Apple released Security Update 2013-003 for Mountain Lion. The patch notes offer nothing specific, saying that the update is, "recommended for all users and improves the security of Mac OS X."
In addition, Apple has not yet added this update to its Security Update site, meaning we don't yet know what's in it.
The update showed up earlier in the day on Tuesday, but quickly disappeared from Apple's site. It reappeared late in the afternoon with a "1.0" appended to the update's name in Software Update on the Mac App Store, where it's available now.
You can download a standalone updater as a 20.4MB download through Apple's support site.