A researchers said that a vulnerability was found within how the company uses SMS text messages to sign up new devices to the service. Anderson and Guarnieri claim that when a user logs into Telegram from a new smartphone, authorization codes are sent via SMS which in turn can be intercepted by the phone company and shared with cyberattackers.
This is particularly a problem when communications providers are heavily monitored or owned by states which want to keep track of their citizens. This year in Iran, for example, the country's government demanded that foreign messaging service providers must store Iranian citizen data within the country -- where law enforcement has easy access.
Once compromised SMS codes have been acquired, the cyberattacker can add new devices to the Telegram account, they can read chat histories and also intercept new messages.