The company didn't specify what changes were made, other than saying it was recommended for all users and it improves the security of Mac OS X.
Apple's website that details the contents of security updates has yet to be updated with the fixes that are contained in Security Update 2013-001.
The updates for Snow Leopard, Snow Leopard Server, Lion and Lion Server are available from Apple's Software Update website, or select Software Update in the Apple Menu.
Article Link: Apple Releases Security Update 2013-001 for Snow Leopard and Lion