TROJAN-HORSE

Computer worms have been replaced by Trojan horse malware programs as the weapon of choice for hackers. Trojans masquerade as legitimate programs, but they contain malicious instructions. They've been around forever, even longer than computer viruses, but have taken hold of current computers more than any other type of malware.

A Trojan must be executed by its victim to do its work. Trojans usually arrive via email or are pushed on users when they visit infected websites. The most popular Trojan type is the fake antivirus program, which pops up and claims you're infected, then instructs you to run a program to clean your PC. Users swallow the bait and the Trojan takes root.

Trojans are hard to defend against for two reasons: They're easy to write (cyber criminals routinely produce and hawk Trojan-building kits) and spread by tricking end-users — which a patch, firewall, and other traditional defense cannot stop. Malware writers pump out Trojans by the millions each month. Anti-malware vendors try their best to fight Trojans, but there are too many signatures to keep up with. 

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