Apple released security updates for OS X's NTP (network time protocol) service. The patch notes don't specify the changes, but do characterize the security flaw as "critical."
Apple's sparse patch notes:
This update addresses a critical security issue with the software that provides the Network Time Protocol service on OS X, and is recommended for all users.
The specifics will be included on Apple's Security Updates page, but these updates have not yet been added as of this writing.
On Yosemite, the download is 2.1MB through Apple's support site, or 1.4MB through the Mac App Store.
On Mavericks, the download is 2.0MB
On Mountain Lion, the download is 1.9MB.
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