Multi-stage malware sneaking into Google Play

Another set of malicious apps has made it into the official Android app store. Detected by ESET security systems as Android/TrojanDropper.Agent.BKY, these apps form a new family of multi-stage Android malware, legitimate-looking and with delayed onset of malicious activity.

Play Protect 

Google Play Protect helps you keep your device safe and secure.

  • It runs a safety check on apps from the Google Play Store before you download them.
  • It checks your device for potentially harmful apps from other sources. These harmful apps are sometimes called malware.
  • It warns you about any detected potentially harmful apps found, and removes known harmful apps from your device.

How Google Play Protect works

Google Play Protect checks apps when you download them. It also periodically scans your device. If it finds a potentially harmful app, it stops the app from running, and may:

  • Warn you. If an app is detected that may be harmful to your device, you’ll get a notification. To remove the app, tap Uninstall on the notification.
  • Remove the app automatically. In some cases, if a harmful app has been detected, you may get a notification saying the app was removed.

Check your app security status

You can check the status of Google Play Protect on your device:

  1. Open your Android device’s Google Play Store app Google Play
  2. Tap Menu Menu and then Play Protect
  3. Look for information about the status of your device

Read more about Play Protect HERE

Welivesecurity have discovered eight apps of this malware family on Google Play and notified Google’s security team about the issue. Google has removed all eight apps from its store; users with Google Play Protect enabled are protected via this mechanism. SOURCE

Interesting thing is none of the apps in question had reached more than a few hundred downloads. Regardless, their advanced anti-detection features make this malware family interesting.

Anti-detection features

These malware samples all employ a multi-stage architecture and encryption to stay under the radar.

After being downloaded and installed, these apps do not request any suspicious permissions and even mimic the activity the user expects them to exhibit.

Along with this, the malicious app also decrypts and executes its payload – that is, the first-stage payload. This payload decrypts and executes the second-stage payload, which is stored in the assets of the initial app downloaded from Google Play. These steps are invisible to the user and serve as obfuscatory measures.

The second-stage payload contains a hardcoded URL, from which it downloads another malicious app (that is, the third-stage payload) without the victim’s knowledge. After a pre-defined delay of approximately five minutes, the user is prompted to install the downloaded app.

The app downloaded by the second-stage payload is disguised as well-known software like Adobe Flash Player or as something legitimate-sounding yet completely fictional – for example “Android Update” or “Adobe Update”. In any case, this app’s purpose is to drop the final payload and obtain all the permissions that payload needs for its malicious actions.

Once installed and having the requested permissions granted, the malicious app serving as the third-stage payload decrypts and executes the fourth-stage – and final – payload.

In all the cases investigated, the final payload was a mobile banking trojan. Once installed, it behaves like a typical malicious app of this kind: it may present the user with fake login forms to steal credentials or credit card details.

One of the malicious apps downloads its final payload using the URL shortener. Thanks to this, we were able to obtain download stats: as of November 14, 2017, the link had been used almost 3000 times with the vast majority of hits coming from the Netherlands.

Two of most recent samples of Android/TrojanDropper.Agent.BKY were caught downloading either MazarBot, a notorious banking trojan, or spyware.

Given its nature, this downloader can deliver any payload of the criminals’ choice as long as it doesn’t get flagged by the Google Protect mechanism.

How to get rid of it

If you’ve downloaded any of these apps, you need to (i) deactivate admin rights for the installed payload, (ii) uninstall the surreptitiously-installed payload and (iii) uninstall the app downloaded from the Play Store.

  • To deactivate admin rights for the installed payload, go to Settings > (General) > Security > Device administrators and search for Adobe Flash Player, Adobe Update or Android Update.
  • To uninstall the installed payload, go to Settings > (General) > Application manager/Apps and search for the particular apps (Adobe Flash Player, Adobe Update or Android Update) to uninstall them.
  • To uninstall the malicious app downloaded from the Play store, go to Settings > (General) > Application manager/Apps and search for apps going by the following names: MEX Tools, Clear Android, Cleaner for Android, World News, WORLD NEWS, World News PRO, Игровые Автоматы Слоты Онлайн or Слоты Онлайн Клуб Игровые Автоматы.

How to stay protected

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.


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