Although it is against app store policy, any ios app can upload your contact data to its servers without asking you.
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Over the course of the past week, a firestorm has erupted in the world of iOS apps, thanks to the discovery that Path was uploading data from your iPhone's address book without asking for explicit permission. Upon opening the app and registering, Path automatically uploaded your contact data in order to "find friends" that you might want to connect to. Path has since apologized and updated its app, but the problem exposed by the episode remains.
Stated simply: any iOS app has complete access to a large amount of data stored on your iPhone, including your address book and calendar. Any iOS app can, without asking for your permission, upload all of the information stored in your address book to its servers. From there, the app developer...