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 Sunday morning, 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, a research engineer at nCircle Network Security, Tyler Reguly, said. He has blogged about the issue.
"Once the firewall is down, where's your line of defense?" he said, in an interview.
By knocking off the Windows Firewall, a criminal could open the door to new types of attacks, but there were 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 was 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 would also kill the shared Internet connection.
An easier solution might be for ICS users to simply move their networks onto a router or Network Address Translation device, chief technology officer with Secure Network, Stefano Zanero, said.
"They are so cheap right now, and in many cases they offer better protection and a easier administration of your LAN," he said. Windows XP appears to be the only platform affected by this attack, which had not been successfully reproduced on Windows Server 2003, Reguly said.
Microsoft 's initial investigation into the matter concluded that the issue only impacted users of Windows XP, the company's public relations agency said in a statement.
"Microsoft is not aware of any attacks attempting to use the reported vulnerability or of customer impact at this time," it said.