diff --git a/lib/redmine/wiki_formatting/html_sanitizer.rb b/lib/redmine/wiki_formatting/html_sanitizer.rb
index ea816b289..8247c181c 100644
--- a/lib/redmine/wiki_formatting/html_sanitizer.rb
+++ b/lib/redmine/wiki_formatting/html_sanitizer.rb
@@ -1,3 +1,22 @@
+# frozen_string_literal: true
+
+# Redmine - project management software
+# Copyright (C) 2006-2022  Jean-Philippe Lang
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+
 module Redmine
   module WikiFormatting