h a c k into network shares of an internal network

Dear friends,

Please tell me whether it is possible for someone on the external network to h a c k into the network folder shares of an internal network in DHCP (192.168.x.x)
through a Comp-A on an external network in DHCP (123.123.x.x)
both networks are on different subnets

but Comp-A has access to both networks and can see all the shared drives of both the internal and external networks
-
using firewall is not an option because file sharing is required everywhere.
care is taken from virusesetc.
remote desktop etc is disabled on Comp-A

Please let me know.

Thanks and Regards
cingo