Don't process MR refs for guests in the notes

app/policies/project_policy.rb

@@ -392,7 +392,7 @@ class ProjectPolicy < BasePolicy
   end.enable :read_issue_iid
 
   rule do
-    (can?(:read_project_for_iids) & merge_requests_visible_to_user) | can?(:read_merge_request)
+    (~guest & can?(:read_project_for_iids) & merge_requests_visible_to_user) | can?(:read_merge_request)
   end.enable :read_merge_request_iid
 
   rule { ~can_have_multiple_clusters & has_clusters }.prevent :add_cluster

changelogs/unreleased/security-do-not-process-mr-ref-for-guests.yml

+---
+title: Don't process MR refs for guests in the notes
+merge_request: 2771
+author:
+type: security

spec/policies/project_policy_spec.rb

@@ -12,7 +12,7 @@ describe ProjectPolicy do
   let(:base_guest_permissions) do
     %i[
       read_project read_board read_list read_wiki read_issue
-      read_project_for_iids read_issue_iid read_merge_request_iid read_label
+      read_project_for_iids read_issue_iid read_label
       read_milestone read_project_snippet read_project_member read_note
       create_project create_issue create_note upload_file create_merge_request_in
       award_emoji read_release
@@ -152,6 +152,16 @@ describe ProjectPolicy do
     end
   end
 
+  context 'for a guest in a private project' do
+    let(:project) { create(:project, :private) }
+    subject { described_class.new(guest, project) }
+
+    it 'disallows the guest from reading the merge request and merge request iid' do
+      expect_disallowed(:read_merge_request)
+      expect_disallowed(:read_merge_request_iid)
+    end
+  end
+
   context 'builds feature' do
     subject { described_class.new(owner, project) }