Known as Henpeck, the worm used MSN's chat network to send messages containing a link to a malicious online file, called BR2002.exe. People who clicked the link triggered a download of the file and inadvertently ran the infectious program. The worm then sent instant messages to everyone on a victim's buddy list.
"Recipients of the message are not automatically infected with the worm," said antivirus company Trend Micro in its advisory. "This happens when the recipient clicks the URL, which downloads the worm and executes it in the system."
The malicious file has since been removed from the Web, however, effectively halting the spread of the program.
Still, victims whose computers have been infected with the worm are likely to have had a "backdoor" program installed. The program, known as BKDR_EVILBOT.A, allows an online vandal to use the infected computer as a platform from which to launch denial-of-service attacks. Such attacks attempt to block another computer's or a network's access to the Internet by flooding the connection to the Net with data.
Rival antivirus Symantec firm rated the Henpeck worm a 2 out of 5, where a 5 is the most severe outbreak.
