Apple has issued an update for its newly-released MacOS 10.5.1, better known as Leopard.

The update addresses Leopard's firewall, the most highly-publicised shortcoming in the operating system.

Shortly after Leopard's public release in October, researchers claimed that the firewall was not doing its job.

Heise Security researcher Jurgen Schmidt said that users would not be able to rely on the firewall to block potentially harmful traffic, even at its most secure setting.

Apple explained that the issues Schmidt had highlighted were down to the way Leopard classifies its 'block all incoming connections' setting.

When users select the option to block all incoming connections, processes running at the root level are not blocked by the firewall.

"The 'block all incoming connections' setting for the firewall is misleading, " Apple admitted.

Apple is changing the option from 'block all incoming connections' to 'allow only essential services' in an effort to provide a more accurate description.

Among the processes not blocked under the setting are components for DHCP network configurations, IPsec security protocols and Bonjour networking software.

The update will also provide the option to further enhance Leopard's firewall protection by allowing users to block all connections for a specific application, including root-level connections which had previously been allowed.

Apple also corrected an issue in which some firewall preferences would not take effect until certain process had been restarted.

The update only effects OS X Leopard. Users can obtain the update through the Apple Downloads site or through the OS X Software Update component.