For the first six months of the year, Microsoft found that more than 5 million computers were infected with Conficker, according to its latest Security Intelligence Report.
[ Other security experts estimate the Conficker worm now infects 7 million computers. | Microsoft's security chief says the rising number of bug fixes is a sign of security success. | Learn how to secure your systems with Roger Grimes' Security Adviser blog and Security Central newsletter, both from InfoWorld. ]
Conficker spreads either by exploiting a vulnerability in the Microsoft Windows Server service, through infected removable media or brute-forcing weak passwords on other PCs.
Conficker alarmed Microsoft so much when it appeared that Microsoft issued an emergency patch in October 2008 for the software vulnerability that allowed it to spread rapidly.
The worm is still circulating, mainly in enterprises, said Vinny Gullotto, general manager of the Microsoft Malware Protection Center. Due to its password-cracking ability, if Conficker gets on one PC in a company, it can often then rapidly spread.
Microsoft collects data on infections from its free security products such as Windows Defender, the Malicious Software Removal Tool (MSRT), Security Essentials as well as ones the company sells.
Another worm -- called Taterf -- took the number two spot for the most infections at 4.9 million. Taterf steals authentication and account information for massively multiplayer online games such as World of Warcraft and Lineage, among others, and spreads through infected drives such as a USB stick or an infected network drive.
Microsoft did see a decline of machines infected with Zlob, a notorious Trojan horse that spread by tricking people into believing it was actually a media codec, which is software used to encode and decode audio or video.
Microsoft's free tools such as MSRT will remove Zlob. For the first half of the year, Microsoft saw only 2.3 million infections, dropping drastically from the 21.1 million infections the company counted for the same period a year prior.
Gullotto said that Microsoft received an e-mail from the supposed creators of Zlob saying that they were now "closing soon." The e-mail, in broken English allegedly from "Russia," complimented Microsoft on responding quickly to the threats.
But it's just a small victory, as there are plenty of other security problems. Fake antivirus programs are among those.
The programs, which look like legitimate security software but do not work, badger people with pop-up menus saying their computer is infected. The annoying messages only subside after buying the software for as much as $60.
Microsoft continues to add detection for new families of rogue antivirus software in products such as Windows Defender and the MSRT, Gullotto said. Fewer PCs were found to have been infected in the latest reporting period: 13.7 million versus 16.8 million computers for the second half of 2008.
Different regions faced different threats. Trojan horse programs -- which can download other malicious software -- were the biggest threat in the U.S., U.K., France and Italy. Malicious software aimed at online banking was problematic in Brazil, while worms were a dominant threat in Spain and South Korea, Microsoft said.
Gullotto said Microsoft also saw a continuing trend for hackers to attack third-party software. For computers running Windows Vista, 84.5 percent of the browser-based exploits targeted third-party software rather than Microsoft's.
Vista implemented an array of new security features designed to reduced its vulnerability to attack. For the weaker Windows XP OS, 56.4 percent of the browser-based exploits targeted Microsoft software rather than that of third parties. Windows 7, which was just released commercially last month, is not included in the figures.