Forty-five percent of the executable files downloaded through Kazaa, the most popular file-sharing program, contain malicious code like viruses and Trojan horses, according to a new study.

Out of 4,778 files downloaded in one month, Bruce Hughes, director of malicious-code research at security firm TruSecure, found that nearly half of them contained various types of nefarious code.

Some code was designed to infect every file in a computer user's Kazaa download directory with a virus. Other code would steal users' AOL Instant Messenger password or install a program on their computer to allow the attacker to surreptitiously send spam through it or otherwise take over the machine remotely to steal personal data and files on the computer.

Hughes said the code he found in shared files got there in one of three ways: The person hosting the shared file embedded the malicious code in a file on purpose; the code was a peer-to-peer worm designed to scour the network and drop itself into download directories; or, in the case of some viruses, once the user downloaded an infected file, the malicious code automatically infected other files in the user's file-share directory so that the user inadvertently infected the computers of other users who downloaded those files.

By Kim Zetter at Wired.

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