12.2 Backport for CE MR

https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/3419

app/models/concerns/mentionable/reference_regexes.rb

@@ -13,7 +13,9 @@ module Mentionable
     def self.other_patterns
       [
         Commit.reference_pattern,
-        MergeRequest.reference_pattern
+        MergeRequest.reference_pattern,
+        Label.reference_pattern,
+        Milestone.reference_pattern
       ]
     end

app/models/system_note_metadata.rb

@@ -10,6 +10,7 @@ class SystemNoteMetadata < ApplicationRecord
     commit cross_reference
     close duplicate
     moved merge
+    label milestone
   ].freeze
   ICON_TYPES = %w[

changelogs/unreleased/security-2920-fix-notes-with-label-cross-reference.yml

+---
+title: Show cross-referenced label and milestones in issues' activities only to authorized users
+merge_request:
+author:
+type: security

spec/models/note_spec.rb

@@ -340,6 +340,63 @@ describe Note do
         expect(label_note.cross_reference?).to be_falsy
       end
     end
+
+    context 'when system note metadata is not present' do
+      let(:note) { build(:note, :system) }
+
+      before do
+        allow(note).to receive(:system_note_metadata).and_return(nil)
+      end
+
+      it 'delegates to the system note service' do
+        expect(SystemNoteService).to receive(:cross_reference?).with(note.note)
+
+        note.cross_reference?
+      end
+    end
+
+    context 'with a system note' do
+      let(:issue)     { create(:issue, project: create(:project, :repository)) }
+      let(:note)      { create(:system_note, note: "test", noteable: issue, project: issue.project) }
+
+      shared_examples 'system_note_metadata includes note action' do
+        it 'delegates to the cross-reference regex' do
+          expect(note).to receive(:matches_cross_reference_regex?)
+
+          note.cross_reference?
+        end
+      end
+
+      context 'with :label action' do
+        let!(:metadata) {create(:system_note_metadata, note: note, action: :label)}
+
+        it_behaves_like 'system_note_metadata includes note action'
+
+        it { expect(note.cross_reference?).to be_falsy }
+
+        context 'with cross reference label note' do
+          let(:label) { create(:label, project: issue.project)}
+          let(:note) { create(:system_note, note: "added #{label.to_reference} label", noteable: issue, project: issue.project) }
+
+          it { expect(note.cross_reference?).to be_truthy }
+        end
+      end
+
+      context 'with :milestone action' do
+        let!(:metadata) {create(:system_note_metadata, note: note, action: :milestone)}
+
+        it_behaves_like 'system_note_metadata includes note action'
+
+        it { expect(note.cross_reference?).to be_falsy }
+
+        context 'with cross reference milestone note' do
+          let(:milestone) { create(:milestone, project: issue.project)}
+          let(:note) { create(:system_note, note: "added #{milestone.to_reference} milestone", noteable: issue, project: issue.project) }
+
+          it { expect(note.cross_reference?).to be_truthy }
+        end
+      end
+    end
   end
   describe 'clear_blank_line_code!' do