Don't call `#send` in form object to build file inputs

Before this commit, Simple Form was calling `#send` in the form object
to check whether the resulting object was an attachment. That made the
library open to DOS, information disclousure and execution of unintended
action attacks if a form was built with user input.

```erb
<%= simple_form_for @user do |f| %>
  <%= f.label @user_supplied_string %>
  ...
<% end %>
```

The solution is try to figure out if an input is of type file by
checking for methods present in the most popular Ruby Gems for file
uploads. The current supported Gems are: `activestorage`, `carrierwave`,
`paperclip`, `shrine` and `refile`.

The code is relying on public APIs so it should be fine for now.
It would be nice to have a single API to perform this check, so we'll
suggest one for those libraries.

Co-Authored-By: Felipe Renan <feelipe.renan@gmail.com>

CHANGELOG.md

@@ -1,5 +1,7 @@
 ## Unreleased
 
+## 5.0.0
+
 ### Enhancements
 * Set multiple attribute for grouped selects also. [@ollym](https://github.com/ollym)
 * Removes or renames label classes. [Abduvakilov](https://github.com/Abduvakilov)
@@ -7,6 +9,18 @@
 * Update bootstrap generator template to match v4.3.x. [@m5o](https://github.com/m5o)
 * Allow "required" attribute in generated select elements of PriorityInput. [@mcountis](https://github.com/mcountis)
 
+### Bug fix
+* Do not call `#send` in form object to check whether the attribute is a file input. [@tegon](https://github.com/tegon)
+
+## Deprecations
+* The config `SimpleForm.file_methods` is deprecated and it has no effect. Simple Form now supports automatically discover of file inputs for the following Gems: activestorage, carrierwave, paperclip, refile and shrine. If you are using a custom method that is not from one of the supported Gems, please change your forms to pass the input type explicitly:
+
+```erb
+ <%= form.input :avatar, as: :file %>
+ ```
+
+See http://blog.plataformatec.com.br/2019/09/incorrect-access-control-in-simple-form-cve-2019-16676 for more information.
+
 ## 4.1.0
 
 ### Enhancements

Gemfile.lock

@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    simple_form (4.1.0)
+    simple_form (5.0.0)
       actionpack (>= 5.0)
       activemodel (>= 5.0)
 
@@ -299,4 +299,4 @@ DEPENDENCIES
   simple_form!
 
 BUNDLED WITH
-   1.17.1
+   1.17.3

lib/generators/simple_form/templates/config/initializers/simple_form.rb

@@ -129,9 +129,6 @@ SimpleForm.setup do |config|
   # change this configuration to true.
   config.browser_validations = false
 
-  # Collection of methods to detect if a file type was given.
-  # config.file_methods = [ :mounted_as, :file?, :public_filename, :attached? ]
-
   # Custom mappings for input types. This should be a hash containing a regexp
   # to match as key, and the input type that will be used when the field name
   # matches the regexp as value.

lib/simple_form.rb

@@ -38,6 +38,17 @@ to
 See https://github.com/plataformatec/simple_form/pull/997 for more information.
   WARN
 
+  FILE_METHODS_DEPRECATION_WARN = <<-WARN
+[SIMPLE_FORM] SimpleForm.file_methods is deprecated and has no effect.
+
+Since version 5, Simple Form now supports automatically discover of file inputs for the following Gems: activestorage, carrierwave, paperclip, refile and shrine.
+If you are using a custom method that is not from one of the supported Gems, please change your forms to pass the input type explicitly:
+
+    <%= form.input :avatar, as: :file %>
+
+See http://blog.plataformatec.com.br/2019/09/incorrect-access-control-in-simple-form-cve-2019-16676 for more information.
+  WARN
+
   @@configured = false
 
   def self.configured? #:nodoc:
@@ -120,10 +131,6 @@ See https://github.com/plataformatec/simple_form/pull/997 for more information.
   mattr_accessor :browser_validations
   @@browser_validations = true
 
-  # Collection of methods to detect if a file type was given.
-  mattr_accessor :file_methods
-  @@file_methods = %i[mounted_as file? public_filename attached?]
-
   # Custom mappings for input types. This should be a hash containing a regexp
   # to match as key, and the input type that will be used when the field name
   # matches the regexp as value, such as { /count/ => :integer }.
@@ -265,6 +272,16 @@ See https://github.com/plataformatec/simple_form/pull/997 for more information.
     @@form_class = value
   end
 
+  def self.file_methods=(file_methods)
+    ActiveSupport::Deprecation.warn(FILE_METHODS_DEPRECATION_WARN, caller)
+    @@file_methods = file_methods
+  end
+
+  def self.file_methods
+    ActiveSupport::Deprecation.warn(FILE_METHODS_DEPRECATION_WARN, caller)
+    @@file_methods
+  end
+
   # Default way to setup Simple Form. Run rails generate simple_form:install
   # to create a fresh initializer with all configuration values.
   def self.setup

lib/simple_form/form_builder.rb

@@ -572,9 +572,28 @@ module SimpleForm
       }.try(:last) if SimpleForm.input_mappings
     end
 
+    # Internal: Try to discover whether an attribute corresponds to a file or not.
+    #
+    # Most upload Gems add some kind of attributes to the ActiveRecord's model they are included in.
+    # This method tries to guess if an attribute belongs to some of these Gems by checking the presence
+    # of their methods using `#respond_to?`.
+    #
+    # Note: This does not support multiple file upload inputs, as this is very application-specific.
+    #
+    # The order here was choosen based on the popularity of Gems and for commodity - e.g. the method
+    # with the suffix `_url` is present in three Gems, so it's checked with priority:
+    #
+    # - `#{attribute_name}_attachment` - ActiveStorage >= `5.2` and Refile >= `0.2.0` <= `0.4.0`
+    # - `#{attribute_name}_url` - Shrine >= `0.9.0`, Refile >= `0.6.0` and CarrierWave >= `0.2.1`
+    # - `#{attribute_name}_attacher` - Refile >= `0.4.0` and Shrine >= `0.9.0`
+    # - `#{attribute_name}_file_name` - Paperclip ~> `2.0` (added for backwards compatibility)
+    #
+    # Returns a Boolean.
     def file_method?(attribute_name)
-      file = @object.send(attribute_name) if @object.respond_to?(attribute_name)
-      file && SimpleForm.file_methods.any? { |m| file.respond_to?(m) }
+      @object.respond_to?("#{attribute_name}_attachment") ||
+        @object.respond_to?("#{attribute_name}_url") ||
+        @object.respond_to?("#{attribute_name}_attacher") ||
+        @object.respond_to?("#{attribute_name}_file_name")
     end
 
     def find_attribute_column(attribute_name)

lib/simple_form/version.rb

@@ -1,4 +1,4 @@
 # frozen_string_literal: true
 module SimpleForm
-  VERSION = "4.1.0".freeze
+  VERSION = "5.0.0".freeze
 end

test/form_builder/general_test.rb

@@ -239,31 +239,24 @@ class FormBuilderTest < ActionView::TestCase
     assert_select 'form select#user_updated_at_1i.datetime'
   end
 
-  test 'builder generates file for file columns' do
-    @user.avatar = MiniTest::Mock.new
-    @user.avatar.expect(:public_filename, true)
-    @user.avatar.expect(:!, false)
-
-    with_form_for @user, :avatar
-    assert_select 'form input#user_avatar.file'
+  test 'builder generates file input for ActiveStorage >= 5.2 and Refile >= 0.2.0 <= 0.4.0' do
+    with_form_for UserWithAttachment.build, :avatar
+    assert_select 'form input#user_with_attachment_avatar.file'
   end
 
-  test 'builder generates file for activestorage entries' do
-    @user.avatar = MiniTest::Mock.new
-    @user.avatar.expect(:attached?, false)
-    @user.avatar.expect(:!, false)
-
-    with_form_for @user, :avatar
-    assert_select 'form input#user_avatar.file'
+  test 'builder generates file input for Shrine >= 0.9.0, Refile >= 0.6.0 and CarrierWave >= 0.2.1' do
+    with_form_for UserWithAttachment.build, :cover
+    assert_select 'form input#user_with_attachment_cover.file'
   end
 
-  test 'builder generates file for attributes that are real db columns but have file methods' do
-    @user.home_picture = MiniTest::Mock.new
-    @user.home_picture.expect(:mounted_as, true)
-    @user.home_picture.expect(:!, false)
+  test 'builder generates file input for Refile >= 0.4.0 and Shrine >= 0.9.0' do
+    with_form_for UserWithAttachment.build, :profile_image
+    assert_select 'form input#user_with_attachment_profile_image.file'
+  end
 
-    with_form_for @user, :home_picture
-    assert_select 'form input#user_home_picture.file'
+  test 'builder generates file input for Paperclip ~> 2.0' do
+    with_form_for UserWithAttachment.build, :portrait
+    assert_select 'form input#user_with_attachment_portrait.file'
   end
 
   test 'build generates select if a collection is given' do

test/support/models.rb

@@ -333,3 +333,21 @@ end
 
 class UserNumber1And2 < User
 end
+
+class UserWithAttachment < User
+  def avatar_attachment
+    OpenStruct.new
+  end
+
+  def cover_url
+    "/uploads/cover.png"
+  end
+
+  def profile_image_attacher
+    OpenStruct.new
+  end
+
+  def portrait_file_name
+    "portrait.png"
+  end
+end

test/test_helper.rb

@@ -91,4 +91,5 @@ class ActionView::TestCase
   alias :validating_user_path :user_path
   alias :validating_users_path :user_path
   alias :other_validating_user_path :user_path
+  alias :user_with_attachment_path :user_path
 end