T 5.21 Trojan horses

A Trojan horse is a program with a hidden, undocumented function or effect. A user, therefore, has no influence on the execution of that function and for that reason there is a similarity to computer viruses. However, self-reproduction does not take place. Any kind of application software can be used as a carrier for Trojan horses. However, also script languages like batch files, ANSI control sequences, Postscript etc., interpreted by the operating system or by an application, can be misused for harmful functions.

The more privileges are granted to the originator program, the more effective the harmful effects of a Trojan horse are.

Example:

A modified log-in program can contain a Trojan horse which will ask for the user's name and password, send it to an offender and hand it over to the correct log-in program afterwards. Such Trojan horses have, for instance, occurred in the online service AOL.