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Microsoft has issued a software patch for what it described as a "critical" new security vulnerability affecting most versions of Windows and certain versions of the Internet Explorer Web browser.

Affected are certain versions of Windows NT, Windows 2000, and Windows Me, Microsoft said. Other versions of Windows may also be vulnerable. However, Microsoft said Windows XP users are not affected and need take no action.

Systems running Internet Explorer versions 5.01, 5.5, and 6.0 are also affected by the new vulnerability, Microsoft said.

A free downloadable patch is available now. Microsoft describes the flaw in security bulletin MS02-065.

Buffer Problem

This latest 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, California, 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 extra data. The surplus would cause the buffer to overflow, and in the process would place and run the attacker's data on the victim's PC.

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.

Earlier versions of either MDAC or Internet Explorer that are no longer supported may also be vulnerable, the software maker said.

Danger Described

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."

The security hole is particularly menacing, notes Stuart McClure, Foundstone's president and chief technical officer. A large number of systems are vulnerable to it, and existing worms such as Code Red or Nimda could easily be modified to take advantage of the newly disclosed flaw.

"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."

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

Patches Collected

The same day that Microsoft warned of the MDAC flaw, the vendor released a cumulative patch for IE that patches six other new flaws in addition to offering all previously issued security patches for the Web browser in one update. The cumulative patch was released in bulletin MS02-066.

Microsoft deemed the cumulative patch "important," as opposed to the "critical" rating it gave the MDAC flaw. 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.

Four of the newly patched vulnerabilities would allow an attacker to read certain files on a user's computer, while the other two would let an attacker see what a user is entering on Web sites or would cause IE to fail, Microsoft said.

(Joris Evers, in Amsterdam, contributed to this report.)

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