Software is vulnerable to modifications that either cause it to fail or cause it to perform an unintended task. Indeed, because software is so susceptible to ‘off by one’ errors, it is quite easy to modify. Changing a bit or two can convert a working program into a failing one. Depending on which bit was changed, the program may crash when it begins, or it may execute for some time before it falters.

Other categories of software modification include:

A Trojan horse:  A Trojan horse is a program that overtly does one thing while covertly doing another.

A Virus:  A Virus is a specific type of Trojan horse that can be used to spread its ‘infection’ from one computer to another.

A Trapdoor:  A Trapdoor is a program that has a secret entry point.

Information leaks: Information leaks in a program: code that makes information accessible to unauthorized people or programs.