  A new attack against the Kirchhoff-Law-Johnson-Noise (KLJN) key distribution
system is explored. The attack is based on utilizing a parasitic
dc-voltage-source in the loop. Relevant situations often exist in the
low-frequency limit in practical systems, especially when the communication is
over a distance, due to a ground loop and/or electromagnetic interference
(EMI). Surprisingly, the usual current/voltage comparison based defense method
that exposes active attacks or parasitic features (such as wire resistance
allowing information leak) does not function here. The attack is successfully
demonstrated. Proposed defense methods against it are shown.
