The U.S. Department of Homeland Security (DHS) warned Wednesday that a recently patched Microsoft Windows vulnerability could put the nation\u2019s critical infrastructure at risk.The patch, described in Microsoft Security Bulletin MS06-040, relates to Windows Server services. It was one of 12 updates issued Tuesday by the software giant, but security experts are particularly concerned with the bug because hackers have already exploited the vulnerability. The vulnerability is described here.Microsoft is advising customers to give this update priority, said Christopher Budd, a security program manager with Microsoft\u2019s security response center. "The top thing that we\u2019re trying to help people understand is we want them to take 06-040 and put it at the top of the stack," he said late Tuesday.The DHS statement echoed Microsoft\u2019s sentiments warning that the vulnerability "could impact government systems, private industry and critical infrastructure, as well as individual and home users." The statement can be found here.Attackers have already started exploiting the vulnerability in a limited manner, Budd said. A sample exploit has been published within Immunity\u2019s security testing toolkit, and snippets of the malware are beginning to circulate in public, security vendors said.The bug is of particular concern because Windows Server services are generally enabled by default on Windows systems, and a worm based on the flaw could end up being widespread. Windows Server services are used for common network applications like file sharing and printing.The fact that DHS has taken the rare step of warning about MS06-040 underscores the severity of the situation, said Jonathan Bitle, manager of technical accounts with Qualys.But because security-conscious companies are blocking the Internet ports used by this malware\u2014ports 139 and 445\u2014any worm will have a hard time jumping from one corporate network to another, Bitle said. "It will probably be the type of situation where if a worm does come out, it will hit sporadically through different companies where they haven\u2019t been able to apply the patches or put the controls in place."-Robert McMillan, IDG News Service (San Francisco Bureau)Keep checking in at our CSO Security Feed page for updated news coverage.