Conficker, a worm that targets the Windows operating system, is the biggest ongoing threat to enterprises, according to a study conducted by Microsoft.
The Microsoft Security Intelligence Report volume 12 found that the worm was detected approximately 220 million times worldwide over the past two and a half years. In the fourth quarter of 2011 alone, it was detected on 1.7 million systems.
The report analysed data from 600 million systems from 100 countries and regions between July 2011 and December 2011.
Microsoft warned that the worm has continued to spread because of weak or stolen passwords and vulnerabilities for which a security update exists.
"Conficker is one of the biggest security problems we face, yet it is well within our power to defend against," said Tim Rains, director of Microsoft Trustworthy Computing. "It is critically important that organisations focus on the security fundamentals to help protect against the most common threats."
To tackle the issue, Rains said that enterprises should use strong passwords, educate employees on their importance, regularly update their systems, use antivirus software from a trusted source, and consider the cloud as a business resource.