Add notification email on invalid second authenticator

app/controllers/auth/sessions_controller.rb

@@ -181,6 +181,11 @@ def on_authentication_failure(user, security_measure, failure_reason)
       ip: request.remote_ip,
       user_agent: request.user_agent
     )
+
+    # Only send a notification email every hour at most
+    return if redis.set("2fa_failure_notification:#{user.id}", '1', ex: 1.hour, get: true).present?
+
+    UserMailer.failed_2fa(user, request.remote_ip, request.user_agent, Time.now.utc).deliver_later!
   end
 
   def second_factor_attempts_key(user)

app/mailers/user_mailer.rb

@@ -191,6 +191,18 @@ def suspicious_sign_in(user, remote_ip, user_agent, timestamp)
     end
   end
 
+  def failed_2fa(user, remote_ip, user_agent, timestamp)
+    @resource   = user
+    @remote_ip  = remote_ip
+    @user_agent = user_agent
+    @detection  = Browser.new(user_agent)
+    @timestamp  = timestamp.to_time.utc
+
+    I18n.with_locale(locale) do
+      mail subject: default_i18n_subject
+    end
+  end
+
   private
 
   def default_devise_subject

app/views/user_mailer/failed_2fa.html.haml

@@ -0,0 +1,24 @@
+= content_for :heading do
+  = render 'application/mailer/heading', heading_title: t('user_mailer.failed_2fa.title'), heading_subtitle: t('user_mailer.failed_2fa.explanation'), heading_image_url: frontend_asset_url('images/mailer-new/heading/login.png')
+%table.email-w-full{ cellspacing: 0, cellpadding: 0, border: 0, role: 'presentation' }
+  %tr
+    %td.email-body-padding-td
+      %table.email-inner-card-table{ cellspacing: 0, cellpadding: 0, border: 0, role: 'presentation' }
+        %tr
+          %td.email-inner-card-td.email-prose
+            %p= t 'user_mailer.failed_2fa.details'
+            %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.to_s),
+                    platform: t("sessions.platforms.#{@detection.platform.id}", default: @detection.platform.id.to_s)
+              %br/
+              %strong #{t('sessions.date')}:
+              = l(@timestamp.in_time_zone(@resource.time_zone.presence), format: :with_time_zone)
+            = render 'application/mailer/button', text: t('settings.account_settings'), url: edit_user_registration_url
+      %p= t 'user_mailer.failed_2fa.further_actions_html',
+            action: link_to(t('user_mailer.suspicious_sign_in.change_password'), edit_user_registration_url)

app/views/user_mailer/failed_2fa.text.erb

@@ -0,0 +1,15 @@
+<%= t 'user_mailer.failed_2fa.title' %>
+
+===
+
+<%= t 'user_mailer.failed_2fa.explanation' %>
+
+<%= t 'user_mailer.failed_2fa.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.in_time_zone(@resource.time_zone.presence), format: :with_time_zone) %>
+
+<%= t 'user_mailer.failed_2fa.further_actions_html', action: t('user_mailer.suspicious_sign_in.change_password') %>
+
+=> <%= edit_user_registration_url %>

config/locales/en.yml

@@ -1791,6 +1791,12 @@ en:
       extra: It's now ready for download!
       subject: Your archive is ready for download
       title: Archive takeout
+    failed_2fa:
+      details: 'Here are details of the sign-in attempt:'
+      explanation: Someone has tried to sign in to your account but provided an invalid second authentication factor.
+      further_actions_html: If this wasn't you, we recommend that you %{action} immediately as it may be compromised.
+      subject: Second factor authentication failure
+      title: Failed second factor authentication
     suspicious_sign_in:
       change_password: change your password
       details: 'Here are details of the sign-in:'

spec/controllers/auth/sessions_controller_spec.rb

@@ -265,21 +265,35 @@
         context 'when repeatedly using an invalid TOTP code before using a valid code' do
           before do
             stub_const('Auth::SessionsController::MAX_2FA_ATTEMPTS_PER_HOUR', 2)
-          end
 
-          it 'does not log the user in' do
             # Travel to the beginning of an hour to avoid crossing rate-limit buckets
             travel_to '2023-12-20T10:00:00Z'
+          end
 
+          it 'does not log the user in' do
             Auth::SessionsController::MAX_2FA_ATTEMPTS_PER_HOUR.times do
               post :create, params: { user: { otp_attempt: '1234' } }, session: { attempt_user_id: user.id, attempt_user_updated_at: user.updated_at.to_s }
               expect(controller.current_user).to be_nil
             end
 
             post :create, params: { user: { otp_attempt: user.current_otp } }, session: { attempt_user_id: user.id, attempt_user_updated_at: user.updated_at.to_s }
+
             expect(controller.current_user).to be_nil
             expect(flash[:alert]).to match I18n.t('users.rate_limited')
           end
+
+          it 'sends a suspicious sign-in mail', :sidekiq_inline do
+            Auth::SessionsController::MAX_2FA_ATTEMPTS_PER_HOUR.times do
+              post :create, params: { user: { otp_attempt: '1234' } }, session: { attempt_user_id: user.id, attempt_user_updated_at: user.updated_at.to_s }
+              expect(controller.current_user).to be_nil
+            end
+
+            post :create, params: { user: { otp_attempt: user.current_otp } }, session: { attempt_user_id: user.id, attempt_user_updated_at: user.updated_at.to_s }
+
+            expect(UserMailer.deliveries.size).to eq(1)
+            expect(UserMailer.deliveries.first.to.first).to eq(user.email)
+            expect(UserMailer.deliveries.first.subject).to eq(I18n.t('user_mailer.failed_2fa.subject'))
+          end
         end
 
         context 'when using a valid OTP' do

spec/mailers/previews/user_mailer_preview.rb

@@ -93,4 +93,9 @@ def appeal_approved
   def suspicious_sign_in
     UserMailer.suspicious_sign_in(User.first, '127.0.0.1', 'Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:75.0) Gecko/20100101 Firefox/75.0', Time.now.utc)
   end
+
+  # Preview this email at http://localhost:3000/rails/mailers/user_mailer/failed_2fa
+  def failed_2fa
+    UserMailer.failed_2fa(User.first, '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

spec/mailers/user_mailer_spec.rb

@@ -135,6 +135,24 @@
                      'user_mailer.suspicious_sign_in.subject'
   end
 
+  describe '#failed_2fa' do
+    let(:ip) { '192.168.0.1' }
+    let(:agent) { 'NCSA_Mosaic/2.0 (Windows 3.1)' }
+    let(:timestamp) { Time.now.utc }
+    let(:mail) { described_class.failed_2fa(receiver, ip, agent, timestamp) }
+
+    it 'renders failed 2FA notification' do
+      receiver.update!(locale: nil)
+
+      expect(mail)
+        .to be_present
+        .and(have_body_text(I18n.t('user_mailer.failed_2fa.explanation')))
+    end
+
+    include_examples 'localized subject',
+                     'user_mailer.failed_2fa.subject'
+  end
+
   describe '#appeal_approved' do
     let(:appeal) { Fabricate(:appeal, account: receiver.account, approved_at: Time.now.utc) }
     let(:mail) { described_class.appeal_approved(receiver, appeal) }