Apple has today pushed one security update, and it seems likely that a second may be just starting too. These updates are the normal, silent, pushed updates which maintain data files used by the tools built into El Capitan, Sierra, and High Sierra.
Gatekeeper’s data files have been updated to version 134. Apple neither announces these updates, nor informs us what they update, although in this case it is likely to ensure that Gatekeeper blocks recently revoked security certificates.
I also hear that the configuration data for Apple’s malware removal tool MRT may also be updated to 1.27. I will confirm this as soon as I see it here, by updating this article.
You can check whether these updates have 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 for Sierra and High Sierra, available from Downloads above.