A login system backdoor can mask into the form of the heavily-coded user and password which allows the access to the system. The plot of the famous movie - WarGames (1983), was based on this scheme. The designer of a computer system had installed the never documented  password, deriving from his son's name, to get the unauthorized accessed to the system and its hidden features.

There was an attempt done to implement a backdoor in the Linux kernel (November 2003), which showed that the code change applied by a backdoor may be really subtle. It may need to change only two-three lines, often masking under a typical typographic error, to allow its designer get access to the most deep processes running on the backdoor infected machine (root processes etc).

A backdoor may appear in prevalence in the systems, designed under the proprietary method (which means their source code is not supplied along with the basic software and thus can not be inspected). Programmers working on such software often succeed at secretly implementing additional code, which is sometimes called after as "Easter Eggs". These pieces of code are really harsh to detect, though they can be activated by a certain combination of keystrokes or different activity. These varieties of code may be found even in Microsoft Windows, certainly being inserted by the Russian-speaking programmers, as the messages often appear in Russian language.

There is also a possibility to create a backdoor without any changes to the source code of an application or sometimes even modify the code after it has been already compiled. This is done through changing the compiler itself. When such changed compiler finds the code, it compiles at as usual, however inserting a backdoor into the final code (very often it is a password-stealing application). This way the creator of the backdoor gains access to the system infected.

Lots of computer worms, particularly Sobig and Mydoom, try to install a backdoor on the computer they have infected. They particularly like the computers with a broadband connection, functioning on the insecure versions of Microsoft Windows and also running the full-of-bugs Microsoft Outlook. Such backdoors allow spammer to send bulk e-mails from the computers infected.

Backdoor irc clients problem is now disturbing many of those who use irc clients fro private and business purposes. It is a proven fact that many of the irc clients, especially those distibuted freely, may contain a backdoor so the developers of the client are able to gain access to your computer and use it for their own, often malicious, purposes.

Backdoor removal process is not an easy one. Some backdoors are inserted in a very smart way so that the most up-to-date anti virus and anti malware applications are unable to track them and therefore there is no way to combat it for the ordinary user. However, the experienced system administrators are in known of how to remove these applications so if you experience some problems with your system and are suspicious of them being cause by a backdoor activity (even if your anti virus software finds nothing), maybe the best decision is to contact some system administrator for him to take a look on your system.

Remember that backdoors may be really dangerous, if of course you don't enjoy the idea of someone being able to access the most hidden corners of your system.