Escape namespace in label references

When referencing cross-namespace labels, we append the namespace name
to the rendered label.

This MR escapes the name to prevent XSS attacks.

changelogs/unreleased/security-fix-xss-in-label-namespace.yml

+---
+title: Escape namespace in label references to prevent XSS
+merge_request:
+author:
+type: security

lib/banzai/filter/label_reference_filter.rb

@@ -89,7 +89,7 @@ module Banzai
           parent_from_ref = from_ref_cached(project_path)
           reference       = parent_from_ref.to_human_reference(parent)
 
-          label_suffix = " <i>in #{reference}</i>" if reference.present?
+          label_suffix = " <i>in #{ERB::Util.html_escape(reference)}</i>" if reference.present?
         end
 
         presenter = object.present(issuable_subject: parent)

spec/lib/banzai/filter/label_reference_filter_spec.rb

@@ -521,6 +521,15 @@ describe Banzai::Filter::LabelReferenceFilter do
 
       expect(reference_filter(act).to_html).to eq exp
     end
+
+    context 'when group name has HTML entities' do
+      let(:another_group) { create(:group, name: '<img src=x onerror=alert(1)>', path: 'another_group') }
+
+      it 'escapes the HTML entities' do
+        expect(result.text)
+          .to eq "See #{group_label.name} in #{another_project.full_name}"
+      end
+    end
   end
 
   describe 'cross-project / same-group_label complete reference' do