The new Apple TV 5.1.1 update was issued alongside the new iTunes 11 available for Mac and Windows. Apple TV 5.1.1 can be downloaded through Software Update in the Settings application.
Apple said the update addresses a few security issues for the set-top box, including one that could allow an attacker to terminate arbitrary code. The build is identified as 10A831.
The last Apple TV software update was issued in October, but some users have reported serious issues after installing. It's unknown whether the latest build addresses those issues.