Named after the Trojan Horse of ancient Greek history, it is a particular kind of network software application developed to stay hidden on the computer where it has been installed. As with worms, Trojans generally serve malicious purposes and are in the “malware” classification. Trojans sometimes access personal information stored on home or business computers and then send it to a remote party via the Internet. Alternatively, Trojans may serve merely as a back door application. Trojans can also launch DoS attacks.

A combination of firewalls and anti-virus software should be used to protect networks against Trojans.

New Trojans are released on a frequent basis. For example, on March 3, 2005, security experts at McAfee and SophosLabs issued alerts of a new Trojan virus called Troj/BagleD1-L. This Trojan tries to prevent various security applications (such as anti-virus and firewall software) from working by renaming files belonging to security applications so that they can no longer load. It then attempts to block access to a range of security-related Websites by altering the Windows HOSTS file. The virus is said to arrive on email messages having a ZIP attachment. After it is opened, the ZIP attachment includes a program file named “doc—01.exe” or “prs—03.exe” or some other name. If the program inside the ZIP attachment is opened, the Troj/BagleD1-L attempts to connect to one of many Websites to download more code.

See Also: Anti-Virus Software; Denial of Service (DoS); Firewall; Internet; Network; Trojan.