Logic Bomb Virus.
The first use of a time bomb in software may have been with the scribe markup language and word processing system, developed by Brian Reid. Reid sold Scribe to a software company called Unilogic, and agreed to insert a set of time-dependent functions (called "time bombs") that would deactivate freely copied versions of the program after a 90-day expiration date. To avoid deactivation, users paid the software company, which then issued a code that defused the internal time bomb feature. Richard Stallman saw this as a betrayal of the programmer ethos. Instead of honoring the notion of share-and-share alike, Reid had inserted a way for companies to compel programmers to pay for information access.History
A logic bomb is a piece of code intentionally inserted into a software system that will set off a malicious function when specified conditions are met. For example, a programmer may hide a piece of code that starts deleting files (such as a salary database trigger), should they ever be terminated from the company. Software that is inherently malicious, such as viruses and worms, often contain logic bombs that execute a certain payload at a pre-defined time or when some other condition is met. This technique can be used by a virus or worm to gain momentum and spread before being noticed. Some viruses attack their host systems on specific dates, such as Friday the 13th or April Fool's Day. Trojans that activate on certain dates are often called "time bombs". To be considered a logic bomb, the payload should be unwanted and unknown to the user of the software. As an example, trial programs with code that disables certain functionality after a set time are not normally regarded as logic bombs.Definition
A DESTRUCTIVE time-bomb computer virus is set to detonate today, users are being warned. The bug, known as BlackWorm, Kama Sutra, Mywife or CME-24, has spread through e-mail attachments. If an attachment is opened the computer becomes infected, and on the third day of every month it will seek and delete certain file types on Windows computers, including Adobe PDFs and Microsoft Word, Excel and PowerPoint documents. It also tries to disable anti-virus software that is out of date. BlackWorm's spread is said to be relatively low compared to other viruses.Example:
A logic bomb, also called slag code because all that's left after it detonates is computer slag, is not the same thing as a virus, although it often behaves in a similar manner. It is a piece of computer code that executes a malicious task, such as clearing a hard drive or deleting specific files, when it is triggered by a specific event. It's secretly inserted into the code of a computer's existing software, where it lies dormant until that event occurs. This event might be a positive trigger, such as a specific date and time or the removal of an employee's name from the salary database; or it might be a negative trigger, such as a particular employee failing to input a command by a certain time -- meaning he or she is probably not at the company anymore. Negative triggers are considered to be more dangerous than positive ones, since the risk of accidentally triggering the bomb -- say, if the employee is suddenly hospitalized with appendicitis -- increases dramatically. And when the bomb goes off, the damage is done -- files are deleted, secret information is sent to the wrong people, the network is crippled for days ...How does it work