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A Trojan is introducing malware into thousands of computer systems worldwide, and the number is growing by the hour.

The malware is being introduced by MSN Messenger files posing as pictures, mostly seeming to come from known acquaintances.

The files are a new type of Trojan that has snared several thousand PCs for a bot network within hours of its launch earlier on Nov. 18 and is being used to discover virtual PCs as a means of increasing its growth vector.

The eSafe CSRT (Content Security Response Team) at Aladdin—a security company—detected the new threat propagating around noon EST on Nov. 18. At 18:00 UTC (Coordinated Universal Time), eSafe had detected 1 operator and more than 500 on-command bots in the network. Less than three hours later, or by 2:30 EST, when eWEEK spoke with Roei Lichtman, eSafe director of product management, the number had soared to several thousand PCs and was growing by several hundred systems per hour.

eSafe is monitoring the IRC channel used to control the botnet. The only inhabitants of the network besides the operator are in fact infected PCs.

The Trojan is an IRC bot that's spreading through MSN Messenger by sending itself in a .zip file with two names. One of the names includes the word "pics" as a double extension executable—a name generally used by scanners and digital cameras: for example, DSC00432.jpg.exe. The Trojan is also contained in a .zip file with the name "images" as a .pif executable—for example, IMG34814.pif.

The files are infiltrating new systems by using either known contacts from which the Trojan has harvested instant messaging names, as well as from the systems of unknown users.

The infection vector—an IM program—isn't new. But the Trojan is the first that eSafe has tracked that has tried to scan for VNC (Virtual Network Computing) instances, likely in order to multiply the botnet's number of connections.