The KeRanger virus is ransomware. Usually, it infiltrates your Mac bundled with unreliable software and encrypts the files you store onto the system. If this happens, you have two options – either say “Good bye” to your data for good or pay the ransom asked by the virus developers.
KeRanger has been infecting Mac users since March 2016 and it has been distributed together with a piece of a legitimate software named Transmission torrent client.
KeRanger starts its attack with a file called OSX.KeRanger.A. Once the file sneaks into the Transmission 2.90 update, it gets installed alongside it. This means that every Mac user who has downloaded and runs the Transmission 2.90 update, also runs the KeRanger file.
In order to block KeRanger, Apple revoked the GateKeeper signature and updated its XProtect system (part of File Quarantine). However, if you are using Transmission, you must upgrade your Mac to the latest version, Transmission 2.92, at once.