10.9 Mavericks is your security update for 10.6 through 10.8.5

Mavericks is free.

Its system requirements are the same as for Mountain Lion



It is also a security update:



I surmise that Apple does not intend to provide security updates for older OS versions.  At least not for 10.8.

If you want to get patched, start upgrading to 10.9…

Update: 10/4/2013

While it was fun to speculate about Apple forcing us to upgrade to 10.9.  It was just speculation.  I now believe I was wrong and I expect Apple to release updates for older versions similar to its past behavior.