Do not show emails of users in confirmation page

This change does not show the user email in
confirmations/show page.

app/views/devise/confirmations/new.html.haml

@@ -6,7 +6,7 @@
         = render "devise/shared/error_messages", resource: resource
       .form-group
         = f.label :email
-        = f.email_field :email, class: "form-control", required: true, title: 'Please provide a valid email address.'
+        = f.email_field :email, class: "form-control", required: true, title: 'Please provide a valid email address.', value: nil
       .clearfix
         = f.submit "Resend", class: 'gl-button btn btn-success'
 

changelogs/unreleased/security-hide-email-in-confirmation-page.yml

+---
+title: Do not show emails of users in confirmation page
+merge_request:
+author:
+type: security

spec/controllers/confirmations_controller_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe ConfirmationsController do
+  include DeviseHelpers
+
+  before do
+    set_devise_mapping(context: @request)
+  end
+
+  describe '#show' do
+    render_views
+
+    subject { get :show, params: { confirmation_token: confirmation_token } }
+
+    context 'user is already confirmed' do
+      let_it_be_with_reload(:user) { create(:user, :unconfirmed) }
+      let(:confirmation_token) { user.confirmation_token }
+
+      before do
+        user.confirm
+        subject
+      end
+
+      it 'renders `new`' do
+        expect(response).to render_template(:new)
+      end
+
+      it 'displays an error message' do
+        expect(response.body).to include('Email was already confirmed, please try signing in')
+      end
+
+      it 'does not display the email of the user' do
+        expect(response.body).not_to include(user.email)
+      end
+    end
+
+    context 'user accesses the link after the expiry of confirmation token has passed' do
+      let_it_be_with_reload(:user) { create(:user, :unconfirmed) }
+      let(:confirmation_token) { user.confirmation_token }
+
+      before do
+        allow(Devise).to receive(:confirm_within).and_return(1.day)
+
+        travel_to(3.days.from_now) do
+          subject
+        end
+      end
+
+      it 'renders `new`' do
+        expect(response).to render_template(:new)
+      end
+
+      it 'displays an error message' do
+        expect(response.body).to include('Email needs to be confirmed within 1 day, please request a new one below')
+      end
+
+      it 'does not display the email of the user' do
+        expect(response.body).not_to include(user.email)
+      end
+    end
+
+    context 'with an invalid confirmation token' do
+      let(:confirmation_token) { 'invalid_confirmation_token' }
+
+      before do
+        subject
+      end
+
+      it 'renders `new`' do
+        expect(response).to render_template(:new)
+      end
+
+      it 'displays an error message' do
+        expect(response.body).to include('Confirmation token is invalid')
+      end
+    end
+  end
+end