For the second time in five months, the Transmission BitTorrent client for Mac has been infected with malware.
The malware, dubbed OSX/Keydnap, is designed to steal the contents of the keychain on OS X systems and to keep a permanent backdoor open. 
According to the ESET researchers who discovered the malware:
“Over the past few hours, OSX/Keydnap has begun to be distributed from a trusted website, via a recompilation of the open source BitTorrent client Transmission.”
The good news is that “within minutes” of being notified, the Transmission team removed the malicious files from their server. The bad news is that it is known how many people have downloaded the app.
The malware has a digital signature dated August 28th, so ESET advises anyone who downloaded Transmission 2.92 between August 28th and 29th to remove it from their system immediately.
If you think you may be infected, check for the existence of any of the following files or folders in the paths:
/Applications/Transmission.app/Contents/Resources/License.rtf /Volumes/Transmission/Transmission.app/Contents/Resources/License.rtf $HOME/Library/Application Support/com.apple.iCloud.sync.daemon/icloudsyncd $HOME/Library/Application Support/com.apple.iCloud.sync.daemon/process.id $HOME/Library/LaunchAgents/com.apple.iCloud.sync.daemon.plist /Library/Application Support/com.apple.iCloud.sync.daemon/ $HOME/Library/LaunchAgents/com.geticloud.icloud.photo.plist
If you see these files, according to ESET, your system is infected.
If you have OSX/Keydnap on your system, you can remove it with a reputable antivirus. There is also a script on GitHub that you can run via the OS X terminal to delete the malware.
