Strong Parameters

With this plugin Action Controller parameters are forbidden to be used in Active Model mass assignments until they have been whitelisted. This means you'll have to make a conscious choice about which attributes to allow for mass updating and thus prevent accidentally exposing that which shouldn't be exposed.

In addition, parameters can be marked as required and flow through a predefined raise/rescue flow to end up as a 400 Bad Request with no effort.

class PeopleController < ActionController::Base
  # This will raise an ActiveModel::ForbiddenAttributes exception because it's using mass assignment
  # without an explicit permit step.
  def create
    Person.create(params[:person])
  end

  # This will pass with flying colors as long as there's a person key in the parameters, otherwise
  # it'll raise a ActionController::MissingParameter exception, which will get caught by
  # ActionController::Base and turned into that 400 Bad Request reply.
  def update
    person = current_account.people.find(params[:id])
    person.update_attributes!(person_params)
    redirect_to person
  end

  private
    # Using a private method to encapsulate the permissible parameters is just a good pattern
    # since you'll be able to reuse the same permit list between create and update. Also, you
    # can specialize this method with per-user checking of permissible attributes.
    def person_params
      params.require(:person).permit(:name, :age)
    end
end

Permitted Scalar Values

Given

params.permit(:id)

the key :id will pass the whitelisting if it appears in params and it has a permitted scalar value associated. Otherwise the key is going to be filtered out, so arrays, hashes, or any other objects cannot be injected.

The permitted scalar types are String, Symbol, NilClass, Numeric, TrueClass, FalseClass, Date, Time, DateTime, StringIO, IO, ActionDispatch::Http::UploadedFile and Rack::Test::UploadedFile.

To declare that the value in params must be an array of permitted scalar values map the key to an empty array:

params.permit(:id => [])

To whitelist an entire hash of parameters, the permit! method can be used

params.require(:log_entry).permit!

This will mark the :log_entry parameters hash and any subhash of it permitted. Extreme care should be taken when using permit! as it will allow all current and future model attributes to be mass-assigned.

Nested Parameters

You can also use permit on nested parameters, like:

params.permit(:name, {:emails => []}, :friends => [ :name, { :family => [ :name ], :hobbies => [] }])

This declaration whitelists the name, emails and friends attributes. It is expected that emails will be an array of permitted scalar values and that friends will be an array of resources with specific attributes : they should have a name attribute (any permitted scalar values allowed), a hobbies attribute as an array of permitted scalar values, and a family attribute which is restricted to having a name (any permitted scalar values allowed, too).

Thanks to Nick Kallen for the permit idea!

Handling of Unpermitted Keys

By default parameter keys that are not explicitly permitted will be logged in the development and test environment. In other environments these parameters will simply be filtered out and ignored.

Additionally, this behaviour can be changed by changing the config.action_controller.action_on_unpermitted_parameters property in your environment files. If set to :log the unpermitted attributes will be logged, if set to :raise an exception will be raised.

Use Outside of Controllers

While Strong Parameters will enforce permitted and required values in your application controllers, keep in mind that you will need to sanitize untrusted data used for mass assignment when in use outside of controllers.

For example, if you retrieve JSON data from a third party API call and pass the unchecked parsed result on to Model.create, undesired mass assignments could take place. You can alleviate this risk by slicing the hash data, or wrapping the data in a new instance of ActionController::Parameters and declaring permissions the same as you would in a controller. For example:

raw_parameters = { :email => "john@example.com", :name => "John", :admin => true }
parameters = ActionController::Parameters.new(raw_parameters)
user = User.create(parameters.permit(:name, :email))

Installation

In Gemfile:

gem 'strong_parameters'

and then run bundle. To activate the strong parameters, you need to include this module in every model you want protected.

class Post < ActiveRecord::Base
  include ActiveModel::ForbiddenAttributesProtection
end

Alternatively, you can protect all Active Record resources by default by creating an initializer and pasting the line:

ActiveRecord::Base.send(:include, ActiveModel::ForbiddenAttributesProtection)

If you want to now disable the default whitelisting that occurs in Rails 3.2, change the config.active_record.whitelist_attributes property in your config/application.rb:

config.active_record.whitelist_attributes = false

This will allow you to remove / not have to use attr_accessible and do mass assignment inside your code and tests.

Compatibility

This plugin is only fully compatible with Rails versions 3.0, 3.1 and 3.2 but not 4.0+, as it is part of Rails Core in 4.0. An unofficial Rails 2 version is strong_parameters_rails2.