Update: Microsoft updates free tool to remove persistent worm

Microsoft has updated its free security tool to remove a persistent worm that is targeting a now-patched but severe vulnerability that affects several server products.

The latest update to the Malicious Software Removal Tool (MSRT) can now remove infections of Conficker, a worm that infects a server and then tries to download other malicious software, according to a company blog.

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

Microsoft has observed 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, wrote Cristian Craioveanu and Ziv Mador, on the Microsoft Malware Protection Center 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.

The outbreak of Conficker.B is mostly affecting customers who are running large networks. Countries with affected systems include the U.S., Mexico, France, Spain, Canada, Italy, Brazil, South Korea, Germany, Malaysia and the Czech Republic, Microsoft said.

The company's 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 make the passwords for shared networks stronger and then 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.

Many companies throughout Europe 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 analyzed 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 Web site 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 Web sites, since it's hard to know which ones of hundreds could potentially go live, Hypponen said.