Microsoft's Malicious Software Removal Tool (MSRT) has been upgraded so it can remove a worm that tries to download malicious software.

Conficker targets a flaw in Windows Server Service and Microsoft thought the flaw was so severe that it issued an emergency patch, outside of its usual cycle, on October 23 for Windows 2000, XP, Vista, Server 2003 and Server 2008.

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Microsoft said in a blog, it has identified a new variation of the worm, called Win32/Conficker.B, that has been infecting servers. Systems become infected when a hacker constructs a malicious Remote Procedure Call (RPC) to an unpatched server, which then allows arbitrary code to run on a machine

Conficker.B uses other methods to spread, including trying to copy itself to other shared network machines by guessing passwords, said Cristian Craioveanu and Ziv Mador, in the blog. It can also spread via removable media.

Conficker uses several tricks to avoid detection. It uses a technique called polymorphism, a mechanism that can use compression and encryption to make the code appear different to antivirus software and more difficult to detect. It also makes its files hard to detect and changes key access rights, Microsoft said.

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MSRT is a simple security tool that scans a PC and can remove some malicious software. It is far short of a full antivirus suite, but Microsoft has invested in supporting the tool to help remove some of the most flagrant and nagging malicious software affecting Windows PCs and servers.

The company is recommending that administrators run a MSRT scan. Infected computers, however, may not be able to access Windows Update, the built-in update tool for Windows. Microsoft has given instructions for how to download the MSRT with a clean machine and then distribute MSRT.

NEXT PAGE: Conflicker is spreading rapidly

Microsoft's Malicious Software Removal Tool (MSRT) has been upgraded so it can remove a worm that tries to download malicious software.

Many companies have seen Conficker rapidly spread on their networks over the last few weeks, said Mikko Hypponen, chief research officer for the Finnish security company F-Secure.

F-Secure has analysed the malware and found it contains an algorithm that generates domain names for command-and-control servers. The malware authors can then turn one of those domain names into a live website where the infected PCs report to for updated malware or instructions, he said.

The technique has been used by other botnets, such as Mebroot. It's very difficult to shut down the command-and-control websites, since it's hard to know which ones of hundreds could potentially go live, Hypponen said.

"It's a pretty nasty mechanism," Hypponen said.

F-Secure has registered some of those domain names generated by the algorithm to try to get an estimate of how many computers may be infected. On Tuesday, the number stood at more than 2.5 million. On Wednesday, Hypponen said F-Secure has seen more than 3.5 million machines polling the registered domain name for instructions. But F-Secure analysts think the real number of infected machines could be much higher.

Other than infecting computers, Hypponen said F-Secure hasn't seen other malicious activity from Conficker.B's network of computers. However, those machines form a massive botnet that could be used for other havoc.

An earlier version of Conficker tampered with PC's Domain Name System (DNS) settings. That can cause a computer to visit a different website than the one shown in a browser's address box.

Hypponen said in those instances, Conficker redirected users from Google to Russian websites stuffed with advertisements. The tampering also caused advertising pop-ups to appear. In both examples, Conficker's controllers could be directing masses of traffic on those advertisements in order to generate fraudulent revenue, he said.

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