The 10.6 version of the operating system is reportedly still used by about 20 percent of the world's Apple computer users.
The Cupertino, Calif., tech giant released a patch for the latest version of OS X, Mavericks (10.9) Wednesday, along with patches for Mountain Lion (10.8) and Lion (10.7).
The patches plugged 21 security holes in the three version of the software, theregister.co.uk reported.
However, no similar patch was issued for Snow Leopard (10.6), likely leaving those security holes in that version and signaling -- or at least suggesting -- Apple's intention to let the software become obsolete, security analysts said.
"Let's face it, Apple doesn't go out of their way to ensure users are aware when products are going end of life," Andrew Storms, director of DevOps at security company CloudPassage, said.
Snow Leopard users concerned about security can upgrade to Mavericks easily -- and probably for free. Users can download the latest OS X version from the Mac App Store, so long as they have the latest Snow Leopard update installed and their Mac was purchased in 2007 or later.