Phishers have been using compromised MySpace.com accounts to attack unsuspecting Web surfers, security experts said Thursday.

The attack is thought to have infected several thousand PCs according to reports from ISPs (Internet service providers), said Johannes Ullrich, chief research officer for the SANS Institute. Ullrich has documented the issue on the SANS Internet Storm Center blog.

Lawrence Baldwin, chief forensics officer with security vendor MyNetWatchman, discovered the threat Tuesday and The Washington Post reported on it late Wednesday.

Criminals have managed to install fake navigation bars on the top of MySpace.com user profile pages that, when clicked, lead to malicious computers that attempt to infect the victim's computer. The attack uses several known Internet Explorer flaws that have been fixed, so users who have installed the latest Microsoft patches are not at risk, security experts said.

The code was installed on "maybe a few dozen," MySpace.com pages, most of which have been removed by administrators at the social-networking site, Ullrich said. MySpace.com representatives did not respond to requests for comment on Thursday.

Two components comprise the attack. It attempts to install malicious botnet software on victims' computers, and it also uses these infected computers to try to steal MySpace credentials in a phishing attack.

Computers that are compromised by the attack become infected with malicious botnet software known as "flux bot," which makes them unwitting participants in the phishing scam. After the malicious Web site attempts to install the flux bot code, it then presents victims with a fake MySpace.com log-in page, which tries to extract their MySpace.com user name and password.

Baldwin allowed one of his test computers to be infected with flux bot and found that attackers were remarkably successful at stealing passwords. "I operated as a flux node for about 12 hours and did a full audit of all the traffic coming into my machine. I was probably getting close to 60 MySpace users an hour surfing to my flux node. And at least a quarter of those actually gave up their credentials."

Baldwin estimates that the attackers were using another 200 compromised flux bot machines in their attack.

Because MySpace.com allows users to install their own HTML (Hypertext Markup Language) code and is used by such a large number of technically unsophisticated users, it has become an attractive target for these types of attacks.

Last December, hackers created a worm that quickly spread across MySpace.com, stealing user names and passwords. That worm exploited a flaw in Apple's QuickTime media player.