so, how would I go about doing that?

I'll probably keep this updated as I get ideas myself, but I'm very eager to hear yours!

Thanks in advance!

Edit

Just to add a bit of detail here. The issue I'm having is that I want the business rule "Only users with permission may edit products" to be contained in one and only one place. I feel that the same code which determines whether a user can GET or POST to the Edit action should also be responsible for determining whether to render the "Edit" link on the Index or Details views. Maybe that's not possible/not feasible, but I feel like it should be...

Edit 2

Starting a bounty on this one. I've received some good and helpful answers, but nothing that I feel comfortable "accepting". Bear in mind that I'm looking for a nice clean method to keep the business logic that determines whether or not the "Edit" link on the index view will be displayed in the same place that determines whether or not a request to Products/Edit/1 is authorised or not. I'd like to keep the pollution in my action method to an absolute minimum. Ideally, I'm looking for an attribute-based solution, but I accept that may be impossible.

Fuchsia

Answer 1

First of all, I think you already half-way figured it, becuase you stated that

as first I'd need a different role for every product, and second I won't know which role to check for until I've retrieved my Product from the Repository

I've seen so many attempts at making role-based security do something it was never intended to do, but you are already past that point, so that's cool :)

The alternative to role-based security is ACL-based security, and I think that is what you need here.

You will still need to retrieve the ACL for a product and then check if the user has the right permission for the product. This is so context-sensitive and interaction-heavy that I think that a purely declarative approach is both too inflexible and too implicit (i.e. you may not realize how many database reads are involved in adding a single attribute to some code).

I think scenarios like this are best modeled by a class that encapsulates the ACL logic, allowing you to either Query for decision or making an Assertion based on the current context - something like this:

var p = this.ProductRepository.GetProductById(id);
var user = this.GetUser();
var permission = new ProductEditPermission(p);

If you just want to know whether the user can edit the product, you can issue a Query:

bool canEdit = permission.IsGrantedTo(user);

If you just want to ensure that the user has rights to continue, you can issue an Assertion:

permission.Demand(user);

This should then throw an exception if the permission is not granted.

This all assumes that the Product class (the variable p) has an associated ACL, like this:

public class Product
{
    public IEnumerable<ProductAccessRule> AccessRules { get; }

    // other members...
}

You might want to take a look at System.Security.AccessControl.FileSystemSecurity for inspiration about modeling ACLs.

If the current user is the same as Thread.CurrentPrincipal (which is the case in ASP.NET MVC, IIRC), you can simplyfy the above permission methods to:

bool canEdit = permission.IsGranted();

or

permission.Demand();

because the user would be implicit. You can take a look at System.Security.Permissions.PrincipalPermission for inspiration.

Viper

Do you have or know of an example that uses ACL-based security in an MVC app? – Perfectly

Why exactly do we need to have a List? I am in the same predicament but I don't want to be storing an entire list of users that can edit a product. I want to just see quickly (for the sake of Views as well as service layer methods) if a user can edit a product. And the comparison is simply "does users tenant own product". Why do we need to be storing entire lists? – Proficient

Answer 2

From what you are describing it sounds like you need some form of user access control rather than role based permissions. If this is the case then it needs to be implemented throughout your business logic. Your scenario sounds like you can implement it in your service layer.

Basically you have to implement all functions in your ProductRepository from the perspective of the current user and the products are tagged with permissions for that user.

It sounds more difficult than it actually is. First off you need a user token interface that contains the user information of uid and role list (if you want to use roles). You can use IPrincipal or create your own along the lines of

public interface IUserToken {
  public int Uid { get; }
  public bool IsInRole(string role);
}

Then in your controller you parse the user token into your Repository constructor.

IProductRepository ProductRepository = new ProductRepository(User);  //using IPrincipal

If you're using FormsAuthentication and a custom IUserToken then you can create a Wrapper around the IPrincipal so your ProductRepository is created like:

IProductRepository ProductRepository = new ProductRepository(new IUserTokenWrapper(User));

Now all your IProductRepository functions should access the user token to check permissions. For example:

public Product GetProductById(productId) {
  Product product = InternalGetProductById(UserToken.uid, productId);
  if (product == null) {
    throw new NotAuthorizedException();
  }
  product.CanEdit = (
    UserToken.IsInRole("admin") || //user is administrator
    UserToken.Uid == product.CreatedByID || //user is creator
    HasUserPermissionToEdit(UserToken.Uid, productId)  //other custom permissions
    );
}

If you wondering about getting a list of all products, in your data access code you can query based on permission. In your case a left join to see if the many-to-many table contains the UserToken.Uid and the productId. If the right side of the join is present you know the user has permission to that product and then you can set your Product.CanEdit boolean.

Using this method you can then use the following, if you like, in your View (where Model is your Product).

<% if(Model.CanEdit) { %>
  <a href="/Products/1/Edit">Edit</a>
<% } %>

or in your controller

public ActionResult Get(int id) {
  Product p = ProductRepository.GetProductById(id);
  if (p.CanEdit) {
    return View("EditProduct");
  }
  else {
    return View("Product");
  }
}

The benefit to this method is that the security is built in to your service layer (ProductRepository) so it is not handled by your controllers and cannot be bypassed by your controllers.

The main point is that the security is placed in your business logic and not in your controller.

Orobanchaceous

The examples here really drive the point home. Thank you for that. – Unchristian

In terms of MVC, would this CanEdit attribute be on a DTO? It seems that you would want to have CanEdit available in many many places, and so naturally would want the CanEdit property to be on the ProductDto, so all records which have this information. But, for example if I load a list of 100 products, the application would have to perform this mad calculation for each one just to get the CanEdit flag. This seems a bit ridiculous... what am I missing? – Proficient

Answer 3

The copy paste solutions really become tedious after a while, and is really annoying to maintain. I would probably go with a custom attribute doing what you need. You can use the excellent .NET Reflector to see how the AuthorizeAttribute is implemented and perform your own logic to it.

What it does is inheriting FilterAttribute and implementing IAuthorizationFilter. I can't test this at the moment, but something like this should work.

[AttributeUsage(AttributeTargets.Method | AttributeTargets.Class, Inherited = true, AllowMultiple = true)]
public class ProductAuthorizeAttribute : FilterAttribute, IAuthorizationFilter
{
    public void OnAuthorization(AuthorizationContext filterContext)
    {
        if (filterContext == null)
        {
            throw new ArgumentNullException("filterContext");
        }

        object productId;
        if (!filterContext.RouteData.Values.TryGetValue("productId", out productId))
        {
            filterContext.Result = new HttpUnauthorizedResult();
            return;
        }

        // Fetch product and check for accessrights

        if (user.IsAuthorizedFor(productId))
        {
            HttpCachePolicyBase cache = filterContext.HttpContext.Response.Cache;
            cache.SetProxyMaxAge(new TimeSpan(0L));
            cache.AddValidationCallback(new HttpCacheValidateHandler(this.Validate), null);
        }
        else
            filterContext.Result = new HttpUnauthorizedResult();
    }

    private void Validate(HttpContext context, object data, ref HttpValidationStatus validationStatus)
    {
        // The original attribute performs some validation in here as well, not sure it is needed though
        validationStatus = HttpValidationStatus.Valid;
    }
}

You could probably also store the product/user that you fetch in the filterContext.Controller.TempData so you can fetch it in the controller, or store it in some cache.

Edit: I just noticed the part about the edit link. The best way I can think of is factoring out the authorization part from the attribute and make a HttpHelper for it that you can use in your view.

Withoutdoors

Answer 4

I tend to think that authorization is part of your business logic (or at least outside of your controller logic anyway). I agree with kevingessner above, in that the authorization check should be part of the call to fetch the item. In his OnException method, you could show the login page (or whatever you have configured in the web.config) by something like this:

if (...)
{
    Response.StatusCode = 401;
    Response.StatusDescription = "Unauthorized";
    HttpContext.Response.End();
}

And instead of making UserRepository.GetUserSomehowFromTheRequest() calls in all the action methods, I would do this once (in an override of the Controller.OnAuthorization method for example), then stick that data somewhere in your controller base class for later use (e.g. a property).

Felicidad

Answer 5

I think that it's unrealistic, and a violation of the separation of concerns, to expect to have controller/model code control what the view renders. The controller/model code can set a flag, in the view model, that the view can use to determine what it should do, but I don't think that you should expect a single method to be used by both controller/model and view to control both access to and rendering of the model.

Having said that you could approach this in either of two ways -- both would involve a view model that carries some annotations used by the view in addition to the actual model. In the first case, you can use an attribute to control access to the action. This would be my preference, but would involve decorating each method independently -- unless all of the actions in a controller have the same access attributes.

I've developed a "role or owner" attribute for just this purpose. It verifies that the user is in a particular role or is the owner of the data being produced by the method. Ownership, in my case, is controlled by the presence of a foreign key relationship between the user and the data in question -- that is, you have a ProductOwner table and there needs to be a row containing the product/owner pair for the product and current user. It differs from the normal AuthorizeAttribute in that when the ownership or role check fails, the user is directed to an error page, not the login page. In this case, each method would need to set a flag in the view model that indicates that the model can be edited.

Alternatively, you could implement similar code in the ActionExecuting/ActionExecuted methods of the controller (or a base controller so that it applies consistently across all controllers). In this case, you would need to write some code to detect what kind of action is being executed so you know whether to abort the action based on the ownership of the product in question. The same method would set the flag to indicate that the model can be edited. In this case, you'd probably need a model hierarchy so you could cast the model as an editable model so that you can set the property regardless of the specific model type.

This option seems more coupled to me than using the attribute and arguably more complicated. In the case of the attribute you can design it so that it takes the various table and property names as attributes to the attribute and uses reflection to get the proper data from your repository based on the attribute's properties.

Epistle

It doesn't matter if the "flag" gets set in the controller, so long as the code the controller uses to set the flag is the same code that determines that other requests are authorised. I don't mind how the result gets to where it's going, I just mind that the code which implements the business rule isn't repeated. You've got DRY up against SoC here. In the NerdDinner tutorial, if the business rule saying "only the person who made the dinner can edit it" changed, you'd have to edit code in five places (by my count). That's not ideal. – Fuchsia

If you use the attribute on your edit action, when the attribute succeeds you can assume that the user is allowed to edit -- the attribute will run before the method and can set the result -- in which case the method will never get executed. Given that, you can set the flag in the view model in the edit method -- note that if you have a separate view for edit don't even need to do this. – Epistle

Answer 6

Answering my own question (eep!), Chapter 1 of Professional ASP.NET MVC 1.0 (the NerdDinner tutorial) recommends a similar solution to mine above:

public ActionResult Edit(int id)
{
  Dinner dinner = dinnerRepositor.GetDinner(id);
  if(!dinner.IsHostedBy(User.Identity.Name))
    return View("InvalidOwner");

  return View(new DinnerFormViewModel(dinner));
}

Asides from making me hungry for my dinner, this doesn't really add anything as the tutorial goes on to repeat the code implementing the business rule immediately in the matching POST Action Method, and in the Details view (actually in a child partial of the Details view)

Does that violate SRP? If the business rule changed (so that e.g. anyone who had RSVP'd could edit the dinner), you'd have to change both GET and POST methods, and the View (and the GET and POST methods and View for the Delete operation too, although that's technically a seperate business rule).

Is pulling the logic out into some kind of permissions arbitrator object (as I've done above) as good as it gets?

Fuchsia

Answer 7

You're on the right track, but you can encapsulate all of the permission check into a single method like GetProductForUser, which takes a product, user, and the required permission. By throwing an exception that's caught in the controller's OnException handler, the handling is all in one place:

enum Permission
{
  Forbidden = 0,
  Access = 1,
  Admin = 2
}

public class ProductForbiddenException : Exception
{ }

public class ProductsController
{
  public Product GetProductForUser(int id, User u, Permission perm)
  {
    Product p = ProductRepository.GetProductById(id);
    if (ProductPermissionService.UserPermission(u, p) < perm)
    {
      throw new ProductForbiddenException();
    }
    return p;
  }

  public ActionResult Edit(int id)
  {
    User u = UserRepository.GetUserSomehowFromTheRequest();
    Product p = GetProductForUser(id, u, Permission.Admin);
    return View(p);
  }

  public ActionResult View(int id)
  {
    User u = UserRepository.GetUserSomehowFromTheRequest();
    Product p = GetProductForUser(id, u, Permission.Access);
    return View(p);
  }

  public override void OnException(ExceptionContext filterContext)
  {
    if (typeof(filterContext.Exception) == typeof(ProductForbiddenException))
    {
      // handle me!
    }
    base.OnException(filterContext);
  }
}

You just have to provide ProductPermissionService.UserPermission, to return a user's permission on a given product.By using a Permission enum (I think I've got the right syntax...) and comparing permissions with <, Admin permissions imply Access permissions, which is pretty much always right.

Hangnail

The only big problem I see here is that rolling up the permission into the request somewhat prevents you using the same code to generate the view. Imagine the Index (List) method of your controller there. The client has come to you with the requirement that the List view has "Edit" links next to products the current user is allowed to edit. How would you reconcile that so the same code which determines that a user can GET or POST the Edit action also determines whether or not to render the "Edit" link on the List or Details views? – Fuchsia

The permissions associated with each action method (i.e the permissions that control access to the page and access to the model, which may be the same) could be stored in an attribute on the action. Then the code at <msdn.microsoft.com/en-us/library/…> could be used to retrieve that permission from the attribute for the call to GetProductForUser. – Hangnail

Continuing... In the view, a method like Controller.UserCanAccessAction(User u, string action) could return true or false if the user can access the view, based on the permission in the attribute. So the view code could be like: <% if (ProductController.UserCanAccessAction(CurrentUser, "Edit")) { Response.Write(Html.ActionLink("Edit", ....)); } %> This technique doesn't duplicate the permission in the action and the view. You could use two attributes (e.g. ModelPermissionAttribute and ActionPermissionAttribute) if the permissions differ. – Hangnail

Answer 8

You can use a XACML based implementation. This way you can externalize authorization and also have a repository for your policies outside of your code.

Waki