Toyota Motor said the vehicle data of about 2.15 million users was publicly available in Japan for about a decade from November 2013 to mid-April.
The leak, attributed to a setting error in the cloud environment, could encompass details such as vehicle locations and identification numbers of vehicle devices, but there were no reports of malicious use, Toyota Motor said.
Affected customers included those who signed up for the T-Connect network service from the start of 2012 until April 17, Toyota said.
Also affected were users of G-Link, a similar service for owners of the luxury Lexus-branded vehicles, that provides features such as emergency support.
Toyota said it took steps to block outside access to the data after the issue was discovered. Toyota Motor started an investigation into all cloud environments managed by Toyota Connected Corp.