Apple has pushed two updates today, to the data files used by XProtect, bringing its version number to 2106, dated 1 October 2019, and to its malware removal tool MRT, bringing it to version 1.50, also dated 1 October 2019. XProtect version 2105 wasn’t generally released, but was included in Catalina beta releases.
Apple doesn’t release information about what these updates add or change. Looking at the XProtect data files, there don’t appear to be any changes to the ‘Yara’ signature file, which is surprising, but around 100 identifiers have been added to the Extension blacklist in XProtect.meta.plist. The significance of these isn’t currently apparent, but this does represent a substantial change for once.
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 SilentKnight, LockRattler and SystHist for El Capitan, Sierra, High Sierra, Mojave and Catalina, available from their product page. If your Mac has not yet installed this update, you can force an update using SilentKnight, LockRattler, or at the command line.
I have updated the reference pages here which are accessed directly from LockRattler 4.2 and later using its Check blog button.
I maintain lists of the current versions of security data files for Mojave on this page, High Sierra on this page, Sierra on this page, and El Capitan on this page.