Trojan horse: act with foresight
The 'Trojan horse' or 'Trojan' for short has long since replaced the virus as a threat to IT systems. Today, 55% of malware is a Trojan horse and only ten percent are viruses.
As a rule, Trojans do not aim to destroy computers, they want to 'spy' on them or 'use' them to carry out other functions. The programs are called 'Trojans' because they disguise themselves as useful 'workhorses', but then install additional malicious programs unnoticed. These are often 'sniffers', which spy on the content of systems, or 'keyloggers', which register keyboard entries (e.g. passwords).
Trojans are usually structured as a combined program. The useful part of the program also introduces the malware via a 'linker' or 'binder'. Any program can be abused in this way. Once introduced, the host program is no longer required. The introduced malware usually starts automatically as soon as the computer starts up. Recently, plug-ins for the browser or apps that run on mobile devices have also become particularly susceptible to the 'smuggling' of malware.
In principle, Trojans can use any route that loads data onto the computer: Data carriers, network connections, e-mails, etc. The executability of a file is often deliberately disguised so that the user believes, for example, that he is opening a harmless image file with the extension 'Dingsbums.jpg' because the 'executable' does not appear in the display on the monitor. In fact, however, the file bore the identifier 'Dingsbums.jpg.exe'.
Other Trojans exploit the vulnerabilities of existing programs, the 'exploits'. Nowadays, everyone is familiar with the requests to quickly update sensitive programs in order to close the security gap.
The only protection against Trojans is to refrain from using programs from unknown or unsafe sources, especially from the 'grey area' of the Internet - this also applies to seemingly harmless photo, video or sound files. The usual virus scanners, on the other hand, are only of limited help against Trojans. They can often successfully eliminate unopened Trojans, but once installed their capabilities are usually limited. In such cases, the only thing that helps is transferring the data medium with the last clean backup.