Eight malicious applications hiding the new threat were found in the official application store, all legitimate-looking but delaying the malicious activity to hide their true intent. Google has removed all eight programs after being alerted of the threat.
Detected as Android/TrojanDropper.Agent.BKY, the applications form a new family of multi-stage Android malware, ESET says. Although the most popular of these apps reached only several hundred downloads, the use of advanced anti-detection features makes this malware family interesting.
All samples of the mobile Trojan employ a multi-stage architecture and make use of encryption to stay under the radar, the security researchers say. The applications managed to keep their malicious intent hidden by not requesting suspicious permissions after installation and by mimicking the activity they were supposed to exhibit.
However, the apps also decrypt and execute a first-stage payload designed to decrypt and execute the second-stage payload from the assets of the app downloaded from Google Play. These steps, however, are not visible to the user but serve as obfuscatory measures, ESET says.
The second-stage payload downloads a malicious app from a hardcoded URL without the victim’s knowledge. After a delay of around 5 minutes, however, the victim is prompted to install this third-stage payload.
This application masquerades as Adobe Flash Player or another popular app. To appear legitimate to the user, the app uses a name such as Android Update or Adobe Update to trick the user into allowing it to execute and into granting the necessary permissions for the payload to perform nefarious actions.
Once installed and with the requested permissions granted, the app decrypts and executes a final, fourth-stage payload. According to ESET, this payload was a mobile banking Trojan in all analyzed cases.
The Trojan was designed to present the victim with fake login forms to steal their credentials or credit card details.
Because one of the malicious apps downloads the final payload using the bit.ly URL shortener, the security researchers discovered that the link had been used almost 3000 times as of November 14, and that most of the hits came from the Netherlands.
Two of most recent samples of this malware downloader were observed dropping either the notorious MazarBot banking Trojan or spyware. According to ESET, the downloader’s nature allows its operators to deliver any payload through it, “as long as it doesn’t get flagged by the Google Protect mechanism.”
Impacted users are advised to first deactivate the admin rights for the installed payload, and then uninstall the surreptitiously-installed apps, along with the application initially downloaded from the Play Store.
Users should head to Settings > (General) > Security > Device administrators and deactivate the admin rights that Adobe Flash Player, Adobe Update, or Android Update might have. The installed payload can be removed from the Application manager.
The nefarious apps involved in this malicious campaign include MEX Tools, Clear Android, Cleaner for Android, World News, WORLD NEWS, World News PRO, Игровые Автоматы Слоты Онлайн, and Слоты Онлайн Клуб Игровые Автоматы.
“Unfortunately, multi-stage downloaders, with their improved obfuscation features, have a better chance of sneaking into official app stores than common Android malware does. Users who want to stay protected should not rely fully on the stores’ protections; instead, it’s crucial for users to check app ratings and comments, pay attention to what permissions they grant to apps, and run a quality security solution on their mobile devices,” ESET concludes.
