The agency, which also runs the United States Computer Emergency Readiness Team (US-CERT), sent out a news release on Wednesday recommending that people apply Microsoft's MS06-040 patch as quickly as possible. The software maker as part of its monthly patch cycle.
"Users are encouraged to avoid delay in applying this security patch," the Department of Homeland Security said in the statement. The patch fixes a serious flaw that, if exploited, could enable an attacker to remotely take complete control of an affected system, the agency said.
Microsoft on Tuesday issued a dozen security bulletins, nine of which were tagged "critical," the company's highest severity rating. However, the flaw addressed in MS06-040 is the only one among the updates that could let an anonymous attacker remotely commandeer a Windows PC without any user interaction.
The flaw has some similarities to the Windows bug thatin 2003. Both security vulnerabilities are related to a Windows component called "remote procedure call," which provides support for networking features such as file sharing and printer sharing.
"Blaster took advantage of a vulnerability in the same service. We recognize that this is something that is easily exploitable," said Amol Sarwate, the manager of vulnerability research lab at Qualys. "It is excellent that DHS sent out this alert, because I think a lot of people are vulnerable."
Microsoft has seen a "very limited attack" that already used the newly disclosed flaw, the software maker said Tuesday.
Overnight, some hacker toolkits were updated with code that allows researchers to check for the flaw and exploit it, said Neel Mehta, a security expert at Internet Security Systems in Atlanta.
"This is a very serious vulnerability," Mehta said. "At the moment, this exploit is being used in targeted attacks to compromise specific systems. However, there is nothing about the nature of the vulnerability that prevents it from being used in a much more widespread fashion as part of a worm."
Microsoft worked with the Department of Homeland Security on the alert, a company representative said. "Microsoft...encourages customers to deploy this update on their systems as soon as possible, given that we are aware of targeted exploitation of the vulnerability," the representative said.
Microsoft deems the vulnerability critical for all versions of Windows. However, users of Windows XP with Service Pack 2 and Windows Server 2003 with Service Pack 1 should be protected by the Windows Firewall if they do not use file sharing and printer sharing, Christopher Budd, a security program manager at Microsoft, said in an interview Tuesday.
The Microsoft updates are available via the Windows Update and Automatic Updates tools as well as from Microsoft's Web site. Temporary workarounds are outlined in the security bulletins for those who can't immediately apply the patches.