Or the Mac App Store itself. Its enforced sandboxing would have provided a decent first line of defense against this, but torrent clients can't be submitted to

Or the Mac App Store itself. Its enforced sandboxing would have provided a decent first line of defense against this, but torrent clients can't be submitted to the App Store due to Apple not liking the legal aspects, not to mention the other issues people have with it. (Outside the store, apps can still opt into sandboxing, but that wouldn't help with a malicious installer.)