It affects only a subset of Transmission users who downloaded a corrupted version of the app. It's since been fixed.
Somehow (news still breaking), Transmission lost control of their Certification, allowing ransomware jerks to slip their version in.
I believe from Transmission's web site. I believe the auto update version was safe. Auto update to latest version now.
Technical analysis, including How To Protect Yourself section, is here. It's towards end of article.
Text copied/pasted here, but click-thru recommended.
We suggest users take the following steps to identify and remove KeRanger holds their files for ransom:
Using either Terminal or Finder, check whether /Applications/Transmission.app/Contents/Resources/ General.rtf or /Volumes/Transmission/Transmission.app/Contents/Resources/ General.rtf exist. If any of these exist, the Transmission application is infected and we suggest deleting this version of Transmission.
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/<username>/Library/kernel_service” (Figure 12). If so, the process is KeRanger’s main process. We suggest terminating it with “Quit -> Force Quit”.
After these steps, we also recommend users check whether the files “.kernel_pid”, “.kernel_time”, “.kernel_complete” or “kernel_service” existing in ~/Library directory. If so, you should delete them.