Trojan horse spreads quickly through Microsoft's IM

Compromises 11,000 PCs in first 24 hours, says researcher
A new Trojan horse that started to spread early Sunday via Microsoft Corp.'s instant messaging client has already infected about 11,000 PCs, a security company said today.

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 late today, the botnet had been built out to 12,000 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 network computing (VNC) clients, the generic term for remote control programs used to access one computer's files and desktop from another.

Once the Trojan horse has installed itself on a PC through IM, it can sniff out a VNC client, then use it to infect a remotely controlled system, perhaps one inside a corporation's firewall. "You increase your reach to these PCs as well, as if you infected them," Lichtman said, momentarily taking the hacker's point of view. To his knowledge, the Trojan's use of a VNC 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.