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Subsystems are unique among other APF‑authorized programs because of the special treatment that they receive. Subsystems are notified when certain events occur, such as when a job starts or ends or when an operator command is entered. Some access control software packages accord them special treatment, and they can be active without showing up in standard console displays. This special treatment poses a number of security issues.
For example, consider the treatment of operator commands. When the operator enters a command, it is passed to all active subsystems, one at a time. Each subsystem can ignore the command or act on it, and can keep it for itself or pass it on to the next subsystem. If no subsystem claims a command, z/OS processes it. The IBM virtual telecommunications access method (VTAM) uses this method to accept its commands. A clandestine subsystem might also use it to intercept commands and produce fake replies or just to look at commands as they go by. For example, a clandestine subsystem might watch for replies to requests for passwords and record them. The Subsystem Display (3.1) shows you the status of each subsystem, whether it is Active or Dormant, and the number of functions it supports. Subsystems that are Dormant do not intercept or monitor operator commands or other system events.
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