Smartphone apps are very useful but users often have to pay for them with their privacy.

The German consumer protection group Stiftung Warentest looked at 63 applications and graded 28 of them as "critical" in regards to data protection. Another nine were determined to be "very critical" -including widely-popular apps such as WhatsApp, Facebook, Foodspotting and iTranslate.

The applications take personal data such as names, telephone numbers, email addresses or even passwords and pass it along in annon-anonymized or un-encrypted form to the servers of the corresponding providers - usually without asking the users for permission, said the consumer group.

In the case of some apps unnecessary app usage statistics or the smartphone identifier is transferred without the knowledge of users. With help of the identifier, transferred information from the various apps can be assigned to a specific mobile phone.

This makes it possible for app providers or analysis and advertising networks which are given the data to create a user profile, for example for individual advertisements.

The users are unaware of which information is provided to whom as well as not knowing the retention and deletion periods of these mobile data collectors.

Because an unencrypted transfer can easily be read by hackers inan insecure Wi-Fi network, the testers recommend certain apps not be used in public hotspots. Should an app require a password, users definitely should not use the same one as they use for online banking or email accounts.