Java – Rest API and resource authorization

For the first question, your approach seems correct to me. You consume a token in the sendsms entry that can only be generated by the login endpoint. You could also use a generic token created by login, and not easily guessable (it would be equivalent to a cryptographically generated session ID), used as primary key to a database tuple. There you can store whatever you want (for example the number of SMS still allowed to be sent).

You can also store the "state" in a variable and protect it using, say, HMAC, continuously exchanging it between the server and the client, but you still require some server state to defend against replay attacks on non-idempotent endpoints; at minimum, you need a per-session incremental counter.

The second problem is trickier, and you need first to ask yourself what the difference in behaviour (as seen by the server) would be between the app with its logic, and a malicious user sending repeated requests. You might enforce a minimum delay between requests that is comparable to the maximum rate at which the app can be driven, so that there's no longer an advantage in using tricks rather than going through the app.

Please for the love of God do not create a SecurityUtils class!

Your class will become 10k lines of spaghetti code in a matter of months! You would need to have an Action type (create, read, update, destroy, list, etc.) passed into your isUserAuthorized() method, which would quickly become a thousand-line-long switch statement with increasingly complex logic that would be difficult to unit test. Don't do it.

Generally what I do, at least in Ruby on Rails, is have each domain object be responsible for its own access privileges by having a policy class for each model. Then, the controller asks the policy class if the current user for the request has access to the resource or not. Here's an example in Ruby, since I've never implemented anything like it in Java, but the idea should come across cleanly:

class OrderPolicy

    class Scope < Struct.new(:user, :scope)

        def resolve

            # A user must be logged in to interact with this resource at all
            raise NotAuthorizedException unless user

            # Admin/moderator can see all orders
            if (user.admin? || user.moderator?)
                scope.all
            else
                # Only allow the user to see their own orders
                scope.where(orderer_id: user.id)
            end
        end
    end

    # Constructor, if you don't know Ruby
    def initialize(user, order)
        raise NotAuthorizedException unless user
        @user = user
        @order= order
    end

    # Whitelist what data can be manipulated by each type of user
    def valid_attributes
        if @user.admin?
            [:probably, :want, :to, :let, :admin, :update, :everything]
        elsif @user.moderator?
            [:fewer, :attributes, :but, :still, :most]
        else
            [:regualar, :user, :attributes]
        end
    end

    # Maybe restrict updatable attributes further
    def valid_update_attributes
    end

    # Who can create new orders
    def create?
        true # anyone, and they would have been authenticated already by #initialize
    end

    # Read operation
    def show?
        @user.admin? || @user.moderator? || owns_order
    end

    # Only superusers can update resources
    def update?
        @user.admin? || @user.moderator?
    end

    # Only admins can delete, because it's extremely destructive or whatever
    def destroy?
        @user.admin?
    end

    private

    # A user 'owns' an order if they were the person who submitted the order
    # E.g. superusers can access the order, but they didn't create it
    def owns_order
        @order.orderer_id == @user.id
    end
end

Even if you have complex nested resources, some resource must 'own' the nested resources, so the top-level logic inherently bubbles down. However, those nested resources need their own policy classes in the event that they can be updated independently of the 'parent' resource.

In my application, which is for my university's department, everything revolves around the Course object. It's not a User-centric application. However, Users are enrolled in Course, so I can simply ensure that:

@course.users.include? current_user && (whatever_other_logic_I_need)

for any resource that a particular User needs to modify, since almost all resources are tied to a Course. This is done in the policy class in the owns_whatever method.

I haven't done much Java architecture, but it seems that you could make a Policy interface, where the different resources that need to be authenticated must implement the interface. Then, you have all the necessary methods which can become as complex as you need them to be per domain object. The important thing is to tie the logic to the model itself, but at the same time keep it in a separate class (single responsibility principle (SRP)).

Your controller actions could look something like:

public List<Order> index(OrderQuery query) {

    authorize(Order.class)
    // you should be auto-rescuing the NotAuthorizedException thrown by
    //the policy class at the controller level (or application level)

    // if the authorization didn't fail/rescue from exception, just render the resource
    List<Order> orders = db.search(query);
    return renderJSON(orders);
}

public Order show(int orderId) {

    authorize(Order.class)
    Order order = db.find(orderId);
    return renderJSON(order);
}