Report anything you find privately, not as a public issue: open a security advisory, or message @ham1235i on Telegram.

Please include what you did, what happened, and what you expected. A first reply should come within a few days.

The bot runs on one server, owned by one person, and answers to one Telegram account. The things worth protecting are the API keys, the panel, and the database of who talks where.

• The panel exists only in the owner's private chat and is gated on their numeric Telegram id. Every button press is re-checked, because a panel message can be forwarded and its buttons travel with it. MASTER_ID settles the id outright; without it, a username is used exactly once to learn the id and never consulted again.
• API keys are stored encrypted in the database with a key file beside it, are shown only masked, and the message that delivers one is deleted from the chat. They are never written to the log or the audit trail.
• The server is not reachable from a chat. The bot runs unprivileged and can ask a root helper for exactly two things — restart and update — by writing one word to a spool file. Nothing from Telegram is ever passed to a shell.
• Message text is never stored. The database keeps who is in which chat and how active they are, as counts. A test checks the database file and its write-ahead log to keep it that way.

Said plainly, because a security file that only lists strengths is not useful.

• Someone who already has the server. The encryption key sits next to the database, so root, or the bot's own user, can read the keys. Encryption is there to make a copied database — a backup, a file pulled to a laptop — useless on its own.
• Whoever controls the repository. update runs the code on the branch. That is what an auto-update is, and it is why the helper accepts nothing else.
• A compromised Telegram account. Whoever is signed in as the owner has the panel. Use two-factor authentication on that account.

Revoke it at the provider first, then add the new one from the panel (services → the service → add a key) and remove the old one. A service can hold several keys, so the replacement can go in before the old one comes out and the chat never sees a gap. Nothing else has to be edited and the bot does not need a restart.

The Telegram token is the exception: it is read from the environment at startup and cannot be set from the panel, because whoever could change it could point the bot at a different account. Revoke it with /revoke in BotFather, update .env, and restart.