Asian cybercriminals have figured out an unusual way to use the architecture of a mobile ad network to siphon money from their victims.
The new method represents another step in the evolution of mobile malware, which is booming with more smartphones shipping than PCs. Mobile ad networks open up the perfect backdoor for downloading code.
"It's a very, very clean infection vector," said Wade Williamson, a senior security analyst at Palo Alto Networks who discovered the new trickery.
In legitimate partnerships between ad distributors and developers, the latter embeds the former's software development kit (SDK) into the app, so it can download and track ads in order to split revenue.
Unfortunately, how well developers vet the ad networks they side with varies from one app maker to another. If the developer does not care or simply goes with the highest bidder, then the chances of siding with a malicious ad network is high.
Wiliamson found one such network's SDK embedded in legitimate apps provided through online Android stores across Asian countries, such as Malaysia, Taiwan and China. Once installed, the SDK pulls down an Android application package file (APK) and runs it in memory where the user cannot easily discover it.
The APK typically waits until another app is being installed before triggering a popup window that seeks permission to access Android's SMS service.
"It doesn't have to go through the whole process of doing a full install," Williamson said. "It just sits there and waits on the smartphone to install something else and then piggybacks in."
Once installed, the APK takes control of the phone's messaging service to send text to premium rate numbers and to download instructions from a command and control server. The majority of Android malware today, 77 percent, wring money from victims through paid messaging services, said Juniper Networks' latest mobile threat report.
Williamson has seen more than a half dozen samples of the latest malware, which he believes is coming from one criminal group, while acknowledging multiple groups is possible.
Android users in Asia and Russia are more susceptible to Android malware, because many apps are downloaded from independent online stores. In the U.S., most Android users take apps from the Google Play store, which scans for malware and malicious ad networks.
Because of the effectiveness of the latest malware, Williamson expects criminals in the future to use the same scheme to download more insidious malware capable of stealing credentials to online banking and retail sites where credit card numbers are stored.
The same pathway could also be used to steal credentials for entering corporate networks.
"As soon as you have a vector like this, the difference between creating malware that sends spoof SMS messages versus looks for the network and tries to break in is just malware functionality," Williamson said.