Hackers have published code that could let an attacker disable the Microsoft Windows Firewall on certain Windows XP machines.The code, which was posted on the Internet early Sunday morning, could be used to disable the Windows Firewall on a fully patched Windows XP PC that was running Windows\u2019 Internet Connection Service (ICS). This service allows Windows users to essentially turn their PC into a router and share their Internet connection with other computers on the local area network (LAN). It is typically used by home and small-business users.The attacker could send a malicious data packet to another PC using ICS that would cause the service to terminate. Because this service is connected to the Windows firewall, this packet would also cause the firewall to stop working, said Tyler Reguly, a research engineer at nCircle Network Security, who has blogged about the issue."Once the firewall is down, where\u2019s your line of defense?" he said Monday in an interview.By knocking off the Windows Firewall, a criminal could open the door to new types of attacks, but a number of factors make such an attack scenario unlikely, Reguly said.For example, the attacker would have to be within the LAN in order to make the attack work, and of course it would work only on systems using ICS, which is disabled by default. Furthermore, the attack would have no effect on any third-party firewall being used by the PC, Reguly said.Users can avoid the attack by disabling ICS, Reguly said. But this will also kill the shared Internet connection.An easier solution may be for ICS users to simply move their networks onto a router or network address translation device, said Stefano Zanero, chief technology officer with Secure Network SRL. "They are so cheap right now, and in many cases they offer better protection and an easier administration of your LAN," he said via instant message.Windows XP appears to be the only platform affected by this attack, which has not been successfully reproduced on Windows Server 2003, Reguly said.Microsoft\u2019s initial investigation into the matter "has concluded that the issue only impacts users of Windows XP," the company\u2019s public relations agency said Monday in a statement. "Microsoft is not aware of any attacks attempting to use the reported vulnerability or of customer impact at this time."By Robert McMillan, IDG News Service (San Francisco Bureau)Keep checking in at our Security Feed for updated news coverage.