New Windows Flaw Disclosed by Microsoft

'Zero-day exploit' involving WebDAV vulnerability hits U.S. Army web server.

Security experts say the latest Windows vulnerability revealed last week by Microsoft Corp. has already been used by crackers to attack at least one machine belonging to the U.S. Army. And, it turns out, the flaw used to attack the Web server was discovered not by Microsoft or independent researchers but by the attacker himself.

Microsoft released a patch for the critical vulnerability in a Windows 2000 component used by the WebDAV (Web-based Distributed Authoring and Versioning) protocol. The vulnerability gives an attacker control of a vulnerable machine, officials at the Redmond, Wash., company said.

Experts at TruSecure Corp., based in Herndon, Va., received word of the attack on the Armys Web server two weeks ago through contacts within the Army. A Web server was attacked using a URL that was 4KB in length, and the machine was subsequently compromised. The server then immediately began mapping the network around it, looking for other vulnerable machines and seeing what else of interest was within reach. It then started sending the results of its mapping to a remote machine through TCP port 3389 using terminal services, said Russ Cooper, surgeon general at TruSecure.

Once Army security staff realized the server had been compromised, it took the machine offline and rebuilt it. But as soon as it was reconnected to the Internet, the server was compromised again. At that point, Army personnel realized they were dealing with something new and went to Microsofts support site and filled out a Web form describing the issue.

"Weve had some reports of it being actively exploited, and thats the reason we went out [with the patch] as soon as we did," said Steve Lipner, director of security assurance at Microsoft.

Military Strike

Chronology of WebDAV vulnerability/ Army hack incident:
  • March 9 or 10 Army Web server compromised
  • March 11 Army notifies Microsoft, outside security experts
  • March 14 Microsoft produces patch for new vulnerability
  • March 17 Patch and security advisories are released by Microsoft and CERT
Still, the vulnerability and its exploitation caught security officials at Microsoft and at independent bodies such as the CERT Coordination Center off guard. Attacks such as this that occur against previously unknown vulnerabilities are known as zero-day exploits, as there is no elapsed time between the discovery of the flaw and its exploitation. Although security experts said they have not seen any other attacks using this exploit, Cooper said he expects to see a worm based on it within a week or so.

"Its absolutely vital that people get rid of WebDAV on their boxes if they dont need it," Cooper said. "If they dont know whether theyre using it, chances are that theyre not, and they should disable it."

And, because there is just one machine known to have been compromised with this attack so far, Cooper said he believes it was the work of an individual cracker and not a nation or other organization. "With the element of surprise like that, Id think a nation state would go after a large number of machines and not just this one," he said.

The flaw affects only Windows 2000 machines that are configured to run as Web servers. To exploit this issue, an attacker must establish a Web connection with the affected computer. The attacker can then send a specially formed HTTP request to Microsofts IIS (Internet Information Services) running on the machine.

The request would either cause the IIS server to fail or run the code of the attackers choice. Any code would run in the security context of the IIS server, which runs as LocalSystem by default, according to Microsofts advisory on the vulnerability.

WebDAV is used to provide a standard for editing and file management among computers on the Internet.