From 72a7cfaa395bbddabd0f0a712165fd7babf5d58c Mon Sep 17 00:00:00 2001
From: Eugen Rochko <eugen@zeonfederated.com>
Date: Tue, 9 Jun 2020 10:23:06 +0200
Subject: [PATCH] Add e-mail-based sign in challenge for users with disabled
 2FA (#14013)

---
 app/controllers/auth/sessions_controller.rb   |  50 +--------
 .../sign_in_token_authentication_concern.rb   |  49 ++++++++
 .../two_factor_authentication_concern.rb      |  47 ++++++++
 app/mailers/user_mailer.rb                    |  17 +++
 app/models/user.rb                            |  28 ++++-
 .../auth/sessions/sign_in_token.html.haml     |  14 +++
 app/views/user_mailer/sign_in_token.html.haml | 105 ++++++++++++++++++
 app/views/user_mailer/sign_in_token.text.erb  |  17 +++
 config/locales/en.yml                         |   9 ++
 config/locales/simple_form.en.yml             |   1 +
 ...200608113046_add_sign_in_token_to_users.rb |   6 +
 db/schema.rb                                  |   5 +-
 .../auth/sessions_controller_spec.rb          |  66 ++++++++++-
 spec/mailers/previews/user_mailer_preview.rb  |   5 +
 14 files changed, 368 insertions(+), 51 deletions(-)
 create mode 100644 app/controllers/concerns/sign_in_token_authentication_concern.rb
 create mode 100644 app/controllers/concerns/two_factor_authentication_concern.rb
 create mode 100644 app/views/auth/sessions/sign_in_token.html.haml
 create mode 100644 app/views/user_mailer/sign_in_token.html.haml
 create mode 100644 app/views/user_mailer/sign_in_token.text.erb
 create mode 100644 db/migrate/20200608113046_add_sign_in_token_to_users.rb

diff --git a/app/controllers/auth/sessions_controller.rb b/app/controllers/auth/sessions_controller.rb
index e95909447..2415e2ef3 100644
--- a/app/controllers/auth/sessions_controller.rb
+++ b/app/controllers/auth/sessions_controller.rb
@@ -8,7 +8,8 @@ class Auth::SessionsController < Devise::SessionsController
   skip_before_action :require_no_authentication, only: [:create]
   skip_before_action :require_functional!
 
-  prepend_before_action :authenticate_with_two_factor, if: :two_factor_enabled?, only: [:create]
+  include TwoFactorAuthenticationConcern
+  include SignInTokenAuthenticationConcern
 
   before_action :set_instance_presenter, only: [:new]
   before_action :set_body_classes
@@ -39,8 +40,8 @@ class Auth::SessionsController < Devise::SessionsController
   protected
 
   def find_user
-    if session[:otp_user_id]
-      User.find(session[:otp_user_id])
+    if session[:attempt_user_id]
+      User.find(session[:attempt_user_id])
     else
       user   = User.authenticate_with_ldap(user_params) if Devise.ldap_authentication
       user ||= User.authenticate_with_pam(user_params) if Devise.pam_authentication
@@ -49,7 +50,7 @@ class Auth::SessionsController < Devise::SessionsController
   end
 
   def user_params
-    params.require(:user).permit(:email, :password, :otp_attempt)
+    params.require(:user).permit(:email, :password, :otp_attempt, :sign_in_token_attempt)
   end
 
   def after_sign_in_path_for(resource)
@@ -70,47 +71,6 @@ class Auth::SessionsController < Devise::SessionsController
     super
   end
 
-  def two_factor_enabled?
-    find_user&.otp_required_for_login?
-  end
-
-  def valid_otp_attempt?(user)
-    user.validate_and_consume_otp!(user_params[:otp_attempt]) ||
-      user.invalidate_otp_backup_code!(user_params[:otp_attempt])
-  rescue OpenSSL::Cipher::CipherError
-    false
-  end
-
-  def authenticate_with_two_factor
-    user = self.resource = find_user
-
-    if user_params[:otp_attempt].present? && session[:otp_user_id]
-      authenticate_with_two_factor_via_otp(user)
-    elsif user.present? && (user.encrypted_password.blank? || user.valid_password?(user_params[:password]))
-      # If encrypted_password is blank, we got the user from LDAP or PAM,
-      # so credentials are already valid
-
-      prompt_for_two_factor(user)
-    end
-  end
-
-  def authenticate_with_two_factor_via_otp(user)
-    if valid_otp_attempt?(user)
-      session.delete(:otp_user_id)
-      remember_me(user)
-      sign_in(user)
-    else
-      flash.now[:alert] = I18n.t('users.invalid_otp_token')
-      prompt_for_two_factor(user)
-    end
-  end
-
-  def prompt_for_two_factor(user)
-    session[:otp_user_id] = user.id
-    @body_classes = 'lighter'
-    render :two_factor
-  end
-
   def require_no_authentication
     super
     # Delete flash message that isn't entirely useful and may be confusing in
diff --git a/app/controllers/concerns/sign_in_token_authentication_concern.rb b/app/controllers/concerns/sign_in_token_authentication_concern.rb
new file mode 100644
index 000000000..a177aacaf
--- /dev/null
+++ b/app/controllers/concerns/sign_in_token_authentication_concern.rb
@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+module SignInTokenAuthenticationConcern
+  extend ActiveSupport::Concern
+
+  included do
+    prepend_before_action :authenticate_with_sign_in_token, if: :sign_in_token_required?, only: [:create]
+  end
+
+  def sign_in_token_required?
+    find_user&.suspicious_sign_in?(request.remote_ip)
+  end
+
+  def valid_sign_in_token_attempt?(user)
+    Devise.secure_compare(user.sign_in_token, user_params[:sign_in_token_attempt])
+  end
+
+  def authenticate_with_sign_in_token
+    user = self.resource = find_user
+
+    if user_params[:sign_in_token_attempt].present? && session[:attempt_user_id]
+      authenticate_with_sign_in_token_attempt(user)
+    elsif user.present? && user.external_or_valid_password?(user_params[:password])
+      prompt_for_sign_in_token(user)
+    end
+  end
+
+  def authenticate_with_sign_in_token_attempt(user)
+    if valid_sign_in_token_attempt?(user)
+      session.delete(:attempt_user_id)
+      remember_me(user)
+      sign_in(user)
+    else
+      flash.now[:alert] = I18n.t('users.invalid_sign_in_token')
+      prompt_for_sign_in_token(user)
+    end
+  end
+
+  def prompt_for_sign_in_token(user)
+    if user.sign_in_token_expired?
+      user.generate_sign_in_token && user.save
+      UserMailer.sign_in_token(user, request.remote_ip, request.user_agent, Time.now.utc.to_s).deliver_later!
+    end
+
+    session[:attempt_user_id] = user.id
+    @body_classes = 'lighter'
+    render :sign_in_token
+  end
+end
diff --git a/app/controllers/concerns/two_factor_authentication_concern.rb b/app/controllers/concerns/two_factor_authentication_concern.rb
new file mode 100644
index 000000000..cdd8d14af
--- /dev/null
+++ b/app/controllers/concerns/two_factor_authentication_concern.rb
@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+module TwoFactorAuthenticationConcern
+  extend ActiveSupport::Concern
+
+  included do
+    prepend_before_action :authenticate_with_two_factor, if: :two_factor_enabled?, only: [:create]
+  end
+
+  def two_factor_enabled?
+    find_user&.otp_required_for_login?
+  end
+
+  def valid_otp_attempt?(user)
+    user.validate_and_consume_otp!(user_params[:otp_attempt]) ||
+      user.invalidate_otp_backup_code!(user_params[:otp_attempt])
+  rescue OpenSSL::Cipher::CipherError
+    false
+  end
+
+  def authenticate_with_two_factor
+    user = self.resource = find_user
+
+    if user_params[:otp_attempt].present? && session[:attempt_user_id]
+      authenticate_with_two_factor_attempt(user)
+    elsif user.present? && user.external_or_valid_password?(user_params[:password])
+      prompt_for_two_factor(user)
+    end
+  end
+
+  def authenticate_with_two_factor_attempt(user)
+    if valid_otp_attempt?(user)
+      session.delete(:attempt_user_id)
+      remember_me(user)
+      sign_in(user)
+    else
+      flash.now[:alert] = I18n.t('users.invalid_otp_token')
+      prompt_for_two_factor(user)
+    end
+  end
+
+  def prompt_for_two_factor(user)
+    session[:attempt_user_id] = user.id
+    @body_classes = 'lighter'
+    render :two_factor
+  end
+end
diff --git a/app/mailers/user_mailer.rb b/app/mailers/user_mailer.rb
index 88a11f761..2cd58e60a 100644
--- a/app/mailers/user_mailer.rb
+++ b/app/mailers/user_mailer.rb
@@ -126,4 +126,21 @@ class UserMailer < Devise::Mailer
            reply_to: Setting.site_contact_email
     end
   end
+
+  def sign_in_token(user, remote_ip, user_agent, timestamp)
+    @resource   = user
+    @instance   = Rails.configuration.x.local_domain
+    @remote_ip  = remote_ip
+    @user_agent = user_agent
+    @detection  = Browser.new(user_agent)
+    @timestamp  = timestamp.to_time.utc
+
+    return if @resource.disabled?
+
+    I18n.with_locale(@resource.locale || I18n.default_locale) do
+      mail to: @resource.email,
+           subject: I18n.t('user_mailer.sign_in_token.subject'),
+           reply_to: Setting.site_contact_email
+    end
+  end
 end
diff --git a/app/models/user.rb b/app/models/user.rb
index ae0cdf29d..306e2d435 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -38,6 +38,8 @@
 #  chosen_languages          :string           is an Array
 #  created_by_application_id :bigint(8)
 #  approved                  :boolean          default(TRUE), not null
+#  sign_in_token             :string
+#  sign_in_token_sent_at     :datetime
 #
 
 class User < ApplicationRecord
@@ -113,7 +115,7 @@ class User < ApplicationRecord
            :advanced_layout, :use_blurhash, :use_pending_items, :trends, :crop_images,
            to: :settings, prefix: :setting, allow_nil: false
 
-  attr_reader :invite_code
+  attr_reader :invite_code, :sign_in_token_attempt
   attr_writer :external
 
   def confirmed?
@@ -167,6 +169,10 @@ class User < ApplicationRecord
     true
   end
 
+  def suspicious_sign_in?(ip)
+    !otp_required_for_login? && current_sign_in_at.present? && current_sign_in_at < 2.weeks.ago && !recent_ip?(ip)
+  end
+
   def functional?
     confirmed? && approved? && !disabled? && !account.suspended? && account.moved_to_account_id.nil?
   end
@@ -269,6 +275,13 @@ class User < ApplicationRecord
     super
   end
 
+  def external_or_valid_password?(compare_password)
+    # If encrypted_password is blank, we got the user from LDAP or PAM,
+    # so credentials are already valid
+
+    encrypted_password.blank? || valid_password?(compare_password)
+  end
+
   def send_reset_password_instructions
     return false if encrypted_password.blank?
 
@@ -304,6 +317,15 @@ class User < ApplicationRecord
     end
   end
 
+  def sign_in_token_expired?
+    sign_in_token_sent_at.nil? || sign_in_token_sent_at < 5.minutes.ago
+  end
+
+  def generate_sign_in_token
+    self.sign_in_token         = Devise.friendly_token(6)
+    self.sign_in_token_sent_at = Time.now.utc
+  end
+
   protected
 
   def send_devise_notification(notification, *args)
@@ -320,6 +342,10 @@ class User < ApplicationRecord
 
   private
 
+  def recent_ip?(ip)
+    recent_ips.any? { |(_, recent_ip)| recent_ip == ip }
+  end
+
   def send_pending_devise_notifications
     pending_devise_notifications.each do |notification, args|
       render_and_send_devise_message(notification, *args)
diff --git a/app/views/auth/sessions/sign_in_token.html.haml b/app/views/auth/sessions/sign_in_token.html.haml
new file mode 100644
index 000000000..8923203cd
--- /dev/null
+++ b/app/views/auth/sessions/sign_in_token.html.haml
@@ -0,0 +1,14 @@
+- content_for :page_title do
+  = t('auth.login')
+
+= simple_form_for(resource, as: resource_name, url: session_path(resource_name), method: :post) do |f|
+  %p.hint.otp-hint= t('users.suspicious_sign_in_confirmation')
+
+  .fields-group
+    = f.input :sign_in_token_attempt, type: :number, wrapper: :with_label, label: t('simple_form.labels.defaults.sign_in_token_attempt'), input_html: { 'aria-label' => t('simple_form.labels.defaults.sign_in_token_attempt'), :autocomplete => 'off' }, autofocus: true
+
+  .actions
+    = f.button :button, t('auth.login'), type: :submit
+
+  - if Setting.site_contact_email.present?
+    %p.hint.subtle-hint= t('users.generic_access_help_html', email: mail_to(Setting.site_contact_email, nil))
diff --git a/app/views/user_mailer/sign_in_token.html.haml b/app/views/user_mailer/sign_in_token.html.haml
new file mode 100644
index 000000000..826b34e7c
--- /dev/null
+++ b/app/views/user_mailer/sign_in_token.html.haml
@@ -0,0 +1,105 @@
+%table.email-table{ cellspacing: 0, cellpadding: 0 }
+  %tbody
+    %tr
+      %td.email-body
+        .email-container
+          %table.content-section{ cellspacing: 0, cellpadding: 0 }
+            %tbody
+              %tr
+                %td.content-cell.hero
+                  .email-row
+                    .col-6
+                      %table.column{ cellspacing: 0, cellpadding: 0 }
+                        %tbody
+                          %tr
+                            %td.column-cell.text-center.padded
+                              %table.hero-icon.alert-icon{ align: 'center', cellspacing: 0, cellpadding: 0 }
+                                %tbody
+                                  %tr
+                                    %td
+                                      = image_tag full_pack_url('media/images/mailer/icon_email.png'), alt: ''
+
+                              %h1= t 'user_mailer.sign_in_token.title'
+                              %p.lead= t 'user_mailer.sign_in_token.explanation'
+
+%table.email-table{ cellspacing: 0, cellpadding: 0 }
+  %tbody
+    %tr
+      %td.email-body
+        .email-container
+          %table.content-section{ cellspacing: 0, cellpadding: 0 }
+            %tbody
+              %tr
+                %td.content-cell.content-start
+                  %table.column{ cellspacing: 0, cellpadding: 0 }
+                    %tbody
+                      %tr
+                        %td.column-cell.input-cell
+                          %table.input{ align: 'center', cellspacing: 0, cellpadding: 0 }
+                            %tbody
+                              %tr
+                                %td= @resource.sign_in_token
+
+%table.email-table{ cellspacing: 0, cellpadding: 0 }
+  %tbody
+    %tr
+      %td.email-body
+        .email-container
+          %table.content-section{ cellspacing: 0, cellpadding: 0 }
+            %tbody
+              %tr
+                %td.content-cell
+                  .email-row
+                    .col-6
+                      %table.column{ cellspacing: 0, cellpadding: 0 }
+                        %tbody
+                          %tr
+                            %td.column-cell.text-center
+                              %p= t 'user_mailer.sign_in_token.details'
+                          %tr
+                            %td.column-cell.text-center
+                              %p
+                                %strong= "#{t('sessions.ip')}:"
+                                = @remote_ip
+                                %br/
+                                %strong= "#{t('sessions.browser')}:"
+                                %span{ title: @user_agent }= t 'sessions.description', browser: t("sessions.browsers.#{@detection.id}", default: "#{@detection.id}"), platform: t("sessions.platforms.#{@detection.platform.id}", default: "#{@detection.platform.id}")
+                                %br/
+                                = l(@timestamp)
+
+%table.email-table{ cellspacing: 0, cellpadding: 0 }
+  %tbody
+    %tr
+      %td.email-body
+        .email-container
+          %table.content-section{ cellspacing: 0, cellpadding: 0 }
+            %tbody
+              %tr
+                %td.content-cell
+                  .email-row
+                    .col-6
+                      %table.column{ cellspacing: 0, cellpadding: 0 }
+                        %tbody
+                          %tr
+                            %td.column-cell.text-center
+                              %p= t 'user_mailer.sign_in_token.further_actions'
+
+%table.email-table{ cellspacing: 0, cellpadding: 0 }
+  %tbody
+    %tr
+      %td.email-body
+        .email-container
+          %table.content-section{ cellspacing: 0, cellpadding: 0 }
+            %tbody
+              %tr
+                %td.content-cell
+                  %table.column{ cellspacing: 0, cellpadding: 0 }
+                    %tbody
+                      %tr
+                        %td.column-cell.button-cell
+                          %table.button{ align: 'center', cellspacing: 0, cellpadding: 0 }
+                            %tbody
+                              %tr
+                                %td.button-primary
+                                  = link_to edit_user_registration_url do
+                                    %span= t 'settings.account_settings'
diff --git a/app/views/user_mailer/sign_in_token.text.erb b/app/views/user_mailer/sign_in_token.text.erb
new file mode 100644
index 000000000..2539ddaf6
--- /dev/null
+++ b/app/views/user_mailer/sign_in_token.text.erb
@@ -0,0 +1,17 @@
+<%= t 'user_mailer.sign_in_token.title' %>
+
+===
+
+<%= t 'user_mailer.sign_in_token.explanation' %>
+
+=> <%= @resource.sign_in_token %>
+
+<%= t 'user_mailer.sign_in_token.details' %>
+
+<%= t('sessions.ip') %>: <%= @remote_ip %>
+<%= t('sessions.browser') %>: <%= t('sessions.description', browser: t("sessions.browsers.#{@detection.id}", default: "#{@detection.id}"), platform: t("sessions.platforms.#{@detection.platform.id}", default: "#{@detection.platform.id}")) %>
+<%= l(@timestamp) %>
+
+<%= t 'user_mailer.sign_in_token.further_actions' %>
+
+=> <%= edit_user_registration_url %>
diff --git a/config/locales/en.yml b/config/locales/en.yml
index 20d87057f..a33605049 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -1273,6 +1273,12 @@ en:
       explanation: You requested a full backup of your Mastodon account. It's now ready for download!
       subject: Your archive is ready for download
       title: Archive takeout
+    sign_in_token:
+      details: 'Here are details of the attempt:'
+      explanation: 'We detected an attempt to sign in to your account from an unrecognized IP address. If this is you, please enter the security code below on the sign in challenge page:'
+      further_actions: 'If this wasn''t you, please change your password and enable two-factor authentication on your account. You can do so here:'
+      subject: Please confirm attempted sign in
+      title: Sign in attempt
     warning:
       explanation:
         disable: While your account is frozen, your account data remains intact, but you cannot perform any actions until it is unlocked.
@@ -1310,11 +1316,14 @@ en:
       title: Welcome aboard, %{name}!
   users:
     follow_limit_reached: You cannot follow more than %{limit} people
+    generic_access_help_html: Trouble accessing your account? You may get in touch with %{email} for assistance
     invalid_email: The e-mail address is invalid
     invalid_otp_token: Invalid two-factor code
+    invalid_sign_in_token: Invalid security code
     otp_lost_help_html: If you lost access to both, you may get in touch with %{email}
     seamless_external_login: You are logged in via an external service, so password and e-mail settings are not available.
     signed_in_as: 'Signed in as:'
+    suspicious_sign_in_confirmation: You appear to not have logged in from this device before, and you haven't logged in for a while, so we're sending a security code to your e-mail address to confirm that it's you.
   verification:
     explanation_html: 'You can <strong>verify yourself as the owner of the links in your profile metadata</strong>. For that, the linked website must contain a link back to your Mastodon profile. The link back <strong>must</strong> have a <code>rel="me"</code> attribute. The text content of the link does not matter. Here is an example:'
     verification: Verification
diff --git a/config/locales/simple_form.en.yml b/config/locales/simple_form.en.yml
index fd56a35bf..f84b6c884 100644
--- a/config/locales/simple_form.en.yml
+++ b/config/locales/simple_form.en.yml
@@ -151,6 +151,7 @@ en:
         setting_use_blurhash: Show colorful gradients for hidden media
         setting_use_pending_items: Slow mode
         severity: Severity
+        sign_in_token_attempt: Security code
         type: Import type
         username: Username
         username_or_email: Username or Email
diff --git a/db/migrate/20200608113046_add_sign_in_token_to_users.rb b/db/migrate/20200608113046_add_sign_in_token_to_users.rb
new file mode 100644
index 000000000..baa63c10f
--- /dev/null
+++ b/db/migrate/20200608113046_add_sign_in_token_to_users.rb
@@ -0,0 +1,6 @@
+class AddSignInTokenToUsers < ActiveRecord::Migration[5.2]
+  def change
+    add_column :users, :sign_in_token, :string
+    add_column :users, :sign_in_token_sent_at, :datetime
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index beda93c01..df5d48f44 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,8 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2020_06_05_155027) do
+ActiveRecord::Schema.define(version: 2020_06_08_113046) do
+
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
 
@@ -869,6 +870,8 @@ ActiveRecord::Schema.define(version: 2020_06_05_155027) do
     t.string "chosen_languages", array: true
     t.bigint "created_by_application_id"
     t.boolean "approved", default: true, null: false
+    t.string "sign_in_token"
+    t.datetime "sign_in_token_sent_at"
     t.index ["account_id"], name: "index_users_on_account_id"
     t.index ["confirmation_token"], name: "index_users_on_confirmation_token", unique: true
     t.index ["created_by_application_id"], name: "index_users_on_created_by_application_id"
diff --git a/spec/controllers/auth/sessions_controller_spec.rb b/spec/controllers/auth/sessions_controller_spec.rb
index 1950c173a..c387842cd 100644
--- a/spec/controllers/auth/sessions_controller_spec.rb
+++ b/spec/controllers/auth/sessions_controller_spec.rb
@@ -215,7 +215,7 @@ RSpec.describe Auth::SessionsController, type: :controller do
 
       context 'using a valid OTP' do
         before do
-          post :create, params: { user: { otp_attempt: user.current_otp } }, session: { otp_user_id: user.id }
+          post :create, params: { user: { otp_attempt: user.current_otp } }, session: { attempt_user_id: user.id }
         end
 
         it 'redirects to home' do
@@ -230,7 +230,7 @@ RSpec.describe Auth::SessionsController, type: :controller do
       context 'when the server has an decryption error' do
         before do
           allow_any_instance_of(User).to receive(:validate_and_consume_otp!).and_raise(OpenSSL::Cipher::CipherError)
-          post :create, params: { user: { otp_attempt: user.current_otp } }, session: { otp_user_id: user.id }
+          post :create, params: { user: { otp_attempt: user.current_otp } }, session: { attempt_user_id: user.id }
         end
 
         it 'shows a login error' do
@@ -244,7 +244,7 @@ RSpec.describe Auth::SessionsController, type: :controller do
 
       context 'using a valid recovery code' do
         before do
-          post :create, params: { user: { otp_attempt: recovery_codes.first } }, session: { otp_user_id: user.id }
+          post :create, params: { user: { otp_attempt: recovery_codes.first } }, session: { attempt_user_id: user.id }
         end
 
         it 'redirects to home' do
@@ -258,7 +258,7 @@ RSpec.describe Auth::SessionsController, type: :controller do
 
       context 'using an invalid OTP' do
         before do
-          post :create, params: { user: { otp_attempt: 'wrongotp' } }, session: { otp_user_id: user.id }
+          post :create, params: { user: { otp_attempt: 'wrongotp' } }, session: { attempt_user_id: user.id }
         end
 
         it 'shows a login error' do
@@ -270,5 +270,63 @@ RSpec.describe Auth::SessionsController, type: :controller do
         end
       end
     end
+
+    context 'when 2FA is disabled and IP is unfamiliar' do
+      let!(:user) { Fabricate(:user, email: 'x@y.com', password: 'abcdefgh', current_sign_in_at: 3.weeks.ago, current_sign_in_ip: '0.0.0.0') }
+
+      before do
+        request.remote_ip  = '10.10.10.10'
+        request.user_agent = 'Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:75.0) Gecko/20100101 Firefox/75.0'
+
+        allow(UserMailer).to receive(:sign_in_token).and_return(double('email', deliver_later!: nil))
+      end
+
+      context 'using email and password' do
+        before do
+          post :create, params: { user: { email: user.email, password: user.password } }
+        end
+
+        it 'renders sign in token authentication page' do
+          expect(controller).to render_template("sign_in_token")
+        end
+
+        it 'generates sign in token' do
+          expect(user.reload.sign_in_token).to_not be_nil
+        end
+
+        it 'sends sign in token e-mail' do
+          expect(UserMailer).to have_received(:sign_in_token)
+        end
+      end
+
+      context 'using a valid sign in token' do
+        before do
+          user.generate_sign_in_token && user.save
+          post :create, params: { user: { sign_in_token_attempt: user.sign_in_token } }, session: { attempt_user_id: user.id }
+        end
+
+        it 'redirects to home' do
+          expect(response).to redirect_to(root_path)
+        end
+
+        it 'logs the user in' do
+          expect(controller.current_user).to eq user
+        end
+      end
+
+      context 'using an invalid sign in token' do
+        before do
+          post :create, params: { user: { sign_in_token_attempt: 'wrongotp' } }, session: { attempt_user_id: user.id }
+        end
+
+        it 'shows a login error' do
+          expect(flash[:alert]).to match I18n.t('users.invalid_sign_in_token')
+        end
+
+        it "doesn't log the user in" do
+          expect(controller.current_user).to be_nil
+        end
+      end
+    end
   end
 end
diff --git a/spec/mailers/previews/user_mailer_preview.rb b/spec/mailers/previews/user_mailer_preview.rb
index 464f177d0..313666412 100644
--- a/spec/mailers/previews/user_mailer_preview.rb
+++ b/spec/mailers/previews/user_mailer_preview.rb
@@ -59,4 +59,9 @@ class UserMailerPreview < ActionMailer::Preview
   def warning
     UserMailer.warning(User.first, AccountWarning.new(text: '', action: :silence), [Status.first.id])
   end
+
+  # Preview this email at http://localhost:3000/rails/mailers/user_mailer/sign_in_token
+  def sign_in_token
+    UserMailer.sign_in_token(User.first.tap { |user| user.generate_sign_in_token }, '127.0.0.1', 'Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:75.0) Gecko/20100101 Firefox/75.0', Time.now.utc)
+  end
 end