A new Trojan horse that started to spread early Sunday via Microsoft's instant messaging client has already infected about 11,000 PCs, a security company said Monday.

The as-yet-unnamed Trojan horse began hitting systems about 7 a.m. EST on Sunday, according to Roei Lichtman, the director of product management at Aladdin Knowledge Systems Ltd. "We still haven't found what it's meant to do, but at the moment, it's creating an army [of bots]," he said. "Eventually, of course, the operator will send commands to do something."

Users of Microsoft's Windows Live Messenger instant messaging program receive a message that includes spoofed Zip files, such as one named "pics" that is actually a double-extension executable in the format "filenamejpg.exe" or a file labeled "images" that in reality is a .pif executable.

"This is really growing rapidly," said Lichtman. Six hours after it first found the Trojan horse, Aladdin put the total number of assembled bots at about 500; three hours later, that had climbed to several thousand. By 12:30 p.m. EST Monday, the botnet had been built out to 11,000 machines.

As with other malware spread through instant messaging software, the messages bearing malicious code appear to come from people on the recipient's IM contact list.

But while its speed in spreading is impressive, Lichtman pointed to another characteristic of the Trojan horse: It can also propagate via virtual private network (VPN) clients, the programs typically used by businesspeople to connect with their employer's networks when they're outside the corporate firewall.

"That just increases the effectiveness of its spreading," Lichtman said. Once the Trojan horse has installed itself on an in-the-field PC -- most likely a laptop -- through IM, it can penetrate a corporation's perimeter via VPN to infect systems in the business. "It's after machines that aren't running Messenger," Lichtman explained, who added that to his knowledge, the VPN vector was a first.

Aladdin will continue to monitor the bot's spread by tapping into the Internet Relay Chat channel being used to command and control the compromised PCs, said Lichtman.

IM-based threats, while still relatively rare compared with those that spread via e-mail or from malicious Web sites, aren't unknown. Neither are vulnerabilities within IM software. In September, for example, Microsoft forced users of its aged MSN Messenger software to upgrade to Windows Live Messenger 8.1 to stymie a vulnerability in the older program.