How would a hacker be able to spoof their IP when logging into a private FTP server?

Solution 1:

Yes, They can spoof their IP.

The obvious one - as mentioned in the comments is to use a VPN, however there are more other ways, which boil down to the same thing - use an intermediary server. These include -

  • Virtual Machines set up with SSH.
  • Proxy servers which support FTP.
  • Hacked websites with scripts to run FTP from those boxes.
  • Use a jump box / Bastion / SSH gateway.
  • Hacked systems the ftp user has compromised.