Apple released a rare update for an old operating system, Mac App Store Update for OS X Snow Leopard. The update addresses some changes in the signing certificate used by the Mac App Store.
Apple's full patch notes for the update:
This update is recommended for all Snow Leopard users.
The Mac App Store Update for OS X Snow Leopard ensures future compatibility of the Mac App Store with OS X Snow Leopard. It is recommended for all Snow Leopard users.
- Installs a renewed intermediate signing certificate required by the Mac App Store. The certificate ensures that you can continue to use the Mac App Store in Snow Leopard to purchase new apps and run any previously purchased apps that use receipt validation.
- Includes improvements to Mac App Store alerts.
Snow Leopard is also OS X 10.6. If you're running Snow Leopard, you can download the update through Apple Menu -> Software Update. You can also download a standalone installer at 3.5MB from the update's KnowledgeBase entry.