Security Risk – Multiple networks on hub

Is the following a security concern? why? why not?

If you have a router connecting your LAN to a network. The router is connect to a standard 10Mbps shared hub and your firewall is connected to the same hub. The router and the firewall use valid Internet IP addresses.

Now the possible security risk:
Someone adds 2 workstations to the hub (they are now outside of the company LAN and firewall) and gives the workstations internal ips (such as 10.10.10.1 and .2) and then enables file sharingon the two pcs.

Can someone on the Internet access, attack, scan, etc. these rogue workstations?

Any comments would be greatly appreciated,

Kevin K