Telegram uses phone numbers as primary account identifiers. During setup or login on a new device, an SMS login code is dispatched. This reliance on SMS makes accounts vulnerable to SIM-swapping attacks and SMS interception tactics employed by sophisticated threat actors.
How SIM-Swapping Attacks Target Telegram
In a SIM swap attack, an adversary tricks a mobile carrier into porting the victim's phone number to a attacker-controlled SIM card. Once completed, the attacker receives the Telegram login SMS code and gains access to all cloud chats immediately.
Essential Countermeasures
1. Enable Two-Step Verification (2FA)
Two-Step Verification adds a local password requirement in addition to the SMS code when logging into a new device. This password is checked on Telegram servers using a SRP (Secure Remote Password) protocol derivative, preventing cloud access even if the SMS is intercepted.
2. Active Session Management
Users should regularly inspect and revoke unrecognized logged-in devices under Settings > Devices.
3. Configure Recovery Email Correctly
Ensure that the secondary recovery email associated with 2FA password resets is protected with strong passwords and physical hardware security keys (e.g., YubiKey).
Security Question for Discussion
How does Telegram's SRP implementation protect user passwords during Two-Step Verification verification from being intercepted by malicious transit nodes?
