The ever-increasing amount of valuable digital data both at home and in business needs to be protected, since its irrevocable loss is unacceptable. Cloud storage services promise to be a solution for this problem. They offer user-friendly, easily accessible and costsaving ways to store and automatically back up arbitrary data, as well as data sharing between users and the synchronization of multiple devices.

However, recent successful attacks on cloud storage providers have shown that the security of cloud storage services is often poor. This is also the result of the study "On the Security of Cloud Storage Services" of the Fraunhofer Institute for Secure Information Technology that tested different cloud storage providers. None of the providers tested was able to fully meet all the security requirements. The study was updated recently, the alterations are summarized in an addendum.