Microsoft Wednesday 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 the Internet Explorer Web browser.

The security flaw affects the Microsoft Data Access Components (MDAC), a collection of components that provide database access for Windows platforms, according to a statement from Foundstone, a software vulnerability management company in Mission Viejo, Calif., that discovered the flaw.

The vulnerability involves what's known as an "unchecked buffer" in the Remote Data Services (RDS) component of MDAC. The faulty code is in a function called the RDS Data Stub, which is used to pull information from incoming HTTP requests and create RDS commands, according to Microsoft.

An attacker could exploit the security weakness by sending an improperly formatted HTTP request to the Data Stub that contained a surplus of data, causing the buffer to overflow and the attacker's data to be placed and run on the affected machine.

The vulnerability affects certain versions of Windows NT, Windows 2000 and Windows Me, Microsoft said, and potentially other versions of its operating system. It said Windows XP users are not affected and need take no action.

The vulnerability received a severity rating of "critical" from Microsoft, the highest possible rating under the company's new vulnerability rating system, which was announced 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." Many issues that were previously rated critical are now rated "important," a new category in the rating system. Important vulnerabilities could expose user data or threaten system resources, according to Microsoft's new definitions.

The security hole in MDAC 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, Foundstone President and CTO Stuart McClure said.

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

The IDG News Service is a Network World affiliate.