Trojans are malicious programmes pretending to be legitimate software, but they actually carry out hidden, harmful functions.
It pretends to do one thing, but actually does something different, without your knowledge. Popular examples are video codecs that some sites require to view online videos.
When a Trojan codec is installed, it may also install spyware or other malicious software. Another example is a malicious link that says “Cool Game.” When you download and install the game program, it turns out not to be a game, but a harmful Trojan that compromises your computer or erases the data on your hard drive.
Trojans are often distributed with pirated software applications and keygens that create illegal license codes for downloadable software. (See Backdoor Trojan)
A backdoor Trojan allows someone to take control of a user’s computer without their permission.
It may pose as legitimate software to fool users into running it. Alternatively—as is increasingly common—users may unknowingly allow Trojans onto their computer by following a link in spam email or visiting a malicious webpage.
Once the Trojan runs, it adds itself to the computer’s startup routine. It can then monitor the computer until the user is connected to the Internet. When the computer goes online, the person who sent the Trojan can perform many actions—for example, run programs on the infected computer, access personal files, modify and upload files, track the user’s keystrokes, or send out spam email.
Well-known backdoor Trojans include Netbus, OptixPro, Subseven, BackOrifice and, more recently, Zbot or ZeuS.
To avoid backdoor Trojans, you should keep your computers up to date with the latest patches (to close down vulnerabilities in the operating system), and run anti-spam and antivirus software.
Take note that information below is an extract from the Sophos Threatsaurus, compiled by Sophos, a security software and hardware company.
Tags: trojan