Researchers recently found at least 50 apps in the official Google Play market that made charges for fee-based services without the knowledge or permission of users. The apps were downloaded as many as 4.2 million times. Google quickly removed the apps after the researchers reported them, but within days, apps from the same malicious family were back and infected more than 5,000 devices.
The apps, all from a family of malware that security firm Check Point calls ExpensiveWall, surreptitiously uploaded phone numbers, locations, and unique hardware identifiers to attacker-controlled servers. The apps then used the phone numbers to sign up unwitting users to premium services and to send fraudulent premium text messages, a move that caused users to be billed. Check Point researchers didn't know how much revenue was generated by the apps. Google Play showed the apps had from 1 million to 4.2 million downloads.
ExpensiveWall—named after one of the individual apps called LovelyWall—used a common obfuscation technique known as packing. By compressing or encrypting the executable file before it's uploaded to Play, attackers can hide its maliciousness from Google's malware scanners. A key included in the package then reassembled the executable once the file was safely on the targeted device. Although packing is more than a decade old, Google's failure to catch the apps, even after the first batch was removed, underscores how effective the technique remains.