The team of researchers found 56 million items of unprotected data in the applications it studied in detail, which included games, social networks, messaging, medical and bank transfer apps.

Eric Bodden, of the team from the Fraunhofer Institute for Secure Information Technology and Darmstadt University of Technology, Germany, said the number of records affected “will likely be in the billions”.

The vulnerable applications, which they declined to name, number in the tens of thousands, and include some of the most popular on the Apple and Google app stores.

The problem, Bodden said, is in the way developers — those who write and sell the applications — authenticate users when storing their data in online databases.

Most such apps use services like Amazon’s Web Services or Facebook’s Parse to store, share or back up users’ data. While such services offer ways for developers to protect the data, most choose the default option, based on a string of letters and numbers embedded in the software’s code, called a token.

Attackers can easily extract and tweak those tokens in the app, which then gives them access to the private data of all users of that app stored on the server.

Google declined to comment, while Apple and Amazon did not respond to queries.