Hackers have released code that could let an aggressor render Windows Firewall inoperative on some Windows XP machines.
The code, which was uploaded on the Internet early Sunday morning, could be used to disable Windows Firewall on a fully patched Windows XP PC running Windows' Internet Connection Service (ICS). This service allows Windows users to basically turn theirs PCs into routers and share their Internet connections with other systems on a LAN. It is usually used by home and small-business users.
The attacker could send a nasty data packet to another PC using ICS that would cause the service to end. 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 Inc., who has blogged about the issue.
"Once the firewall is down, where's your line of defense?" he said Monday in an interview.
By knocking off the Windows Firewall feature, an attacker could open the door to new types of attacks, but there are a number of issues 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 work only on systems using ICS, which is disabled by default. Additionally, 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 way out may be for ICS users to basically move their networks onto a router or Network Address Translation device, said Stefano Zanero, chief technology officer at 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 alleged 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 inquiry into the matter "has concluded that the issue only impacts users of Windows XP," the company's public relations agency said Monday in a declaration. "Microsoft is not aware of any attacks attempting to use the reported vulnerability or of customer impact at this time."
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