Zombie armies battering Windows

Malicious remote control software continues to be one of the biggest threats to Windows PCs, according to a new Microsoft security report.

More than 43,000 new variants of such insidious software were found in the first half of 2006, making them the most active category of malicious software, Microsoft said in a Security Intelligence Report published on Monday. In June Microsoft also flagged zombies as the most prevalent threat to Windows PCs.

It said in the report: "Attackers, with financial gain in mind, are clearly concentrating a significant amount of development focus on this category of malware."

Of four million Windows PCs found to be infected with some kind of malicious software in the first half of this year, about two million were running malicious remote control software, Microsoft said. The data is collected by Microsoft's free Windows Malicious Software Removal Tool, which runs when security updates are installed on Windows PCs.

While the number is high, it is actually a decrease from the second half of 2005, when Microsoft found that 68 per cent of infected PCs contained a backdoor Trojan. Meanwhile, hackers are trying harder to make their networks of hijacked computers go unnoticed by moving to new web-based techniques.

Rootkits, which make system changes to hide another piece of possibly malicious software, remain an uncommon threat. There has been a 50 per cent reduction in this kind of attack against computers running Windows during the past six months, Microsoft said.

Microsoft introduced the Windows Malicious Software Removal Tool in January last year. An updated version of the program ships monthly with Microsoft's security updates. The tool aims to identify and remove prevalent malicious software from PCs.

Joris Evers writes for CNET News.com