ActionController::StashedRedirects

Pass between different controller flows via stashed redirects

Stash a redirect to execute a controller flow within another and return to the original flow later.

Usage

class ApplicationController < ActionController::Base
  before_action :authenticate

  private
    def authenticate
      # Pass `redirect_url:` to pass the URL we're currently on.
      redirect_to new_session_url(redirect_url: request.url) unless Current.user
    end
end

class SessionsController < ApplicationController
  # Stash a redirect at the start of the session authentication flow,
  # from `params[:redirect_url]` automatically.
  stash_redirect_for :sign_in, on: :new

  def new
  end

  def create
    if User.authenticate_by(session_params)
      # On success, redirect the user back to where they first tried to access before being authenticated.
      redirect_from_stashed :sign_in
    else
      render :new, status: :unprocessable_entity
    end
  end
end

See the internal documentation for more usage information.

Only internal redirects are allowed, so attackers can't pass an external redirect_url.

Making a sudo authentication system

Consider a flow where you want to require super-user, or sudo, privileges for a given action, e.g. type in your password before you can change your credit card.

We'll make a require_sudo API that we can annotate our controller with like this:

class Billing::CreditCardsController < ApplicationController
  require_sudo # Require sudo on all actions in this controller.
  # require_sudo_on :edit, :update # Or just for some actions.

  def edit
  end

  def update
    Current.user.billing.credit_cards.find(params[:id]).update!(credit_card_params)
  end
end

require_sudo or require_sudo_on can come from a controller concern like this:

# app/controllers/concerns/sudo/examination.rb
module Sudo::Examination
  def self.included(klass) = klass.singleton_class.class_eval do
    def require_sudo_on(*actions, **) = require_sudo(only: *actions, **)
    def require_sudo(...) = before_action(:require_sudo, ...)
  end

  private
    def require_sudo
      if sudo.exam_needed?
        raise "Non-get: can't redirect back here, make sure you do …something with an interstitial page?" unless request.get?
        redirect_to new_sudo_exams_url(redirect_url: request.url)
      end
    end

    def sudo = Sudo.new(session)
end

# Which we include in ApplicationController:
class ApplicationController < ActionController::Base
  include Sudo::Examination
end

Notice how in redirect_to new_sudo_exams_url(redirect_url: request.url) we're passing the redirect_url: along that ActionController::StashedRedirects will need. It's pointing back to the page we're on, which required sudo authentication, so we can redirect back to it after the sudo exam has been passed.

Next up, we can add an in-memory PORO model to give the behavior some better names:

# app/models/sudo.rb
class Sudo < Data.define(:store)
  def passed!
    store[:sudo_expires_at] = 15.minutes.from_now
  end

  def exam_needed?
    expires_at = store[:sudo_expires_at]
    expires_at.nil? || Time.parse(expires_at).past?
  end
end

Next, we can add the authenticating sudo controller itself, where stash_redirect_for will use the redirect_url: from earlier:

# app/controllers/sudo/exams_controller.rb
class Sudo::ExamsController < ApplicationController
  stash_redirect_for :sudo, on: :new

  def new
    redirect_from_stashed :sudo unless sudo.exam_needed?
  end

  def create
    if pass_sudo_exam?
      sudo.passed!
      redirect_from_stashed :sudo
    else
      render :new, status: :unprocessable_entity
    end
  end
  private def pass_sudo_exam? = Current.user.authenticate_password(params[:password])
end

Finally, we mount the routes for the controller:

# config/routes.rb
namespace :sudo do
  resources :exams
end

Users can now fill-in their password, which will hit sudo/exams#create and redirect them back to the edit form on the credit cards flow if it's the correct password.

Installation

Install the gem and add to the application's Gemfile by executing:

$ bundle add action_controller-stashed_redirects

If bundler is not being used to manage dependencies, install the gem by executing:

$ gem install action_controller-stashed_redirects

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and the created tag, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/kaspth/action_controller-stashed_redirects.

License

The gem is available as open source under the terms of the MIT License.