Dubbed "JS.Menger.Worm" by Symantec and "Coolnow" by F-Secure, the worm sends instant messages, but does no damage to a user's system. F-Secure is trying to shut down the sites hosting the malicious code before it becomes widespread, the company said.
"The worm is a modified version of our example code," admitted Thor Larholm, one of the experts. "We never intended for anybody to copy the code, although we kind of expected it would happen."
"We published the example to put pressure on Microsoft to patch vulnerabilities that they are fully aware of," Larholm claimed.
Larholm and the antivirus software vendors said that Microsoft released a bundle of patches that fix the flaw used by the MSN Messenger worm on 11 February.
However, a PC with IE is all but secure, according to Larholm.
"The patch doesn't fix all the problems in IE. It fixes a lot, Microsoft deserves kudos for that, but there are still publicly known vulnerabilities, some two months old, that allow an attacker to read any local file and execute arbitrary commands on a user's system," Larholm said.
Larholm maintains a list of unplugged holes on his Web site ( jscript.dk/unpatched/).
Both Symantec and F-Secure have updated their antivirus products so they can detect the worm. No one at Microsoft was immediately available to comment.