Frank O'Dwyer writes:

Here we're talking about deliberately inserted back doors. Those can get extremely nasty, and may be unpatchable. Examples include "data kidnap" (encrypting the target's information in situ and demanding a ransom for the decryption key), and "data cancer" (slow corruption of the target's information, ensuring that the backups are also corrupted). ... I haven't heard of any real examples of such attacks, but that's not especially comforting.

The "data kidnap" scenario was tried with the "PC CYBORG (AIDS) virus" (actually a Trojan) scare of 1989, where a disk with a database application was sent to a number of recipients. I think it scrambled the FAT. It gave a post office box in Panama for the ransom payments. -- Jim Gillogly Highday, 3 Foreyule S.R. 1998, 20:23 12.19.5.12.16, 8 Cib 9 Ceh, Fourth Lord of Night