Could be. I just spent some time on the modbus.org site. I haven't looked in a while. There is pretty much no mention of security though they claim that Modbus over TCP is an internet protocol.
/deploying CAN bus without security
That if you have a box that can talk to network A and B, suddenly anything on A can talk to anything on B.
A CAN bus, or older modbus installs, would be airgapped by its very nature.