Apple was widely skewered for being among the last to fix a gaping security hole in the net's address lookup system that could allow the wholesale hijacking of users' internet connections. And now that the company has finally got around to issuing a patch, there's just one problem: it doesn't work on client versions of Mac OS X.

That's the vast majority of Mac installations.

Researchers from security firm nCircle and the SANS Institute both report that fully patched versions of Tiger (10.4.11) and Leopard (10.5.4) remain vulnerable even after running a bevy of patches Apple released Thursday. Other vendors, including Microsoft, Sun Micro, released similar patches weeks ago.

Both researchers found that OS X clients fail to adequately randomize DNS source ports, allowing attackers to poison the caches of DNS servers that run on the operating system.

"So Apple might have fixed some of the more important parts for servers, but is far from done yet as all the clients linked against a DNS client library still need to get the workaround for the protocol weakness," SANS handler Swa Frantzen wrote.

Given the ultra-insular culture at Apple, it's hard to know why engineers chose to patch some Mac versions and not others. It's possible they reckoned clients handle so few DNS queries that it didn't make sense. Or they may have overlooked it.

There's also the issue of performance. Macs use the popular Berkeley Internet Name Domain program to handle domain-name lookups, and Paul Vixie, the person who oversees the project recently said the DNS patch can slow down lookups under heavy loads.

It's not likely we'll find out why clients remain vulnerable to one of the most critical security bugs to come around in years. Apple representatives haven't answered a single one of our security-related queries in more than 18 months. ®

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