"Cashback services collect a lot of data. This would be fine if it stayed within the company, says Jen King, director of consumer privacy at the Center for Internet and Society at Stanford Law School, but she suspects that isn’t the case.
Though major companies like Rakuten do not sell information to third parties, they do share it. And while the data these companies trade is anonymized, identifying users isn’t difficult, especially if you share your location with an app or website.
King says that for cashback users who would like to retain a semblance of privacy, the safest option would be to use a separate browser only for shopping.
The common perception that our information is just “one little point in a giant database,” King says, is misleading. While many of us feel more secure hiding in the crowd of aggregate information, the vast scale of data collection actually makes the problem worse."