Botnet researcher Joe Stewart calls Clampi "the most professional thieving pieces of malware" he's ever seen. According to Stewart, the Clampi Trojan horse is stealing a "tremendous" amount of financial information.

According to Computerworld, hackers get Clampi on computers by tricking users into opening an e-mail attachment or by using a multi-exploit toolkit that tries attack code for Windows vulnerabilities. Clampi targets the user credentials of 4,500 Web sites. So if a user visits one of those sites, it captures usernames, passwords, PINs and other personal information.

Stewart says the scope of Clampi is one aspect that separates it from run-of-the-mill financial malware.

"They're targeting not just banking sites, but a wide variety of sites where people put in credentials that help them steal money somehow."