Any older computer or control system that remains in use despite its age, often because it still does its job, but which typically has outdated hardware or software. In OT, legacy systems are very common, including Windows NT or XP machines running HMI software, or a PLC from the 1990s that’s still controlling a boiler. These systems often cannot be easily patched or may not support modern security, making them vulnerable. But replacing them can be expensive or risky to operations. So, they live on, sometimes isolated or wrapped in additional protective controls. A legacy ICS component might use old protocols, have serial interfaces, or run an OS long out of support, posing a security and maintenance challenge.
