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The configuration problem only affects dial-up users: some dialling software mistakes the entire Internet as part of a local network, and if the user clicks the option to allow connections from within the local (subnet) network, the firewall will allow connections from any IP address out on the Web to shared resources on the machine.

Microsoft said that advice on how to properly configure the Firewall to limit the scope of network access at the Microsoft website

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and added that 'Users who have a properly configured Network Address Translator (NAT), commonly found in wired and wireless home routers, are fully protected from unintended file sharing beyond their private (home or business) network.'

It said Windows Firewall was designed to offer 'substantial' protection for the majority of users and as such uses global settings to make it easier to use, rather than 'per interface'.

'While Windows Firewall doesn't provide absolute protection against malicious hackers, it provides strong protection in most scenarios and was included in SP2 to provide a level of protection well beyond the common pre-SP2 configuration for most users - no personal firewall at all,' said the company in a statement.

A fix for the problem was not included as part of Tuesday's monthly security bulletin as it was a configuration rather than a software issue it said.

A patch is now available. Users with the autoupdate feature turned on will be protected automatically. It is also available from the Windowsupdate site.