Just one day after hackers showed how to exploit a number of flaws in the ActiveX software used by Internet Explorer, Symantec has spotted online criminals using one of the attacks.

The attack, which was first observed in the last few hours, is not widespread at present. Symantec Security Response Director Oliver Friedrichs said Tuesday that the company had identified just three Web sites that were hosting the attack code, all of which seem to be linked to the same criminals. But he believes that more attacks are inevitable as the bad guys work the code into their malicious toolkits of software. "Given the fact that the proof of concept is available and works, it's not exactly rocket science for someone to plug this into [a toolkit]," he said. "That's likely to happen in short order."

Security researchers Elazar Broad and Krystian Kloskowski have disclosed a slew of ActiveX bugs affecting MySpace and Facebook over the past few days, but the attack reported by Symantec takes advantage of a flaw in an ActiveX control used by Yahoo's Music Jukebox.

After the attackers are able to install software on the victim's machine, they then begin installing a number of malicious files on the victim's computer, Symantec said.

Broad and Kloskowski also discovered a second ActiveX bug in Yahoo Jukebox, but that flaw is not yet being exploited by attackers, according to Symantec. Another major source of problems is the Aurigma ImageUploader ActiveX control, which is used by Web sites such as Facebook and MySpace to upload pictures into Internet Explorer.

Because of all the ActiveX bugs, Security experts are warning users to be cautious while Web browsing. On Tuesday, US-CERT (United States Computer Emergency Readiness Team) published a note encouraging users to disable ActiveX controls, which can be done by setting Internet Explorer's security level to "high" for the Internet zone.

That may not be a realistic option for many consumers, who will find their Internet Explorer browsing experience hobbled without ActiveX, Friedrichs said. But tech-savvy users and corporate IT shops can take steps to disable the untrusted ActiveX controls. In fact, the SANS Internet Storm Center has published free software that disables the controls.

Another option for enterprise users is to create a whitelist of approved ActiveX controls, Friedrichs said.

Security problems with browser add-ons are very common. In the first half of 2007, Symantec identified 237 browser plugin security flaws. About 89 percent of centered on ActiveX.

Yahoo did not immediately respond to an e-mail message requesting comment for this story.