Hackers have published code that could let an attacker disable the Windows Firewall on certain Windows XP machines.

The code, which was posted on the Internet early Oct, 29, could be used to disable the Windows Firewall on a fully patched Windows XP PC that was running Windows' 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's your line of defense?" he said today in an interview.

Is An Attack Likely?

By knocking off the Windows Firewall, a criminal could open the door to new types of attacks, but there are a number of factors that 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 only work 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.

A Solution

An easier solution, may be for ICS users to simply move their networks onto a router or NAT (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 a 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's initial investigation into the matter "has concluded that the issue only impacts users of Windows XP," the company's 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."