Software giant Microsoft has issued a software patch for what it described as a "critical" new security vulnerability affecting most versions of its Windows operating systems and certain versions of its IE (Internet Explorer) web browser.

MDAC (Microsoft data access components), which provide database access for Windows platforms, are affected by the flaw according to a statement from Foundstone, the software vulnerability management company that discovered the flaw.

The vulnerability involves an "unchecked buffer" in the data stub of the Remote Data Services component of MDAC. The faulty code pulls information from incoming HTTP (hypertext transfer protocol) requests and creates RDS commands, according to Microsoft.

If an attacker sent an improperly formatted HTTP request to the data stub that contained a surplus of data, it could cause the buffer to overflow and the attacker's data to be placed and run on the affected machine.

Certain versions of Windows NT, 2000 and Me are affected, Microsoft said. Microsoft reassures Windows XP users that they are not affected by the security flaw and need take no action, but it’s possible that other versions of the operating system could be vulnerable.

Microsoft has given the flaw a severity rating of "critical" — the highest possible rating under the new vulnerability rating system it announced on Tuesday. Microsoft defines critical vulnerabilities as those "whose exploitation could allow the propagation of an internet worm such as Code Red or Nimda without user action."

The security hole is particularly menacing because of the large number of systems that are vulnerable to it and because of the ease with which existing worms such as Code Red or Nimda could be modified to take advantage of the newly disclosed flaw, said Foundstone president and chief technical officer Stuart McClure.

"What makes it really quite dangerous is that it can be easily added to a worm," McClure said.

"It's very much in line with Code Red and Nimda because of the attack vectors and the ways that it attacks. But with this vulnerability both the server and client component can be attacked, as opposed to Code Red and Nimda, which basically exploited server-based vulnerabilities and didn't take advantage of a browser-based vulnerability like this," he said.

Foundstone discovered the vulnerability in August and disclosed the information to Microsoft at that time, McClure said.

The vulnerability affects MDAC versions 2.1, 2.5 and 2.6, according to Microsoft. MDAC is installed and implemented by default in Windows 2000 and within the Windows NT 4.0 option pack.

Machines running Internet Explorer versions 5.01, 5.5 and 6.0 are also affected by the new vulnerability, Microsoft said. Earlier versions of either MDAC or Internet Explorer that are no longer supported may also be vulnerable, the software maker said.

Customers who believe their PCs may be at risk should read the bulletin at here before downloading and installing the software patch issued by Microsoft, which is here.