Signed forms for your Ruby On Rails app.
Latest commit 44067d9 Jun 20, 2016 @Spaceghost Spaceghost Merge pull request #26 from schuetzm/use-secret-key-base
Use Rails' `secret_key_base` as signing key if available


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SignedForm brings new convenience and security to your Rails 4 or Rails 3 application.

SignedForm is under active development. Please make sure you're reading the README associated with the version of SignedForm you're using. Click the tag link on GitHub to switch to the version you've installed to get the correct README.

Or be brave and bundle the gem straight from GitHub master.

A nicely displayed version of this README complete with table of contents is available here.

How It Works

Traditionally, when you create a form with Rails you enter your fields using something like f.text_field :name and so on. Once you're done making your form you need to make sure that you've either set those parameters as accessible in the model (Rails 3) or use permit (Rails 4). This is redundant. Why would you make a form for a user to fill out and then not accept their input? You need to always maintain this synchronization.

SignedForm generates a list of attributes that you have in your form and attaches them to be submitted with the form along with a HMAC-SHA1 signature of those attributes to protect them from tampering. That means no more permit and no more attr_accessible. It just works.

What this looks like:

<%= form_for @user, signed: true do |f| %>
  <% f.add_signed_fields :zipcode, :state # Optionally add additional fields to sign %>

  <%= f.text_field :name %>
  <%= f.text_field :address %>
  <%= f.submit %>
<% end %>
UsersController < ApplicationController
  def update
    @user = User.find params[:id]
    @user.update_attributes params[:user]

Disabled fields need to be explicitly signed:

<%= form_for @user, signed: true do |f| %>
<% f.add_signed_fields :name %>

<%= f.text_field :name, disabled: true %>
<%= f.submit %>
<% end %>

That's it. You're done. Need to add a field? Pop it in the form. You don't need to then update a list of attributes.

Of course, you're free to continue using the standard form_for. SignedForm is strictly opt-in. It won't change the way you use standard forms.

Is it any good?


More than just Convenience - Security

SignedForm protects you in 3 ways:

  • Form fields are signed, so no alteration of the fields are allowed.
  • Form actions are signed. That means a form with an action of /admin/users/3 will not work when submitted to /users/3.
  • Form views are digested (see below). So if you remove a field from your form, old forms will not be accepted despite a valid signature.

The second two methods of security are optional and can be turned off globally or on a form by form basis.


SignedForm requires:

  • Ruby 1.9 or later
  • Rails 4 or Rails 3.1+ (strong_parameters gem required for Rails 3)


Add this line to your application's Gemfile:

gem 'signed_form'

And then execute:

$ bundle

If you're using Rails 3, you'll also need to install the strong_parameters gem. Please set it up as instructed on the linked GitHub repo.

If you're using Rails 4, it works out of the box.

You'll need to include SignedForm::ActionController::PermitSignedParams in the controller(s) you want to use SignedForm with. This can be done application wide by adding the include to your ApplicationController.

ApplicationController < ActionController::Base
  include SignedForm::ActionController::PermitSignedParams

  # ...

On Rails versions older than 4.1, you'll also need to create an initializer:

$ echo "SignedForm.secret_key = '$(rake secret)'" > config/initializers/signed_form.rb

You'll probably want to keep this out of version control. Treat this key like you would your session secret, keep it private.

Support for other Builders

Any form that wraps form_for and the default field helpers will work with SignedForm. For example, a signed SimpleForm might look like this:

<%= simple_form_for @user, signed: true do |f| %>
  <%= f.input :name %>
<% end %>

This will create a signed form as expected.

For builders that don't use the standard field helpers under the hood, you can create an adapter like this:

class MyAdapter < SomeOtherBuilder
  include SignedForm::FormBuilder

  def some_helper(field, *other_args)
    add_signed_fields field

Then in your view:

<%= form_for @user, signed: true, builder: MyAdapter do |f| %>
  <%= f.some_helper :name %>
<% end %>

ActiveAdmin support

Gem signed_form-activeadmin integrates SignedForm with Active Admin.

Form Digests

SignedForm will create a digest of all the views/partials involved with rendering your form. If the form is modifed old forms will be expired. This is done to eliminate the possibility of old forms coming back to bite you.

By default, there is a 5 minute grace period before old forms will be rejected. This is done so that if you make a trivial change to a form you won't prevent a form a user is currently filling out from being accepted when you restart your server.

Of course if a critical mistake is made (such as allowing an admin field to be set in the form) you could change the secret key to prevent any old form from getting through.

By default, these digests are not cached. That means that each form that is submitted will have the views be digested again. Most views and partials are relatively small so the cost of computing the MD5 hash of the files is not very expensive. However, if this is something you care about SignedForm also provides a memory store (SignedForm::DigestStores::MemoryStore) that will cache the digests in memory. Other stores could be used as well, as long as the object responds to #fetch taking the cache key as an argument as well as the block that will return the digest.

Example Configuration

An example config/initializers/signed_form.rb might look something like this (these are the defaults, with the exception of the key of course):

SignedForm.config do |c|
  c.options[:sign_destination]    = true
  c.options[:digest]              = true
  c.options[:digest_grace_period] = 300
  c.options[:signed]              = false # If true, sign all forms by default

  c.digest_store =
  c.secret_key   = 'supersecret'

Those options that are in the options hash are the default per-form options. They can be overridden by passing the same option to the form_for method.

Testing Your Controllers

Because your tests won't include a signature you will get a ForbiddenAttributes exception in your tests that do mass assignment. SignedForm includes a test helper method, permit_all_parameters that works with both TestUnit and RSpec.

In your spec_helper file or test_helper file require 'signed_form/test_helper'. Then include SignedForm::TestHelper in tests where you need it. An example is below.

Caution: permit_all_parameters without a block modifies the singleton class of the controller under test which lasts for the duration of the test. If you want permit_all_parameters to be limited to a specific part of the test, pass it a block and only that block will be affected. Example:

describe CarsController do
  include SignedForm::TestHelper

  describe "POST create" do
    it "should create a car" do
      permit_all_parameters do
        # This won't raise ForbiddenAttributesError
        post :create, {:car => valid_attributes}, valid_session

      # This one will raise
      post :create, {:car => valid_attributes}, valid_session

      # ...

Example without a block:

describe CarsController do
  include SignedForm::TestHelper

  describe "POST create" do
    before { permit_all_parameters }

    describe "with valid params" do
      it "assigns a newly created car as @car" do
        post :create, {:car => valid_attributes}, valid_session

        assigns(:car).should be_a(Car)
        assigns(:car).should be_persisted

      # ...

I want to hear from you

If you're using SignedForm, I'd love to hear from you. What do you like? What could be better? I'd love to hear your ideas. Join the mailing list on librelist to join the discussion at


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request