A trojan is usually deliberately designed by its author to appear authentic and attractive. For example, it may appear to be a screensaver, a service pack, an application update and so on.
Trojans rely on tricking the user into believing that the program is authentic, so that they unwittingly install the program themselves. To do this, most trojans mimic or entirely copy the style and branding of popular legitimate programs or files. Some trojans (particularly on the Android platform) are actually copies of legitimate apps that have been repackaged or trojanized to include harmful components.
Trojans are often distributed using the same (or very similar) names and designs as popular programs, to increase the chances that users will mistake the trojan for the legitimate app and install it instead.
Once installed or opened, the trojan may perform its promised function, or display a decoy document to distract the user. In the background however, it also silently performs unauthorized actions (its payload), without the user's knowledge or consent.
Depending on its creator's intent, a trojan's payload can range from:
- Mildly annoying pranks, like changing desktop icon positions, to
- Serious, user-inhibiting actions, like disabling the keyboard or mouse, to
- Critically destructive actions, like erasing files or stealing data
It is often very difficult for users to realize that a trojan is performing harmful actions, as they are usually well camouflaged to keep the system from triggering any notification messages that might arouse the user's suspicions.
Types of trojans
Trojans are often classified into sub-types based on the actions they perform. These are just a few of the possible types:
- Trojan-downloader Contacts a remote server and downloads other harmful programs from it
- Trojan-dropper Contains one or more harmful components that it drops and installs
- Trojan-spy Monitors activity on the device, or steals content from it
- Password-stealerSteals passwords and other confidential details from the device
A specialized type of trojan is ransomware, which attempts to steal, delete or encrypt the user's own files; the user must subsequently 'ransom' these files back from the attacker, usually by monetary payment. For more information about this type of malware, see: