Most computer and communication devices contain backdoors. Some are for maintenece, others for debugging, and still others are for the DOD. The DOD requires backdoors so that in case of war they can gain easy access. 

CPM does not need a backdoor. CPM is a single user system that is NOT securable. The feature that allows a turnkey CPM, can be easily defeated by typing BREAKs (or CNTRL-C's) as the system boots. This aborts the turnkey program or chain file (try it if you have a HDS-300, which is a turnkey CPM68k system).  The file level passwords in CPM-plus (AKA CPM 3) are also easily defeated. Simply placing a disk in a system without password support gives complete and unrestricted access to any file that has a password. Also the passwords are reverse decryptable. The LS program, that comes with the Z80 Emulator for windows, will list the existing passwords (in plain text, decrypted)  if the -P option is used, when run under either CPM 2 or 3. 

Versados has a backdoor. The source for the backdoor can be found in the source file SECURE.SA. Prior to 4.3, the backdoor was distributed, activated and functional. 4.3 and up, the back door was shipped disabled, and had to be enabled by the end user. This was done because customers found the back door and complained. Of course we can only imagine what the DOD did. The backdoor works during the password verification. If enabled, any password or security word request is satisfied by typing a predetermined, 8 character, skeletal pass/secuity word. -:]_CNxx where xx is the major and mior device code for the terminal being used (may also be something like CNSL, for VME10 console prior to 4.4).  

I do not know what the backdoor is, or looks like in UNIX. But other network enabled OSs have network backdoors that work like this. Ports, or a port, is defined as the backdoor port. If normal, correctly formed packets are recieved, then the port responds as though there is NO body listening. This allows network scanner programs to scan a port without the port indicating that it is listening. If, however, specially formed (but invalid for normal packets) packets are recieved, the port established a connection, and special protocol packets are used to communicate commands, and get data from the host. These packets get routed, but are considered not to be any defined protocol. This allows network snooping software to ignore them if configured to look only for valid know protocols. (note that most network snoopers are not programed to look at everything and many can not be programmed thusly. This is a requirement for protecting backdoors.

Of course, both Versados and unix keep passwords in a file, and a file is easily accessed on the disk without using an OS. The firmware debugger can be used to manually search a versados file system or a Unix filesystem for the contents. Versados does NOT encrypt passwords in that file. It is protected during the execution of versados by making it accessible to the Owner Only. 

Unix protects the password (kept in a globally readable file) by encrypting them. While the passwords are not decryptable, they can be determined using the infinite monkeys at a keyboard technique. Simply encrypt all possible permutations of password and check to see if the encyrpted values match.

The Tehran incident (1974) (look it up) is an example of a backdoor's usage. A B2 style stealth spy plane was detected in Iranian airspace. US built interceptors were dispatched. As the interceptors got close to the large triangular UFO, the flight contol computers onboard the interceptors were shut down. This was facilitated by using the radars. Each radar sends out a coded packet that when reflected can be verified as originating from the sending radar. These coded packets, which can be recieved by other radars , can also contain instructions to affect other onboard systems such as the flight control computers. (Yes, this is what Captian Kirk did to Khan's commandeered starship, in the Wrath of Khan; and, is probably where Star Trek got the idea.) Hey, in 1974, the US had some powerful computers for military use, but not allowed for regular people until the secret patents were divulged by others who spent tons of money in development, came up with the same ideas.

Ask yourself why, when AM radios stations were converted to PM modulation, why did no one build PM recievers. PM is properly recieved by AM recievers, but without any of the benefits of PM. Using a PM reciever, PM stations cam be properly recieved clearly, even if multiple PM station are on the same frequency and overlaping. Takes a powerful computer to separate PM stations on the same ferequency.