Users of instant messaging applications from Yahoo and AOL this week are being warned of two new threats spreading via IM.

The first is a worm targeting AOL's Instant Messenger (AIM) software that could potentially allow an attacker to gain control of a user's system, according to security researchers. The other is a phishing scam propagated through Yahoo Messenger, which tries to lure users into revealing their Yahoo credentials.

The AOL worm sends the message "hehe i found this funny movie," encouraging users to click on a hyperlink that downloads malicious code to the user's desktop, according to an advisory released Monday by IMLogic. From there, the worm connects to a public Internet Relay Chat (IRC) server and continues to spread through AIM messages, researchers at MCI security company NetSec explained.

While IMLogic rated the AIM worm as a "medium threat," a NetSec advisory warned that given the sophistication of new worms such as this, "it is a reasonable assumption that near-full system control may be possible."

When contacted at around 2 a.m. GMT Wednesday, NetSec Security Research Coordinator Tom Parker estimated that a maximum of 15,000 PCs might have been affected.

However, by midday GMT the worm appeared to have slowed and there were few reports of it still in the wild, according to Mikko Hyppönen, director of anti-virus research at F-Secure. Most major anti-virus companies have already updated their products to deal with the threat, Hyppönen said.

What's interesting about the worm is that its topical and appears to be timed with the hype around the release of Star Wars Episode III: Revenge of the Sith , Hyppönen said.

A Sober worm variant circulating earlier this month was also topical, apparently coinciding with ceremonies marking the 60th anniversary of the end of World War II in Europe. It sent e-mail carrying right-wing nationalist messages in German.

"It's an old technique but it gets people to open them," Hyppönen said.

AIM users are advised not to click on hyperlinks sent from unknown users, and to make sure they have a firewall in place and get the latest security updates for when a fix is available.

The Yahoo Messenger scam is one of a growing number of so-called phishing attacks - a kind of online fraud in which users are directed to fake Web sites designed to look like authentic sites, and then asked to reveal sensitive information such as credit card numbers and passwords.

The IDG News Service is a Network World affiliate.