Conficker

The Conficker worm is currently the largest security threat to enterprises, continuing to spread due to weak or stolen passwords and vulnerabilities that require security patches, according to a Microsoft report.

According to the Microsoft "Security Intelligence Report volume 12" (SIRv12), which analyses online threat data, the Conficker worm was detected 220 million times worldwide in the past two and a half years, making it the biggest threat to enterprises. In the fourth quarter alone, the worm was detected on 1.7 million systems worldwide.

Two independent sources have now confirmed that at least 600,000 Macs worldwide have been infected with the malware downloader called Flashback. That number is not just an estimate. It’s a count of unique hardware IDs reporting in to a command-and-control server.