Hide issue title on unsubscribe for anonymous users

app/helpers/notifications_helper.rb

@@ -93,4 +93,8 @@ module NotificationsHelper
       s_(event.to_s.humanize)
     end
   end
+
+  def show_unsubscribe_title?(noteable)
+    can?(current_user, "read_#{noteable.to_ability_name}".to_sym, noteable)
+  end
 end

app/views/sent_notifications/unsubscribe.html.haml

 - noteable = @sent_notification.noteable
 - noteable_type = @sent_notification.noteable_type.titleize.downcase
-- noteable_text = %(#{noteable.title} (#{noteable.to_reference}))
+- noteable_text = show_unsubscribe_title?(noteable) ? %(#{noteable.title} (#{noteable.to_reference})) : %(#{noteable.to_reference})
 - page_title _("Unsubscribe"), noteable_text, noteable_type.pluralize, @sent_notification.project.full_name
 %h3.page-title

changelogs/unreleased/security-unsubscribing-from-issue.yml

+---
+title: Hide confidential issue title on unsubscribe for anonymous users
+merge_request:
+author:
+type: security

spec/controllers/sent_notifications_controller_spec.rb

+# frozen_string_literal: true
+
 require 'rails_helper'
 describe SentNotificationsController do
   let(:user) { create(:user) }
-  let(:project) { create(:project) }
-  let(:sent_notification) { create(:sent_notification, project: project, noteable: issue, recipient: user) }
+  let(:project) { create(:project, :public) }
+  let(:private_project) { create(:project, :private) }
+  let(:sent_notification) { create(:sent_notification, project: target_project, noteable: noteable, recipient: user) }
   let(:issue) do
-    create(:issue, project: project, author: user) do |issue|
-      issue.subscriptions.create(user: user, project: project, subscribed: true)
+    create(:issue, project: target_project) do |issue|
+      issue.subscriptions.create(user: user, project: target_project, subscribed: true)
+    end
+  end
+
+  let(:confidential_issue) do
+    create(:issue, project: target_project, confidential: true) do |issue|
+      issue.subscriptions.create(user: user, project: target_project, subscribed: true)
     end
   end
+  let(:merge_request) do
+    create(:merge_request, source_project: target_project, target_project: target_project) do |mr|
+      mr.subscriptions.create(user: user, project: target_project, subscribed: true)
+    end
+  end
+
+  let(:noteable) { issue }
+  let(:target_project) { project }
+
   describe 'GET unsubscribe' do
     context 'when the user is not logged in' do
       context 'when the force param is passed' do
@@ -32,20 +50,93 @@ describe SentNotificationsController do
       end
       context 'when the force param is not passed' do
+        render_views
+
         before do
           get(:unsubscribe, params: { id: sent_notification.reply_key })
         end
-        it 'does not unsubscribe the user' do
-          expect(issue.subscribed?(user, project)).to be_truthy
+        shared_examples 'unsubscribing as anonymous' do
+          it 'does not unsubscribe the user' do
+            expect(noteable.subscribed?(user, target_project)).to be_truthy
+          end
+
+          it 'does not set the flash message' do
+            expect(controller).not_to set_flash[:notice]
+          end
+
+          it 'renders unsubscribe page' do
+            expect(response.status).to eq(200)
+            expect(response).to render_template :unsubscribe
+          end
         end
-        it 'does not set the flash message' do
-          expect(controller).not_to set_flash[:notice]
+        context 'when project is public' do
+          context 'when unsubscribing from issue' do
+            let(:noteable) { issue }
+
+            it 'shows issue title' do
+              expect(response.body).to include(issue.title)
+            end
+
+            it_behaves_like 'unsubscribing as anonymous'
+          end
+
+          context 'when unsubscribing from confidential issue' do
+            let(:noteable) { confidential_issue }
+
+            it 'does not show issue title' do
+              expect(response.body).not_to include(confidential_issue.title)
+              expect(response.body).to include(confidential_issue.to_reference)
+            end
+
+            it_behaves_like 'unsubscribing as anonymous'
+          end
+
+          context 'when unsubscribing from merge request' do
+            let(:noteable) { merge_request }
+
+            it 'shows merge request title' do
+              expect(response.body).to include(merge_request.title)
+            end
+
+            it_behaves_like 'unsubscribing as anonymous'
+          end
         end
-        it 'redirects to the login page' do
-          expect(response).to render_template :unsubscribe
+        context 'when project is not public' do
+          let(:target_project) { private_project }
+
+          context 'when unsubscribing from issue' do
+            let(:noteable) { issue }
+
+            it 'shows issue title' do
+              expect(response.body).not_to include(issue.title)
+            end
+
+            it_behaves_like 'unsubscribing as anonymous'
+          end
+
+          context 'when unsubscribing from confidential issue' do
+            let(:noteable) { confidential_issue }
+
+            it 'does not show issue title' do
+              expect(response.body).not_to include(confidential_issue.title)
+              expect(response.body).to include(confidential_issue.to_reference)
+            end
+
+            it_behaves_like 'unsubscribing as anonymous'
+          end
+
+          context 'when unsubscribing from merge request' do
+            let(:noteable) { merge_request }
+
+            it 'shows merge request title' do
+              expect(response.body).not_to include(merge_request.title)
+            end
+
+            it_behaves_like 'unsubscribing as anonymous'
+          end
         end
       end
     end