Apple has just pushed an update to the configuration data for its XProtect tool, which detects malware on macOS systems. This brings the version number to 2099, and adds protection against new malware named cryptically as OSX.28a9883. Unfortunately this doesn’t appear to match any current name being used by anyone else.
You can check whether this update has been installed by opening System Information via About This Mac, and selecting the Installations item under Software.
A full listing of security data file versions is given by LockRattler and SystHist for El Capitan, Sierra and High Sierra, available from Downloads above. If your Mac has not yet installed this update, you can force an update using LockRattler, or at the command line.