Apple has pushed two updates today, to the data files used by XProtect, bringing its version number to 2133 dated 19 October 2020, and to its malware removal tool MRT, bringing it to version 1.68, also dated 19 October 2020.
Apple doesn’t release information about what these updates add or change, and now obfuscates the identities of malware detected by XProtect using internal code names.
Changes found in the XProtect Yara definitions include the addition of a detection signature for MACOS.1f26189, and modifications to those for MACOS.8f20223, MACOS.1c119be, MACOS.8032420 and MACOS.e79dc35.
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, Catalina and Big Sur, 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.
IMPORTANT NOTE for Sierra and El Capitan users:
There are reports below of MRT 1.68 causing severe problems. You may wish to avoid updating for the moment, by turning Software Update off for the time being. If you experience these problems after the update, try reverting /System/Library/CoreServices/MRT.app to its previous version, 1.67, from a backup. You’ll need to do that when booted from a different system, such as an external disk. Please report any problems to Apple Support so they’re aware of the situation.