What is a back door?
A back door is a means of access to a computer program that bypasses security mechanisms. A programmer may sometimes install a back door so that the program can be accessed for troubleshooting or other purposes. However, attackers often use back doors that they detect or install themselves, as part of an exploit. In some cases, a worm is designed to take advantage of a back door created by an earlier attack. For example, Nimda gained entrance through a back door left by Code Red.
It is important to take note that backdoors are not always simple to get into. Many attackers create backdoors that are much more difficult to access than the normal methods of accessing the system. This is to protect the backdoor from other attackers as well as preventing detection of the backdoor.