Mac app Transmission infected with malware again

01.09.2016

Last month ESET researchers wrote an article about a new OS X malware called OSX/Keydnap, built to steal the content of OS X’s keychain and maintain a permanent backdoor. At that time of the analysis, it was unclear how victims were exposed to OSX/Keydnap. To quote the original article: “It could be through attachments in spam messages, downloads from untrusted websites or something else.”During the last hours, OSX/Keydnap was distributed on a trusted website, which turned out to be “something else”. It spread via a recompiled version of the otherwise legitimate open source BitTorrent client application Transmission and distributed on their official website.

Instant response from the Transmission team

Literally minutes after being notified by ESET, the Transmission team removed the malicious file from their web server and launched an investigation to identify how this happened. At the time of writing, it was impossible to tell exactly when the malicious file was made available for download. According to the signature, the application bundle was signed on August 28th, 2016, but it seems to have been distributed only the next day. Thus, we advise anyone who downloaded Transmission v2.92 between August 28th and August 29th, 2016, inclusively, to verify if their system is compromised by testing the presence of any of the following file or directory:

/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 any of them exists, it means the malicious Transmission application was executed and that Keydnap is most likely running. Also note that the malicious disk image was named Transmission2.92.dmg while the legitimate one is Transmission-2.92.dmg (notice the hyphen).

Similarity with KeRanger

Just like the KeRanger case, a legitimate code signing key was used to sign the malicious Transmission application bundle. It’s different from the legitimate Transmission certificate, but is still signed by Apple and bypasses Gatekeeper protection.ESET has notified Apple about compromised code signing key.