SECURITY: Don't disclose the existence of secret subcategories

app/models/category_list.rb

@@ -206,12 +206,19 @@ class CategoryList
 
     allowed_topic_create = Set.new(Category.topic_create_allowed(@guardian).pluck(:id))
 
+    parent_ids =
+      Category
+        .secured(@guardian)
+        .where(parent_category_id: categories_with_descendants.map(&:id))
+        .pluck("DISTINCT parent_category_id")
+        .to_set
+
     categories_with_descendants.each do |category|
       category.notification_level = notification_levels[category.id] || default_notification_level
       category.permission = CategoryGroup.permission_types[:full] if allowed_topic_create.include?(
         category.id,
       )
-      category.has_children = category.subcategories.present?
+      category.has_children = parent_ids.include?(category.id)
     end
 
     if @topics_by_category_id

spec/models/category_list_spec.rb

@@ -12,6 +12,29 @@ RSpec.describe CategoryList do
   fab!(:admin)
   let(:category_list) { CategoryList.new(Guardian.new(user), include_topics: true) }
 
+  context "when a category has a secret subcategory" do
+    fab!(:category)
+
+    fab!(:secret_subcategory) do
+      cat = Fabricate(:category, parent_category: category)
+      cat.set_permissions(admins: :full)
+      cat.save!
+      cat
+    end
+
+    let(:admin_category_list) { CategoryList.new(Guardian.new(admin), include_topics: true) }
+
+    it "doesn't set has_children when an unpriveleged user is querying" do
+      found = category_list.categories.find { |c| c.id == category.id }
+      expect(found.has_children).to eq(false)
+    end
+
+    it "sets has_children when an admin is querying" do
+      found = admin_category_list.categories.find { |c| c.id == category.id }
+      expect(found.has_children).to eq(true)
+    end
+  end
+
   describe "security" do
     it "properly hide secure categories" do
       cat = Fabricate(:category_with_definition)