May 2017 isn't that long ago.

   "However, our survey shows that Telegram has had serious and simple 
   issues in the protocol (e.g. modified buggy Diffie-Hellman key exchange)
   that any knowledgeable security expert could penetrate.
   By using the command line interface of Telegram we have been able to
   snoop on some of our friends and detect the times when they were conversing
   to each other. We believe that this is a serious privacy issue, because it 
   can be exploited to detect relationships in classroom for example."

see https://courses.csail.mit.edu/6.857/2017/project/19.pdf