natd in network_cmds in Apple Mac OS X before 10.5.6, when Internet Sharing is enabled, allows remote attackers to cause a denial of service (infinite loop) via a crafted TCP packet.

More... (http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2008-4222)

I've just provided additional information to the CVE editorial board of older additionally affected versions of Mac OS X.

I should also object to the statement by Apple that a "maliciously crafted packet" is required. I first encountered this problem using the Cisco VPN Client for Mac OS X, which among it's numerous embarrassing bugs also regularly sends malformed packets.