Backdoor (computing)

Backdoor (also trapdoor or backdoor ) denotes an (often built by the author ) piece of software that allows users to gain access to the computer or an otherwise protected function of a computer program, bypassing the normal access control.

One example is universal passwords for a BIOS or a special (usually secretly installed by a trojan ) software that allows a corresponding remote access to the computer.

Difference between backdoor and Trojan

→ Main article: Trojan horse ( computer program )

As a Trojan horse, or Trojan for short, a computer program or script is called that masquerades as a useful application, but in the background without the user's knowledge fulfills a different function. The simplest example of this is a harmful file as ich_zerstoere_Daten.exe who receives a file name that suggests a different function, such as lustiger_Bildschirmschoner.exe. It is irrelevant whether the " funny screensaver " actually displays the screen saver while it destroys the data, or whether he simply destroys only the data. The use of the misleading file name is quite sufficient to classify the program as a Trojan horse.

Trojans can also be used to install backdoor programs, but need not necessarily contain. Houses and installed a Trojan Backdoor a stand-alone program, the intruder accesses the installed backdoor program and not to the Trojans. The Trojan was used in this case only as a helper application for the secret installation. The Trojan may be deleted at any time, without this having an impact on the further function of the backdoor program.

However, no one holds the developer of a backdoor program them to make use of the technique of a Trojan horse. In a backdoor program that masquerades itself as a useful application (for example as desktop clock that secretly allows remote access to the computer ), it is a hybrid between a backdoor Trojan. If such a program is finished or even deleted, so the secret backdoor feature is no longer available.


A variant consists in a system of fixed, predetermined, install only the creator of the system known passwords, or other hidden functions that allow access without the usual authentication. A well-known example is that of Award Software over several years assigned hash code " LKWpeter " served with the BIOS universal password.

For software that allows remote access to the computer, for example, include programs such as Sub Seven and Back Orifice.

Also the router from Cisco Systems that handle much of the Internet traffic, are provided with backdoors for U.S. intelligence.

Was demonstrated audience effect the use of a back door in movies such as Wargames and Jurassic Park.

Protect against a backdoor through testability of the source code

→ Main article: closed source, open source and free software

For software products, a free access to its source code is an aspect of computer security. This applies, inter alia, to minimize the risk that a product may contain functionalities, from which the user is supposed to know nothing like the secret function of a backdoor.

Open source software can be verified by the public to the effect and furthermore examine with legally acceptable means for vulnerabilities that can be closed faster this way.


While Open Source Software can be even investigated for clandestine functions and vulnerabilities by anyone with the appropriate expertise, which does not mean that the mere availability of the source code is a guarantee that this has been sufficiently reviewed by the computer users. Over a long period existing vulnerabilities in open source software indicate this fact. In addition, a cleverly built-up back door with a knowledge of industry is sometimes difficult to detect. The time required for analysis is often considerable for complex programs.

Whether purchased from an external source executable program was actually created with the published source code, or here if not previously installed a backdoor or other change has been made, it is often difficult for the user to recognize. It applies also here that with appropriate expertise in theory at least, a check is possible. However, this turns out in practice often be difficult, since the resulting compiling binaries can be especially with larger code bases influenced by many factors and it is generally no reliable way is to find out under what conditions an existing executable file is created.

In 1984, the computer pioneer Ken Thompson during his Turing Award speech is an example of a back door in front that would be difficult to detect even with the availability of the source code. There was talk of a login program for Unix that is changed such that it in addition to the normal password, accepts a general password. This backdoor could, so add Thompson, a correspondingly manipulated C compiler when compiling the login program automatically, making the source code of the login program provides no evidence of manipulation. The practice could move to another instance that is responsible for translating the C compiler itself to an executable file, making the manipulation no longer would be apparent from the source code of the C compiler.