The developers of open source video transcoder app Handbrake have issued a security warning to Mac users after a mirror download server hosting the software was hacked.

The alert was issued on Saturday after it was discovered that the original HandBrake-1.0.7.dmg installer file on mirror server download.handbrake.fr had been replaced by a malicious file.

The affected server has been shut down for investigation, but developers are warning that users who downloaded the software from the server between 14:30 UTC May 2 and 11:00 UTC May 6 have a 50/50 chance of their system being infected by a trojan. "If you see a process called 'Activity_agent' in the OS X Activity Monitor application, you are infected," read the alert.

To remove the malware from an infected computer, users need to open up the Terminal application and run the following commands:

launchctl unload ~/Library/LaunchAgents/fr.handbrake.activity_agent.plist

rm -rf ~/Library/RenderFiles/activity_agent.app

if ~/Library/VideoFrameworks/ contains proton.zip, remove the folder

Users should then remove any installs of the Handbrake.app they have on their system. As an extra security recommendation, users should also change all the passwords that may reside in their OSX KeyChain or in any browser password stores.

The malware in question is a new variant of OSX.PROTON, a Mac-based remote access trojan that gives the attacker root-access privileges. Apple updated its macOS security software XProtect in February to defend against the original Proton malware. Apple initiated the process to update its XProtect definitions on Saturday and the update should already be rolling out to machines silently and automatically.

Handbrake users should note that the primary download mirror and the Handbrake website were unaffected by the hack. Downloads via the application's built-in updater with 1.0 and later are also unaffected, since these are verified by a DSA Signature and won't install if they don't pass. However, users with Handbrake 0.10.5 and earlier who used the application's built-in updater should check their system, as these versions don't have the verification feature.

For reference, HandBrake.dmg files with the following checksums are infected:

SHA1: 0935a43ca90c6c419a49e4f8f1d75e68cd70b274 / SHA256: 013623e5e50449bbdf6943549d8224a122aa6c42bd3300a1bd2b743b01ae6793

(Thanks, Alfonso!)