Escapes milestone title shown in flash message when promoting a milestone

app/controllers/projects/milestones_controller.rb

 class Projects::MilestonesController < Projects::ApplicationController
+  include ActionView::Helpers::SanitizeHelper
   include Gitlab::Utils::StrongMemoize
   include MilestoneActions
 
@@ -76,8 +77,8 @@ class Projects::MilestonesController < Projects::ApplicationController
 
   def promote
     promoted_milestone = Milestones::PromoteService.new(project, current_user).execute(milestone)
-
-    flash[:notice] = "#{milestone.title} promoted to <a href=\"#{group_milestone_path(project.group, promoted_milestone.iid)}\"><u>group milestone</u></a>.".html_safe
+    milestone_title = sanitize(milestone.title)
+    flash[:notice] = "#{milestone_title} promoted to <a href=\"#{group_milestone_path(project.group, promoted_milestone.iid)}\"><u>group milestone</u></a>.".html_safe
     respond_to do |format|
       format.html do
         redirect_to project_milestones_path(project)

spec/controllers/projects/milestones_controller_spec.rb

@@ -127,6 +127,14 @@ describe Projects::MilestonesController do
         expect(flash[:notice]).to eq("#{milestone.title} promoted to <a href=\"#{group_milestone_path(project.group, milestone.iid)}\"><u>group milestone</u></a>.")
         expect(response).to redirect_to(project_milestones_path(project))
       end
+
+      it 'renders milestone name without parsing it as HTML' do
+        milestone.update!(name: 'CCC&lt;img src=x onerror=alert(document.domain)&gt;')
+
+        post :promote, namespace_id: project.namespace.id, project_id: project.id, id: milestone.iid
+
+        expect(flash[:notice]).to eq("CCC<img src=\"x\"> promoted to <a href=\"#{group_milestone_path(project.group, milestone.iid)}\"><u>group milestone</u></a>.")
+      end
     end
 
     context 'promotion fails' do