If you use the popular Handbrake app to convert video, beware: it may contain malware. The developers posted a warning, explaining that:
Anyone who has downloaded HandBrake on Mac between [02/May/2017 14:30 UTC] and [06/May/2017 11:00 UTC] needs to verify the SHA1 / 256 sum of the file before running it.
Anyone who has installed HandBrake for Mac needs to verify their system is not infected with a Trojan. You have 50/50 chance if you’ve downloaded HandBrake during this period.
They also point out that:
If you see a process called “Activity_agent” in the OSX [sic] Activity Monitor application. You are infected.
You can check the latter by opening Activity Monitor (it’s in the /Applications/Utilities folder) and searching for the word “activity.”
If you are infected, see the Handbrake forum post for more information.