SECURITY: Filter tags in user notifications for visibility

app/mailers/user_notifications.rb

@@ -529,11 +529,13 @@ class UserNotifications < ActionMailer::Base
 
     # tag names
     if opts[:show_tags_in_subject] && post.topic_id
-
-      tags = Tag.joins(:topic_tags)
-        .where("topic_tags.topic_id = ?", post.topic_id)
-        .limit(3)
-        .pluck(:name)
+      tags =
+        DiscourseTagging
+          .visible_tags(Guardian.new(user))
+          .joins(:topic_tags)
+          .where("topic_tags.topic_id = ?", post.topic_id)
+          .limit(3)
+          .pluck(:name)
 
       show_tags_in_subject = tags.any? ? tags.join(" ") : nil
     end

spec/mailers/user_notifications_spec.rb

@@ -309,7 +309,13 @@ describe UserNotifications do
     let(:category) { Fabricate(:category, name: 'India') }
     let(:tag1) { Fabricate(:tag, name: 'Taggo') }
     let(:tag2) { Fabricate(:tag, name: 'Taggie') }
-    let(:topic) { Fabricate(:topic, category: category, tags: [tag1, tag2], title: "Super cool topic") }
+
+    let(:hidden_tag) { Fabricate(:tag, name: "hidden") }
+    let!(:hidden_tag_group) do
+      Fabricate(:tag_group, permissions: { "staff" => 1 }, tag_names: [hidden_tag.name])
+    end
+
+    let(:topic) { Fabricate(:topic, category: category, tags: [tag1, tag2, hidden_tag], title: "Super cool topic") }
     let(:post) { Fabricate(:post, topic: topic, raw: 'This is My super duper cool topic') }
     let(:response) { Fabricate(:basic_reply, topic: post.topic, user: response_by_user) }
     let(:user) { Fabricate(:user) }
@@ -388,6 +394,9 @@ describe UserNotifications do
       expect(mail_html.scan(/>Bob Marley/).count).to eq(1)
       expect(mail_html.scan(/>bobmarley/).count).to eq(0)
 
+      expect(mail.subject.scan(/#{tag1.name}/).count).to eq(1)
+      expect(mail.subject.scan(/#{hidden_tag.name}/).count).to eq(0)
+
       SiteSetting.prioritize_username_in_ux = true
 
       mail = UserNotifications.user_replied(