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Microsoft said Monday that it discovered a critical security vulnerability in a component of its Windows 2000 operating system that could enable a remote attacker to gain total control of a machine running Windows 2000 and Microsoft's Internet Information Server Web server.

The Redmond, Washington, company said that it has also received isolated reports of attacks that exploit the new vulnerability, according to a spokesperson.

An unchecked buffer in a Windows 2000 component used to handle the World Wide Web Distributed Authoring and Versioning (WebDAV) protocol could enable an attacker to cause a buffer overflow on the machine running IIS, according to the Microsoft security bulletin MS03-007.

WebDAV is a set of extensions to HTTP that allows users to edit and manage files on remote Web servers. The protocol is designed to create interoperable, collaborative applications that facilitate geographically dispersed "virtual" software development teams.

Mounting Attacks

Attackers could mount a denial of service attack against such machines or execute their own malicious code in the security context of the IIS service, giving them unfettered access to the vulnerable system, Microsoft said.

Attacks could come in the form of malformed WebDAV requests to a machine running IIS version 5.0. Because WebDAV requests typically use the same port as other Web traffic (Port 80), attackers would only need to be able to establish a connection with the Web server to exploit the vulnerability, Microsoft said.

Machines running the Windows NT and Windows XP operating systems are not vulnerable, according to Microsoft.

Patch Available

Microsoft provided a patch for the WebDAV vulnerability and recommended that customers using IIS version 5.0 on Windows 2000 apply that patch at the earliest possible opportunity.

Internet Security Systems detected an attack that used the vulnerability on one of its scanners late last week, according to Dan Ingevaldson, team leader of X-Force research and development at ISS.

The company was able to isolate the attack and identify the vulnerability it exploited. ISS informed Microsoft, but said that the problem was already known to Microsoft at that point, according to Ingevaldson.

Because of reports of active attacks exploiting the WebDAV vulnerability, an updated version of Microsoft's IIS Lockdown Tool was also released for organizations that are unable to immediately install the patch, or that do not need to run IIS.

The Lockdown Tool turns off unnecessary features of IIS, reducing the openings available to attackers, Microsoft said.

ISS is warning administrators to familiarize themselves with the Lockdown Tool before using it. The tool's design and complex options can often lead administrators to believe that they have disabled options when they have not, according to Ingevaldson.

ISS included information in its alert that explains how to properly use the Lockdown Tool and verify that WebDAV is disabled, Ingevaldson said.

Other utilities were provided for organizations that require the use of IIS, but could not apply the patch or deploy the Lockdown Tool.

Potential Problems

The latest announcement recalls earlier Microsoft vulnerabilities that set the stage for the devastating Code Red and NIMDA worms, according to Ian Hameroff, a security strategist at Computer Associates International.

Adding to the danger of the new vulnerability is the fact that many administrators may not know that they have the WebDAV service enabled on their IIS server, Hameroff said.

The service is enabled by default on IIS 5, according to Hameroff.

Computer Associates is encouraging its customers to follow Microsoft's instructions for patching IIS or for shutting down the vulnerable WebDAV component on their IIS Web server.

"We're warning our users that this is an open door to their business that needs to be shut," Hameroff said.

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