Belgian law enforcement exploited end-to-end encryption gaps in WhatsApp to intercept messages exchanged between members of a Chechen jihadist group. Facebook who acquired WhatsApp declined to comment on the events, but security experts speculate that there is something wrong in the implementation of WhatsApp’s encryption. One of the suspects detained had recently returned to Belgium after being injured in a fight in Syria. The analysis of metadata allowed law enforcement to identify the network of the alleged suspects. According to the editor Fabian A. Scherschel, authorities are able to intercept and collect messages that could be decrypted since the key seeds could be more easily guessed.”]
Source: http://securityaffairs.co/wordpress/37700/intelligence/terrorists-using-whatsapp.html