Fix confidential issue label disclosure on milestone view

Add changelog entry

Method should be public

Use milestonish method

Use render data to filter labels

Add specs for label visibility on milestone

app/controllers/concerns/milestone_actions.rb

@@ -26,16 +26,22 @@ module MilestoneActions
     end
   end
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def labels
     respond_to do |format|
       format.html { redirect_to milestone_redirect_path }
       format.json do
+        milestone_labels = @milestone.issue_labels_visible_by_user(current_user)
+
         render json: tabs_json("shared/milestones/_labels_tab", {
-          labels: @milestone.labels.map { |label| label.present(issuable_subject: @milestone.parent) } # rubocop:disable Gitlab/ModuleWithInstanceVariables
+          labels: milestone_labels.map do |label|
+            label.present(issuable_subject: @milestone.parent)
+          end
         })
       end
     end
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
   private
 

changelogs/unreleased/security-fix-confidential-issue-label-visibility-master.yml

@@ -0,0 +1,5 @@
+---
+title: Fix confidential issue label disclosure on milestone view
+merge_request:
+author:
+type: security

spec/controllers/projects/milestones_controller_spec.rb

@@ -175,6 +175,40 @@ describe Projects::MilestonesController do
       end
     end
 
+    describe '#labels' do
+      render_views
+
+      context 'as json' do
+        let!(:guest) { create(:user, username: 'guest1') }
+        let!(:group) { create(:group, :public) }
+        let!(:project) { create(:project, :public, group: group) }
+        let!(:label) { create(:label, title: 'test_label_on_private_issue', project: project) }
+        let!(:confidential_issue) { create(:labeled_issue, confidential: true, project: project, milestone: milestone, labels: [label]) }
+
+        it 'does not render labels of private issues if user has no access' do
+          sign_in(guest)
+
+          get :labels, params: { namespace_id: group.id, project_id: project.id, id: milestone.iid }, format: :json
+
+          expect(response).to have_gitlab_http_status(200)
+          expect(response.content_type).to eq 'application/json'
+
+          expect(json_response['html']).not_to include(label.title)
+        end
+
+        it 'does render labels of private issues if user has access' do
+          sign_in(user)
+
+          get :labels, params: { namespace_id: group.id, project_id: project.id, id: milestone.iid }, format: :json
+
+          expect(response).to have_gitlab_http_status(200)
+          expect(response.content_type).to eq 'application/json'
+
+          expect(json_response['html']).to include(label.title)
+        end
+      end
+    end
+
     context 'promotion succeeds' do
       before do
         group.add_developer(user)