The claim in the title is plain wrong, this is not a vulnerability in the authentication protocol itself. Money quote: What should an attacker do to launch

The claim in the title is plain wrong, this is not a vulnerability in the authentication protocol itself. Money quote: What should an attacker do to launch the attack? Briefly, he would have to change Telegram’s public key (and the IP address, or perform a “live” spoofing attack), included in the client, and distribute the malicious client to the victim(s). For comparison, this would be equivalent to editing entries in ~/.ssh/known_hosts of the victim and then make them use ssh to your server for a MITM attack. Or rather, if you get someone to use a manipulated SSH client, they are vulnerable. Would you consider this a flaw in the SSH authentication protocol?