A logic bomb is a portion of a computer program intended to execute a malicious function when certain conditions are met. For example, an employee may write a program including the instructions that important files be deleted if he or she is terminated. Logic bombs may be included in an otherwise innocuous program or in an inherently malicious program like a virus.
Unlike viruses, logic bombs do not replicate themselves or spread to other systems on their own. Logic bombs can therefore be made to target a specific victim without posing any threat to others. The fact that they do not replicate also makes logic bombs easier to write than other malicious software.
Some logic bombs are programmed to execute a certain function on a certain date, rather than after a specific event. This type of logic bomb is often called a time bomb. Malicious programs like viruses, worms, and Trojan horses are sometimes programmed to attack the host computer on a certain date, allowing them to spread undetected throughout the computer system before they become active.
Sometimes, logic bombs are programmed to activate when something does not happen. For example, an employee may design a logic bomb to delete files when he or she has not logged into the system for a month. Logic bombs of this nature are among the most dangerous, since the conditions for their activation can be met in unintentional ways. For example, if the programmer was not fired, but died unexpectedly, there would be no way to prevent the bomb from being activated.
A program that executes a certain function when predetermined conditions are met is usually only considered a logic bomb if the function is both unwanted and unknown to the user. Trial software that ceases to work after a predetermined period of time is not usually referred to as a logic bomb, though it works the same way. Logic bombs often sabotage the host system in some way, deleting important files or functionality, or allowing unauthorized access to the user's system.
One famous incident, allegedly involving a logic bomb, was the Siberian Pipeline incident of 1982. Supposedly, the United States Central Intelligence Agency (CIA) may have used such a technique to sabotage a Soviet natural gas pipeline. According to the account of National Security Council staffer Thomas C. Reed, a logic bomb was written into software that the Soviets stole from a Canadian firm. The Soviets used the stolen software to control the pipeline, and the logic bomb allegedly resulted in a huge explosion. There were no casualties. Reed's account has been disputed, however, and the explosion may have been due to a different cause, such as faulty construction.