When WhatsApp added end-to-end encryption to every conversation for its billion users two years ago, the mobile messaging giant significantly raised the bar for the privacy of digital communications worldwide. But one of the tricky elements of encryption—and even trickier in a group chat setting—has always been ensuring that a secure conversation reaches only the intended audience, rather than some impostor or infiltrator. And according to new research from one team of German cryptographers, flaws in WhatsApp make infiltrating the app's group chats much easier than ought to be possible.