What is a trojan horse?
A trojan horse is a malicious program, usually disguised as something useful or desirable. When activated, they can cause loss, damage or even theft of data. Unlike viruses and worms, Trojans do not reproduce by infecting other files nor do they self-replicate.
Trojan horses spread when people are lured into opening a program because they think it comes from a legitimate source. Trojan horses can also be included in software that you download for free. Never download software from a source that you don’t trust.
A Trojan Horse is full of as much trickery as the mythological Trojan Horse it was named after. Just as the mythological Trojan horse appeared to be a gift, but turned out to contain Greek soldiers who overtook the city of Troy, today's Trojan horses are computer programs that appear to be useful software, but instead they compromise your security and cause a lot of damage. A recent Trojan horse came in the form of an e-mail that included attachments that claimed to be Microsoft security updates, but turned out to be viruses that attempted to disable antivirus and firewall software.
When a Trojan is activated on your computer, the results can vary. Some Trojans are designed to be more annoying than malicious (like changing your desktop, adding silly active desktop icons) or they can cause serious damage by deleting files and destroying information on your system. Trojans are also known to create a backdoor on your computer that gives malicious users access to your system, possibly allowing confidential or personal information to be compromised.
Added into the mix, we also have what is called a blended threat. A blended threat is a sophisticated attack that bundles some of the worst aspects of viruses, worms, Trojan horses and malicious code into one threat.. Blended threats use server and Internet vulnerabilities to initiate, transmit and spread an attack. This combination of method and techniques means blended threats can spread quickly and cause widespread damage. Characteristics of blended threats include: causes harm, propagates by multiple methods, attacks from multiple points and exploits vulnerabilities.
Blended threats are considered to be the worst risk to security since the inception of viruses, as most blended threats require no human intervention to propagate.