Mac OS X Leopard firewall damages programs

The background to all this is that, in contrast to Tiger, the firewall in Leopard no longer operates at the packet level but rather it works with applications, to which it permits or denies specific network activities. In order to unambiguously identify applications, Apple uses code signatures, something which has also been introduced for the first time in Leopard. Certain applications signed by Apple are automatically permitted to communicate with the network past the firewall without showing that in the user interface -- even if the firewall is set to "Block all incoming connections".

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