A trojan, or trojan horse, is a seemingly legitimate program which secretly performs other, usually malicious, functions. It is usually user-initiated and does not replicate.
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Named after the Trojan Horse of Greek legend, a trojan is a malicious program that performs, or appears to perform, one function, but in reality executes another action without the user's knowledge or consent.
Quite often, the trojan will either have or pretend to have a functionality that offers a useful service to the user - like a screensaver, a utility program, a service pack or application update and so on - in order to encourage the user to install the trojan.
Once the trojan is installed however, it performs its actual, unauthorized function, which ranges from mildly annoying pranks (like changing desktop icon positions), to serious, user-inhibiting functions (like disabling the keyboard or mouse) to critically destructive actions (like erasing files or stealing data).
The majority of trojans today carry a payload designed to cause harm.
Trojans can be further subdivided based on how they function once they are installed onto a system, or how they deliver their payload:
A specialized type of trojan is Trojan-Ransom, which may be used to specifically identify malicious programs which attempt to steal, delete or encrypt the user's own data files; the user must subsequently 'ransom' these files back from the attacker, usually by monetary payment.
Generic Trojan Detections
Generic Detections are a type of detection used by antivirus programs to identify files with trojan-like behavior. Unlike single-file detections, Generics do not identify individual malware; instead, they evaluate suspect files based on their similarity to known malicious programs. If a file is similar to a previously identified threat, it is flagged as a potential security risk.
A Generic Detection may use varying naming conventions. Some examples are:
For more information about Generic Detections, please see the Generic Detection description