Internet-Security.com TCP port 3307 details

TCP/3307 is commonly used by MySQL/MariaDB servers as an alternate port for a second instance of the MySQL protocol.
Synology NAS: the “MariaDB 10” package (DSM) listens on 3307 by default, while “MariaDB 5” uses 3306.
QNAP NAS: the “MariaDB 10” app (QTS) defaults to port 3307 so it can coexist with the older MariaDB/MySQL service on 3306.
WampServer on Windows runs MariaDB on 3307 by default, with MySQL on 3306, allowing both to run simultaneously.
Oracle’s MySQL Installer for Windows and many admin guides configure a second MySQL instance to use 3307 when 3306 is already in use.
Because 3307 often hosts MySQL/MariaDB, attackers routinely scan and brute-force it as an alternate MySQL port when 3306 is blocked.

TCP 3307