A newly discovered Android malware that managed to infect at least 50 applications in Google Play has been downloaded between 1 million and 4.2 million times, Check Point researchers warn.
Dubbed ExpensiveWall, the threat was designed to send fraudulent premium SMS messages and to charge users’ accounts for fake services without their knowledge.
The total number of affected users, Check Point says, could be between 5.9 million and 21.1 million, as ExpensiveWall iw a variant of malware found in Google Play earlier this year. Unlike previous iterations, however, the new sample uses advanced obfuscation techniques to evade Google Play’s built-in anti-malware protections.
The first time the malware was detailed was in January 2017, when McAfee warned that a highly popular app called “I Love Filter” was in fact an SMS Trojan. The security researchers discovered that someone infected the free legitimate app Retro Live and that the Trojan would charge users via SMS messages while also leaking device and user information such as phone number, GPS location, installed apps, and IP address.
In a technical report describing the threat, Check Point reveals that this first variant of the malware wasn’t obfuscated. The security firm also notes that, while ExpensiveWall represents the obfuscated variant of the malware, there is also a third version that only contains the malicious code, but isn’t active.
“After analyzing different samples of the malware, Check Point mobile threat researchers believe ExpensiveWall is spread to different apps as an SDK called ‘gtk’, which developers embed in their own apps,” the researchers note.
ExpensiveWall was first observed on August 7, 2017. Check Point informed Google on it and the reported samples were removed from the store. Within days, another sample infiltrated Google Play and was downloaded more than 5,000 times before being removed.
The malware was designed to register victims to premium services without their knowledge, thus generating profits for its operators. However, the same infrastructure could easily be modified by other malware to spy on victims by capturing photos and recording audio, and even to steal sensitive data and send it to a command and control (C&C) server.
“Since the malware is capable of operating silently, all of this illicit activity takes place without the victim’s knowledge, turning it into the ultimate spying tool,” Check Point argues.
Once installed, ExpensiveWall requests several common permissions, such as Internet access, and SMS permissions. Given that many apps might request similar permissions, most users might grant them without questioning the app’s intentions, especially when installing from Google Play.
The malware then sends device data to its C&C server, including location and unique identifiers, such as MAC address, IP addresses, IMSI, and IMEI. Each time the device is switched on or its connectivity changes, the threat connects to the C&C to receive a URL.
The received page contains “malicious JavaScript code that can invoke in-app functions using JavascriptInterface, like subscribing them to premium services and sending SMS messages. The malware initiates the JavaScript code by silently clicking on the links in the webpage, in the same way it clicks on ads in other occasions,” the researchers explain.
After obtaining the device’s phone number, the malware uses it to subscribe the user to different paid services. While in some cases the activity is performed without notifying the user, in others the user is asked to click a “Continue” button to activate the subscription or send a premium SMS.
CheckPoint has provided a list of all infected applications detected to date. Users who might have installed any of them should manually uninstall the apps from their devices. Although Google removed the impacted software from Google Play, the infected devices haven’t been cleaned.
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