It’s looking more and more likely that OS X Leopard is a biting cat rather than a petting one; while many are reporting that general system stability is improved with the Mac upgrade, others are finding that its over-enthusastic firewall is breaking VoIP and other network-dependent software.

Leopard’s firewall attempts to digitally sign applications when launched, a practice caused by Apple’s shift from managing network traffic at the software level rather than at packet level.  Just as Windows Vista requests network authorisation for new software, the latest version of OS X assigns a code signature which is later used to decide access permissions.  Software such as Skype, however, reacts badly to that signature; after running self-integrity checks it recognizes it has been altered and, in future, refuses to load.  Unfortunately no error message is displayed, merely the Skype icon constantly blinking.

A similar issue is being experienced by World of Warcraft players, who have found that Leopard damages software integrity to such an extent that the recommended “workaround” is complete reinstallation.  The news comes as multiple users are reporting ongoing issues with dropped wireless network connections (warning – large page).