Modems are cheap and easy to install, so employee are often tempted to install them on their desktop workstation without seeking permission or notifying anyone that they have done so. This creates a huge hole in perimeter security, because when employees install modems they do so to facilitate remote access from their home and seldom configure any strong authentication controls. Consequently, a single unauthorized (“rogue”) modem connected to an employee’s desktop workstation creates a “back door” through which attackers can successfully compromise an otherwise well protected system. Therefore, either computer security or internal audit staff should periodically check for the existence of rogue modems. This is most efficiently and effectively accomplished by using war dialing software, which calls every telephone number assigned to the organization to identify those which are connected to modems. (Hackers do this also, to identify targets). Any rogue modems discovered by war dialing should be disconnected, with sanctions applied to the employees responsible for installing them.