Use hash-based flash objects to render complex flash messages

Since flash objects can only be String, Hash or Array (notably excluding SafeBuffers), then this approach is necessary to render complex html in a safe manner.

Each local can be treated as an (unsafe) string, and therefore escaped normally when rendered into the template. The template (and translation strings) can
contain html since they are no longer stored in the flash as a plain string.

Fixes #3215

app/controllers/confirmations_controller.rb

@@ -66,7 +66,7 @@ class ConfirmationsController < ApplicationController
       flash[:error] = t "confirmations.confirm_resend.failure", :name => params[:display_name]
     else
       UserMailer.signup_confirm(user, user.tokens.create).deliver_later
-      flash[:notice] = t "confirmations.confirm_resend.success_html", :email => user.email, :sender => Settings.email_from
+      flash[:notice] = { :partial => "confirmations/resend_success_flash", :locals => { :email => user.email, :sender => Settings.email_from } }
     end
 
     redirect_to login_path

app/helpers/application_helper.rb

@@ -68,4 +68,14 @@ module ApplicationHelper
 
     data
   end
+
+  # If the flash is a hash, then it will be a partial with a hash of locals, so we can call `render` on that
+  # This allows us to render html into a flash message in a safe manner.
+  def render_flash(flash)
+    if flash.is_a?(Hash)
+      render flash.with_indifferent_access
+    else
+      flash
+    end
+  end
 end

app/views/confirmations/_resend_success_flash.html.erb

@@ -0,0 +1 @@
+<%= t "confirmations.confirm_resend.success_html", :email => email, :sender => sender %>

app/views/layouts/_flash.html.erb

@@ -4,7 +4,7 @@
       <source srcset="<%= image_path "notice.svg" %>" type="image/svg+xml" />
       <%= image_tag("notice.png", :srcset => image_path("notice.svg"), :class => "small_icon", :border => 0) %>
     </picture>
-    <div class="message"><%= flash[:error] %></div>
+    <div class="message"><%= render_flash(flash[:error]) %></div>
   </div>
 <% end %>
 
@@ -14,7 +14,7 @@
       <source srcset="<%= image_path "notice.svg" %>" type="image/svg+xml"></source>
       <%= image_tag("notice.png", :srcset => image_path("notice.svg"), :class => "small_icon", :border => 0) %>
     </picture>
-    <div class="message"><%= flash[:warning] %></div>
+    <div class="message"><%= render_flash(flash[:warning]) %></div>
   </div>
 <% end %>
 
@@ -24,6 +24,6 @@
       <source srcset="<%= image_path "notice.svg" %>" type="image/svg+xml"></source>
       <%= image_tag("notice.png", :srcset => image_path("notice.svg"), :class => "small_icon", :border => 0) %>
     </picture>
-    <div class="message"><%= flash[:notice] %></div>
+    <div class="message"><%= render_flash(flash[:notice]) %></div>
   </div>
 <% end %>

test/controllers/confirmations_controller_test.rb

@@ -211,7 +211,8 @@ class UsersControllerTest < ActionDispatch::IntegrationTest
 
     assert_response :redirect
     assert_redirected_to login_path
-    assert_match(/sent a new confirmation/, flash[:notice])
+    assert_equal("confirmations/resend_success_flash", flash[:notice][:partial])
+    assert_equal({ :email => user.email, :sender => Settings.email_from }, flash[:notice][:locals])
 
     email = ActionMailer::Base.deliveries.last
 

test/system/confirmation_resend.rb

@@ -0,0 +1,26 @@
+require "application_system_test_case"
+
+class ConfirmationResendSystemTest < ApplicationSystemTestCase
+  def setup
+    @user = build(:user)
+    visit user_new_path
+
+    fill_in "Email", :with => @user.email
+    fill_in "Email Confirmation", :with => @user.email
+    fill_in "Display Name", :with => @user.display_name
+    fill_in "Password", :with => "testtest"
+    fill_in "Confirm Password", :with => "testtest"
+    click_button "Sign Up"
+
+    check "I have read and agree to the above contributor terms"
+    check "I have read and agree to the Terms of Use"
+    click_button "Continue"
+  end
+
+  test "flash message should not contain raw html" do
+    visit user_confirm_resend_path(@user)
+
+    assert page.has_content?("sent a new confirmation")
+    assert_not page.has_content?("<br />")
+  end
+end