Microsoft has issued a temporary fix for a zero-day flaw in Internet Explorer that was used in recent attacks against the U.S. Department of Labor and other websites.

The workaround, which Microsoft calls a one-click Fix it tool, can be used to prevent known attacks exploiting the vulnerability. Microsoft once again confirmed that the bug affects only IE version 8; IE 6, 7, 9, and 10 are not affected.

"We have updated Security Advisory 2847140 with an easy one-click Fix it to help protect Internet Explorer 8 customers. Customers should apply the Fix it or follow the workarounds listed in the advisory to help protect against the known attacks while we continue working on a security update," said Dustin Childs, group manager of response communications for Microsoft Trustworthy Computing in a statement. "Internet Explorer 6, 7, 9 and 10 are not affected.”

The flaw was discovered by researchers investigating a waterhole-style attack on the Department of Labor's website. The attackers were using the DOL attack to turn around and target the U.S. Department of Energy, researchers say. According to Invincea, "the web pages that were compromised on the DoL site are intended for Dept of Energy employees (and their DoL representatives) in dealing with nuclear-related illnesses linked to Dept of Energy facilities and the toxicity levels at each location."

At least nine other websites were also targeted in the attack, including nonprofit organizations and a large European aerospace, defense, and a security company, according to Jaime Blasco, director of AlienVault research labs.

A Metasploit exploit pack is also available for the use-after-free bug that has been assigned as CVE-2013-1347.

Paul Ducklin, a security expert with Sophos, says a temporary patch is the best bet here since rushing out a permanent one for this flaw could break other things. "The problem with rushing out a permanent patch to this sort of bug is that there is a chance that by sealing off the flaw, you might break things that legitimate real-world websites rely on, and thus interfere with the workflow of some of your users," Ducklin said in a blog post today. "Even if the behaviour broken by your patch is a side-effect of the bug, and thus itself relies on broken behaviour, users don't like security "cures" that give the impression of being worse than the disease."

For Microsoft's Fix it to work, users must have April's patches installed and 32-bit versions of IE running. Microsoft's Enhanced Mitigation Experience Toolkit (EMET) combats attacks using this bug.

Microsoft's Fix it for the IE 8 flaw is available for download here.

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