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| After you set up Microsoft Windows Firewall in Microsoft Windows XP Service Pack 2 (SP2), you may discover that your computer can be accessed by anyone on the Internet when you use a dial-up connection to connect to the Internet, this is due to a back in the way Microsoft's Firewall handles local subnets. |
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Credit:
The information has been provided by Nathan Fowler.
The original article can be found at: http://support.microsoft.com/kb/886185
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This problem occurs because of the way that Windows Firewall interprets local subnets when the "My network (subnet) only" option is used. Windows Firewall is included with Windows XP SP2.
Because of the way that some dialing software configures routing tables, Windows Firewall in Windows XP SP2 can sometimes interpret the whole Internet to be a local subnet. This can let anyone on the Internet access the Windows Firewall exceptions. When the "My network (subnet) only" option is enabled, it is automatically selected for file and print sharing. Therefore, your shared drives can be unexpectedly revealed on the Internet when you use a dial-up connection.
Solution:
To resolve this problem, you must download and install the Critical Update for Windows XP: KB886185
After you install the Critical Update for Windows XP (KB886185), Windows Firewall will no longer interpret a dial-up network connection to be on your local subnet.
Specifically, any IP Route Table entry that has an IP address of 0.0.0.0 and has a mask of 0.0.0.0 will not be interpreted to be on the local subnet. This means that any port exceptions or program exceptions that use the "My network (subnet) only" option in Windows Firewall will not be available over a dial-up connection. You will still be able to access exceptions over a dial-up connection if you remove all scope restrictions, or if you create a custom scope for exceptions.
Subnets can be highly variable, depending on the network that they are connected to. Therefore, using the "My network" scope restriction does not guarantee security. We strongly recommend that you use the custom scope option when you want to make sure that no unwanted incoming traffic is permitted to pass through your firewall exceptions.
For more information about configuring Windows Firewall, visit the following Microsoft TechNet Web page: http://www.microsoft.com/technet/community/columns/cableguy/cg0204.mspx
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