Hackers have posted a new version of malicious software that will make it easier for them to exploit an unpatched vulnerability in Microsoft's IE (Internet Explorer) browser. Based on a critical bug disclosed on 22 March, the software was posted by hackers to the Milw0rm.com website on Friday.

The code exploits a flaw in the way IE processes web pages using the createTextRange() method. Hackers have been using malware that takes advantage of this vulnerability to install unauthorised software on victims' computers over the past week, but this new generation is considered to be more dangerous, according to security researchers.

Older versions of the malware could freeze victims' browsers for more than a minute, giving them an opportunity to shut down their computers or stop the malicious software before it could complete its work. But the new software works more quickly, meaning it will be particularly effective on older machines with limited memory and processing capabilities, said Craig Schmugar, researcher at McAfee.

Though hackers had not widely adopted the software as of Friday morning, Schmugar said he expected that to change. "It's still pretty early," he said. "I think it's reasonable to expect that people will shift."

The software also uses new techniques to avoid certain types of signatures used by antivirus vendors, said Aviv Raff, a security researcher based in Israel. "It's much more effective," he said. "I think people should know and understand that now they are more vulnerable."

The fact that the code was released just before the weekend is also worrying, because administrators had to wait until Monday to apply their protections and to give warning to users, pointed out Juha-Matti Laurio, a security researcher in Helsinki.

With a fix for the problem expected as late as 11 April, the date of Microsoft's next scheduled security update, security companies Determina and eEye Digital Security issued unsupported patches for the problem. According to eEye, there have been more than 70,000 downloads of its software since its Monday release.

Microsoft does not recommend that users install these patches. Instead, it recommends that users disable IE's Active Scripting feature as a workaround.

Despite the severity of the TextRange() bug, McAfee says the malware that takes advantage of it is not particularly widespread. This software at present ranks number 13 in McAfee's list of the top 20 pieces of malware being reported, Schmugar said.

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