DEV: Strip unsubscribe links when sending digest previews to arbitrary addresses (#38298)

# Problem

When an admin sends a digest preview to an arbitrary email address, the
email contained functional unsubscribe links belonging to the target
user, allowing the recipient to unsubscribe that user without their
consent.

# Solution

Pass `skip_unsubscribe_links: true` when generating preview digests,
which skips `UnsubscribeKey` creation and suppresses unsubscribe content
from both email headers and body templates at the source.

app/controllers/admin/email_controller.rb

@@ -45,12 +45,19 @@ class Admin::EmailController < Admin::AdminController
     params.require(:username)
     params.require(:email)
     user = User.find_by_username(params[:username])
+    raise Discourse::InvalidParameters unless user
 
     message, skip_reason =
-      UserNotifications.public_send(:digest, user, since: params[:last_seen_at])
+      UserNotifications.public_send(
+        :digest,
+        user,
+        since: params[:last_seen_at],
+        skip_unsubscribe_links: true,
+      )
 
     if message
       message.to = params[:email]
+
       begin
         Email::Sender.new(message, :digest).send
         render json: success_json

app/mailers/user_notifications.rb

@@ -251,7 +251,9 @@ class UserNotifications < ActionMailer::Base
 
   def digest(user, opts = {})
     build_summary_for(user)
-    @unsubscribe_key = UnsubscribeKey.create_key_for(@user, UnsubscribeKey::DIGEST_TYPE)
+    if !opts[:skip_unsubscribe_links]
+      @unsubscribe_key = UnsubscribeKey.create_key_for(@user, UnsubscribeKey::DIGEST_TYPE)
+    end
 
     @since = opts[:since].presence
     @since ||= [user.last_seen_at, user.user_stat&.digest_attempted_at, 1.month.ago].compact.max
@@ -371,7 +373,7 @@ class UserNotifications < ActionMailer::Base
             email_prefix: @email_prefix,
             date: short_date(Time.now),
           ),
-        add_unsubscribe_link: true,
+        add_unsubscribe_link: !opts[:skip_unsubscribe_links],
         unsubscribe_url: "#{Discourse.base_url}/email/unsubscribe/#{@unsubscribe_key}",
         topic_ids: topics_for_digest.pluck(:id),
         post_ids:

app/views/user_notifications/digest/_footer.html.erb

@@ -3,10 +3,12 @@
     <!-- Empty cells pad either side of the email content -->
     <td></td>
     <td width="650" align="center">
-      <%=raw(t 'user_notifications.digest.unsubscribe',
-              site_link: html_site_link,
-              email_preferences_link: link_to(t('user_notifications.digest.your_email_settings'), Discourse.base_url + '/my/preferences/emails'),
-              unsubscribe_link: link_to(t('user_notifications.digest.click_here'), "#{Discourse.base_url}/email/unsubscribe/#{@unsubscribe_key}"))  %>
+      <% if @unsubscribe_key %>
+        <%=raw(t 'user_notifications.digest.unsubscribe',
+                site_link: html_site_link,
+                email_preferences_link: link_to(t('user_notifications.digest.your_email_settings'), Discourse.base_url + '/my/preferences/emails'),
+                unsubscribe_link: link_to(t('user_notifications.digest.click_here'), "#{Discourse.base_url}/email/unsubscribe/#{@unsubscribe_key}"))  %>
+      <% end %>
     </td>
     <td></td>
   </tr>

app/views/user_notifications/digest/_footer.text.erb

@@ -3,10 +3,12 @@
 <%= raw(@markdown_linker.references) %>
 
 <%= digest_custom_text("above_footer") %>
-<%=raw(t :'user_notifications.digest.unsubscribe',
-     site_link: site_link,
-     email_preferences_link: raw(@markdown_linker.create(t('user_notifications.digest.your_email_settings'), '/my/preferences/emails')),
-     unsubscribe_link: raw(@markdown_linker.create(t('user_notifications.digest.click_here'), "/email/unsubscribe/#{@unsubscribe_key}")))  %>
+<%- if @unsubscribe_key %>
+  <%=raw(t :'user_notifications.digest.unsubscribe',
+       site_link: site_link,
+       email_preferences_link: raw(@markdown_linker.create(t('user_notifications.digest.your_email_settings'), '/my/preferences/emails')),
+       unsubscribe_link: raw(@markdown_linker.create(t('user_notifications.digest.click_here'), "/email/unsubscribe/#{@unsubscribe_key}")))  %>
+<%- end %>
 
 <%= raw(@markdown_linker.references) %>
 

frontend/discourse/admin/controllers/admin-email/preview-digest.js

@@ -10,6 +10,7 @@ export default class AdminEmailPreviewDigestController extends Controller {
 
   username = null;
   lastSeen = null;
+  email = null;
 
   @empty("email") emailEmpty;
   @or("emailEmpty", "sendingEmail") sendEmailDisabled;

frontend/discourse/tests/acceptance/admin-email-preview-test.js

@@ -1,4 +1,4 @@
-import { click, visit, waitUntil } from "@ember/test-helpers";
+import { click, fillIn, visit, waitUntil } from "@ember/test-helpers";
 import { test } from "qunit";
 import { acceptance } from "discourse/tests/helpers/qunit-helpers";
 
@@ -36,4 +36,29 @@ acceptance("Admin - email-preview", function (needs) {
         "text content is escaped correctly"
       );
   });
+
+  test("send digest form requires a destination email", async function (assert) {
+    await visit("/admin/email/preview-digest");
+
+    assert
+      .dom(".email-preview-digest .controls")
+      .exists("send email form is shown");
+
+    assert
+      .dom(".email-preview-digest .controls input[type='text']")
+      .exists("an email text field is shown for sending this digest preview");
+
+    assert
+      .dom(".email-preview-digest .controls .btn-default")
+      .isDisabled("send button is disabled until an email address is entered");
+
+    await fillIn(
+      ".email-preview-digest .controls input[type='text']",
+      "preview@example.com"
+    );
+
+    assert
+      .dom(".email-preview-digest .controls .btn-default")
+      .isNotDisabled("send button is enabled once an email address is entered");
+  });
 });

spec/requests/admin/email_controller_spec.rb

@@ -238,10 +238,34 @@ RSpec.describe Admin::EmailController do
              params: {
                last_seen_at: 1.week.ago,
                username: admin.username,
-               email: email("previous_replies"),
+               email: admin.email,
              }
         expect(response.status).to eq(200)
       end
+
+      it "sends the digest to the requested email without unsubscribe links" do
+        Fabricate(:topic, created_at: 1.day.ago)
+        ActionMailer::Base.deliveries.clear
+
+        expect {
+          post "/admin/email/send-digest.json",
+               params: {
+                 last_seen_at: 1.month.ago,
+                 username: user.username,
+                 email: "attacker@evil.com",
+               }
+        }.not_to change { UnsubscribeKey.count }
+
+        expect(response.status).to eq(200)
+        expect(response.parsed_body["success"]).to eq("OK")
+
+        message = ActionMailer::Base.deliveries.last
+        expect(message.to).to eq(["attacker@evil.com"])
+        expect(message.html_part.body.to_s).not_to include("/email/unsubscribe/")
+        expect(message.text_part.body.to_s).not_to include("/email/unsubscribe/")
+        expect(message.header["List-Unsubscribe"].to_s).to be_blank
+        expect(message.header["List-Unsubscribe-Post"].to_s).to be_blank
+      end
     end
   end