A new Mac OS X trojan horse that monitors web browsing traffic in order to steal Bitcoins has been discovered by SecureMac. The trojan, called OSX/CoinThief.A, is disguised as an innocuous Bitcoin app called StealthBit that purports to send and receive anonymous payments.

The app was posted on open-source website GitHub, but the precompiled version of the app had the malicious payload installed. The malware installs browser extensions in Safari and Google Chrome looking for login credentials for a number of Bitcoin related websites including MtGox, BTC-e, and blockchain.info. When the app finds login credentials, it sends those back to the malware's developer.

Initial infection occurs when a user installs and runs an app called "StealthBit," which was recently available for download on GitHub, a website that acts as a repository for open source code. The source code to StealthBit was originally posted on GitHub, along with a precompiled copy of the app for download. The precompiled version of StealthBit did not match a copy generated from the source code, as it contained a malicious payload. Users who downloaded and ran the precompiled version of StealthBit instead ended up with infected systems. A user posting over the weekend on Reddit, the popular discussion site, reported losing 20 Bitcoins (currently worth upwards of $12,000 USD) to the thieves.

Bitcoin users who may have downloaded the app should check their browser extensions in Safari and Google Chrome for generic "Pop-Up Blocker" extensions.