The vulnerability, found by security company Foundstone and confirmed by Microsoft, could allow an Internet attacker to take over a Web server, spread an e-mail virus or create a fast-spreading network worm.
"There are millions of systems and clients that will be affected by this," said George Kurtz, chief executive of Foundstone. "This is huge."
Foundstone originally discovered the flaw and worked with Microsoft to develop a patch.
The flaw, in a component of Windows that allows Web servers and browsers to communicate with online databases, could be as widespread as the flaws that allowed theand to spread, said Kurtz. It likely affects the majority of the more than 4.1 million sites hosted on Microsoft's Internet Information Service (IIS) software. In addition, millions of Windows 95, 98, Me and 2000 PCs could also be vulnerable to the software bug.
Microsoft rated the flaw as critical under its newsystem that is intended to lessen the number of flaws that receive a "critical" rating to help administrators identify the most important vulnerabilities to patch.
"There is a possibility that it might be wormable," said Lynn Terwoerds, security program manager for Microsoft's security response center. "It is clearly critical...we want the patch uptake to be really high."
The flaw affects IIS Web servers
Because the MDAC software is not installed by default, at least one security researcher argued that the flaw wouldn't be as widespread as Foundstone claimed.
"I think that there are enough servers that use the component that some are going to be broken into," said Marc Maiffret, chief hacking officer for vulnerability assessment firm eEye Digital Security. The company found the IIS flaw that led to the Code Red worm, which compromised as many as 400,000 servers.
"Default flaws are a much worse thing," he said. "Rather than having to download a piece of (vulnerable) software, you just set up a IIS Web server, and it's vulnerable."
Windows computers, except those running Windows XP, are also vulnerable if Internet Explorer 5.01, 5.5 and 6 are present, as they also use the data access component. However, attacks on such systems are harder to accomplish, Terwoerds said. Outlook Express 6 and Outlook 2000 are immune to attack in their default configurations, and other versions of the mail client can be made safe by using the Outlook E-mail Security Update, she said.
Microsoft has posted information about the flaw and how to secure Windows PCs and Web servers in an advisory on its TechNet site.
Getting all system administrators to patch their systems will be extremely difficult.
Recenthas shown that, in the case of the Linux Slapper worm, only about 40 percent of administrators patched their systems in the 7 weeks before the Slapper worm was released into the Internet. After the worm started spreading, a new surge of patching resulted in another 25 percent to 30 percent of systems being patched. However, the remaining third of computer systems remains vulnerable.
While a worm or attack program that uses this particular vulnerability hasn't appeared on the Internet yet, it's only a matter of time, said Foundstone's Kurtz, who urged system administrators to patch immediately.
"Sometimes the security tsunamis of the world prompt people to patch things, and that's not good for security," he said. "It is good to be proactive."