/usr/lib/ruby/vendor_ruby/merb-core/rack/ is in ruby-merb-core 1.1.3+dfsg-2.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/vendor_ruby/merb-core/rack/adapter/
/usr/lib/ruby/vendor_ruby/merb-core/rack/adapter.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/adapter/abstract.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/adapter/ebb.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/adapter/evented_mongrel.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/adapter/fcgi.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/adapter/irb.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/adapter/mongrel.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/adapter/runner.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/adapter/swiftiplied_mongrel.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/adapter/thin.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/adapter/thin_turbo.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/adapter/webrick.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/application.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/handler/
/usr/lib/ruby/vendor_ruby/merb-core/rack/handler/mongrel.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/helpers.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/middleware/
/usr/lib/ruby/vendor_ruby/merb-core/rack/middleware.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/middleware/conditional_get.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/middleware/content_length.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/middleware/head.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/middleware/path_prefix.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/middleware/profiler.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/middleware/static.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/middleware/tracer.rb
/usr/lib/ruby/vendor_ruby/merb-core/rack/stream_wrapper.rb