Everyone running 2.90 on OS X should immediately upgrade to 2.91 or delete their copy of 2.90, as they may have downloaded a malware-infected file.
Using “ Activity Monitor ” preinstalled in OS X, check whether any process named “ kernel_service ” is running. If so, double check the process, choose the “ Open Files and Ports ” and check whether there is a file name like “/Users//Library/kernel_service ”. If so, the process is KeRanger ’ s main process. We suggest terminating it with “ Quit -> Force Quit ”
Apple is aware of the issue and has already revoked "a digital certificate from a legitimate Apple developer that enabled the rogue software to install on Macs."
If you don't use the Transmission software, there is nothing you need to do at this time.
via
http://www.macrumors.com/2016/03/06/mac-ransomware-transmission/