Gigabytes of medical data was deleted from one vulnerable database
Thousands of web-based databases have been deleted by cyberthieves seeking a ransom to restore the data.
Gigabytes of medical, payroll and other data held in MongoDB databases have been taken by attackers, say security researchers.
The systems were vulnerable to attack because their administrators accidentally left them easily accessible via the internet.
Attackers are seeking small amounts of bitcoins as payment to restore data.
Mr Gevers said the attacks started before Christmas but had accelerated once the holiday period was over.