Yes, it is possible that a second-hand modem may have been compromised or häçked by the previous owner. This could allow the attacker to access information that is transmitted through the modem, including personal information like usernames, passwords, and credit card details.
To protect yourself from this threat, it is important to change the default login credentials of the modem and to update the firmware to the latest version. You should also avoid using unsecured public Wi-Fi networks and use encryption tools like VPNs to ensure that your online activity is secure.
It is also a good idea to regularly monitor your accounts for any suspicious activity and to enable two-factor authentication wherever possible. This will add an extra layer of security to your online accounts, making them more difficult for häçkers to access.