Programmable logic controllers are key components of industrial control systems that are used across the critical infrastructure. The infamous Stuxnet malware attacked programmable logic controllers that managed uranium hexafluoride centrifuges in Iran’s Natanz facility, causing the centrifuges to operate outside their designed limits while leading plant operators to believe all was well. This attack and others have rendered the task of securing programmable logic controllers an important problem. Most research in the area has focused on network-level intrusion detection and protection mechanisms. Few research efforts have specifically considered threats to the internal networks of industrial control systems, which include connections from the computer platforms that manage programmable logic controllers. This chapter analyzes the threats to the internal environment of an elevator control system that engages a Siemens programmable logic controller. Several approaches for mitigating the threats are presented.