Generally speaking I believe developers should be careful/responsible for
handling what they are sending to their models for mass assignment, and
there's where strong params help. The ideal solution indeed would be for
Parameters not to inherit from Hash, which is something Rails will likely
be changing in the near future. This would avoid people calling methods
that are inherited from Hash in the Parameters object, such as
symbolize_keys. It won't avoid people actually converting the Parameters
object into a Hash with, say, a to_h/to_hash call, but at least that would
make it more explicit.
Thanks for sharing your thoughts on that.
On Wed, Aug 6, 2014 at 1:51 PM, <johannes.schlumber...@appfolio.com> wrote:
> Hello,
>
> Recently I was looking into upgrading one of our Rails 3.2 apps to use
> strong_parameters. I encountered what seems like a flaw to me and I would
> like
> to spark discussion about this, hoping for personal learning and potential
> improvement of the rails framework.
>
> The switch from protected attributes to strong parameters looks from the
> outside
> like access control for mass assignment moves from the model to the
> controllers
> (where, I agree, they belong). However looking at the implementation of
> strong
> parameters, it seems easy for developers to accidentally introduce bugs.
>
> How is that?
> @params in a controller is of type ActiveController::Parameters, which
> inherits
> from HashWithIndifferentAccess which in turn inherits from a Ruby Hash.
> Strong
> Parameters extends ActiveController::Parameters to have a 'permitted?'
> function,
> essentially acting as the whitelist of permitted parameters for mass
> assignment
> [1].
>
> Strong Parameters also overrides ActiveRecords sanitize_for_mass_assignment
> function, where it checks if the attributes respond to 'permitted?'. The
> implementation assumes that if the attributes Hash does not respond to
> 'permitted?' the mass assignment decision should be deferred to
> ActiveRecord. If
> it responds to 'permitted?' and the mass assigned attributes are not on the
> whitelist it will raise an exception [2].
> The 'permitted?' function here acts as a weird capability [3]. It is weird
> because a capability that is not present will usually deny an action (mass
> assignment) while here the action gets permitted in the absence of the
> capability.
>
> Why does that matter?
> It matters because it is possible for a developer to accidentally lose that
> capability accidentally very easily on the way from the controller (where
> permit happened and the capability gets created) to the model (where the
> capability gets used). This loss does happen silently and effectively
> disables
> mass assignment protection.
>
> How does that happen?
> The only class aware of the 'permitted?' capability is
> ActiveController::Parameters, if we call a method that is not aware of the
> capability it can get lost as a side effect:
>
> class SomeModel < ActiveRecord::Base
> #fields :name, :protected_secret_field
> include ActiveModel::ForbiddenAttributesProtection
> end
>
> #imagine a request w/ params = {'name' => 1, 'protected_secret_field' =>
> 2}:
> params.reject!{|k,_|['controller', 'action'].include? k}
> params.permit(:name)
> SomeModel.new(params) #Exception, world is OK
> SomeModel.new(params.symbolize_keys) #No Exception, secret overwritten
>
> symbolize_keys returns an object of type Hash, that no longer has the
> 'permitted?' function, so strong parameter protection is effectively
> disabled.
>
> Both of these patterns are found quite a bit in our codebases - often in a
> not
> such simple form though.
>
> In some sense the problem is, that there is non-framework code on the
> codepath between where the whitelist is defined and where it is used, and
> it is
> easy to accidentally lose the whitelist which disables mass assignment
> protection. This problem did not exist with protected attributes, since
> there
> was no codepath.
>
> Using 'attr_accessible :name' on the model would have prevented these
> problems,
> however I am under the impression that strong_parameters is to replace
> this old
> way.
>
> Am I the only one worried about this (I have not found any discussion
> online) or
> is this a known problem? If it is known, what is the proposed solution?
> I understand that developers need to take care when handling user input,
> and
> such should not call e.g. symbolize_keys. But the same is true for mass
> assignment vulnerabilities as a whole - in a world where programmers do
> not make
> mistakes mass assignment errors do not exist.
>
> A possible solution could be to not attach this capability onto the
> parameters
> Hash but store it separately and then retrieve it at mass assignment time
> - so
> it is not the developers responsibility to babysit it. In an alternative
> implementation 'permit/permit!' could be used to declare a list of mass
> assignable parameters for a given action in a controller (optionally on a
> per
> model basis, default empty list) and store it on the request or as a thread
> local variable. The mass_assignment part on the model would then read this
> variable and decide if the mass assignment should be allowed or not. This
> way
> the capability/whitelist can not be accidentally lost by transforming the
> parameters hash anymore.
> best,
> Johannes
>
> [1] https://github.com/rails/strong_parameters/blob/master/
> lib/action_controller/parameters.rb
> [2] https://github.com/rails/strong_parameters/blob/master/
> lib/active_model/forbidden_attributes_protection.rb
> [3] http://en.wikipedia.org/wiki/Capability-based_security
>
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--
At.
Carlos Antonio
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