A Modbus connection feature on openPDC accepts a caller-specified destination address and port with no restriction on which internal hosts may be targeted. An authenticated user can attempt connections to arbitrary internal network destinations, revealing which destinations are reachable. With repeated attempts, an attacker may be able to map the internal network.
Grid Protection Alliance recommends restricting network access to modbus using a firewall, and disallowing connections to loopback and private (RFC 1918) address ranges unless explicitly required.