In addition to sending private data to other users phones, Joseph Keller reports:
Also of note is how QuizUp handles access to your contacts. The game allows you to invite your friends to the game via text message, which you need to grant QuizUp access to your contacts to allow. Once this is done, QuizUp sends your contact’s emails, in plain text, to their servers, in violation of federal privacy laws. This is the same thing that got social network Path in trouble last year.