A logic bomb[7] is a program, or portion of a program, which lies dormant until a specific piece of program logic is activated. In this way, a logic bomb is very analagous to a real-world land mine.The most common activator for a logic bomb is a date. The logic bomb checks the system date and does nothing until a pre-programmed date and time is reached. At that point, the logic bomb activates and executes it’s code.
A logic bomb could also be programmed to wait for a certain message from the programmer. The logic bomb could, for example, check a web site once a week for a certain message. When the logic bomb sees that message, or when the logic bomb stops seeing that message, it activates and executes it’s code.

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[7]Logic Bomb