Microsoft's Malicious Software Removal Tool - a program that removes malware from Windows machines - detected password-stealing software from more than 2 million PCs in the first week after it was updated.
One password stealer, called Taterf, alone was detected on 700,000 computers in the first day after the update. That's twice as many infections as were spotted during the entire month after Microsoft began detecting the notorious Storm Worm malware last September.
"These are ridiculous numbers of infections my friends, absolutely mind-boggling," said Matt McCormack, from Microsoft's Malware Response Centre.
Between 10 June and 17 June, Microsoft removed Taterf from about 1.3 million machines, he said.
Microsoft's September detections seriously hobbled the Storm Worm botnet, once considered a top Internet threat.
Password stealers such as Taterf are among the most common types of malicious software on the Internet. That's because there's big money to be made selling the virtual currencies used in online games for real-world cash.
Once a criminal learns a gamer's username and password, he can log into the game and sell the victim's virtual possessions for virtual gold coins. Those coins are then handed to another character in the game who sells the gold for real-world dollars at an online exchange such as IGE, said Greg Hoglund, CEO of HBGary and a co-author of the book "Exploiting Online Games."
"There's no way to audit that money transfer, so effectively they're doing money laundering," he said. "There's almost zero risk for the attackers."
The password-stealing programs are often installed via web-based attack code that exploits flaws in multimedia programs such as Adobe's Flash Player or Apple's QuickTime Player, Hoglund said.
The attacks are often technically sophisticated, exploiting previously undisclosed bugs in Windows software, said Roger Thompson, chief research officer with AVG Technologies. "The 'World of Warcraft' password stealers have provided most of the innovation over the last twelve months," he said via instant message.
Microsoft's McCormack provided some data on where most of the password stealer detections occurred. Not surprisingly, China was the top country, with 529,003 detections.
Security experts say Chinese games are frequently the target of these attacks. Rounding out the top five countries for detections were Taiwan with 279,428, Spain with 235,381, the US with 213,374 and Korea with 184,306.
About 330 million copies of the Malicious Software Removal Tool update were downloaded during this June period.
Gamers can make easy targets for criminals because some of them disable anti-virus software to boost gaming performance, while others download free "cracked" versions of games, which can contain malware, McCormack said.
"So how does one avoid being infected?" he asked. "Running an up-to-date anti-virus solution is a good start. Running an up-to-date, patched browser is another necessity," he said. "Enabling Automatic Updates helps a whole bunch, too."