  def install_location filename, destination_dir # :nodoc:
    raise Gem::Package::PathError.new(filename, destination_dir) if
      filename.start_with? '/'

    destination_dir = realpath destination_dir
    destination_dir = File.expand_path destination_dir

    destination = File.join destination_dir, filename
    destination = File.expand_path destination

    raise Gem::Package::PathError.new(destination, destination_dir) unless
      destination.start_with? destination_dir + '/'

    destination.untaint
    destination
  end
The software uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the software does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.