Redmine

The following code is causing the problem.

      # https://github.com/redmine/redmine/blob/df88b9c7ddb485784b1c74c40e7b34675d68f983/app/helpers/application_helper.rb#L62-L63
      name = h(principal.name(options[:format]))
      name = "@" + name if options[:mention]

• h(principal.name(options[:format])) => ActiveSupport::SafeBuffer object
• "@" => String object
• "@" + name => String object

The concatenation of "" changes the class of the object(ActiveSupport::SafeBuffer => String).
Pre-converting "" to an ActiveSupport::SafeBuffer object by html_safe should prevent the class from being changed.

diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
index ced1845ebb..ec97845756 100644
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -60,7 +60,7 @@ module ApplicationHelper
     case principal
     when User
       name = h(principal.name(options[:format]))
-      name = "@" + name if options[:mention]
+      name = "@".html_safe + name if options[:mention]
       css_classes = ''
       if principal.active? || (User.current.admin? && principal.logged?)
         url = user_url(principal, :only_path => only_path)
diff --git a/test/helpers/application_helper_test.rb b/test/helpers/application_helper_test.rb
index a5c533a4fb..d34de2b08c 100644
--- a/test/helpers/application_helper_test.rb
+++ b/test/helpers/application_helper_test.rb
@@ -1841,6 +1841,16 @@ class ApplicationHelperTest < Redmine::HelperTest
     assert_equal result, link_to_principal(unknown_principal, :class => 'bar')
   end

+  def test_link_to_principal_should_escape_principal_name
+    user = User.generate!(firstname: "firstname<>'", lastname: 'lastname&"')
+    group = Group.generate!(lastname: "group<>'&")
+
+    assert_include "firstname&lt;&gt;&#39; lastname&amp;&quot;", link_to_principal(user)
+    assert_include "@firstname&lt;&gt;&#39; lastname&amp;&quot;", link_to_principal(user, {mention: true})
+    assert_include "group&lt;&gt;&#39;&amp;", link_to_principal(group)
+    assert_include "&lt;&gt;&#39;&amp;", link_to_principal("<>'&")
+  end
+
   def test_link_to_group_should_return_only_group_name_for_non_admin_users
     User.current = nil
     group = Group.find(10)