Iranian hackers exploited an SMS text message flaw to hack Telegram accounts. 15 million users in Iran and more than a dozen accounts on the instant messaging service have been compromised by Iranian hackers exploiting an SMS-verification flaw. Such attack can be used against any messaging app, like Whatsapp and Viber, whose registration is based upon SMS-based verification mechanism. While other encrypted services do not offer any security mechanism to combat such hacking and surveillance attempts by hackers and government, Telegram provides some security features that are less known but important to implement.
Source: https://thehackernews.com/2016/08/hack-telegram-account.html