/usr/lib/ruby/ is in ruby-sanitize 2.1.0-2+deb9u1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/vendor_ruby/
/usr/lib/ruby/vendor_ruby/sanitize/
/usr/lib/ruby/vendor_ruby/sanitize.rb
/usr/lib/ruby/vendor_ruby/sanitize/config/
/usr/lib/ruby/vendor_ruby/sanitize/config.rb
/usr/lib/ruby/vendor_ruby/sanitize/config/basic.rb
/usr/lib/ruby/vendor_ruby/sanitize/config/relaxed.rb
/usr/lib/ruby/vendor_ruby/sanitize/config/restricted.rb
/usr/lib/ruby/vendor_ruby/sanitize/transformers/
/usr/lib/ruby/vendor_ruby/sanitize/transformers/clean_cdata.rb
/usr/lib/ruby/vendor_ruby/sanitize/transformers/clean_comment.rb
/usr/lib/ruby/vendor_ruby/sanitize/transformers/clean_element.rb
/usr/lib/ruby/vendor_ruby/sanitize/version.rb