(CNN) A massive online database apparently containing the personal information of up to one billion Chinese citizens was left unsecured and publicly accessible for more than a year — until an anonymous user in a hacker forum offered to sell the data and brought it to wider attention last week. The leak could be one of the biggest ever recorded in history, cybersecurity experts say, highlighting the risks of collecting and storing vast amounts of sensitive personal data online — especially in a country where authorities have broad and unchecked access to such data. The vast trove of Chinese personal...