- Aug 17, 2014
- 11,115
This year we have seen many different malware campaigns trying to compromise users with malicious apps distributed via Google Play. Even though these apps are often removed within days after having been reported to Google, they still manage to infect thousands of users. All apps submitted to Google Play are automatically analyzed in an effort to block malicious applications, but the latest campaigns we have seen use techniques such as legitimate applications containing malicious behavior on a timer (in this case two hours) in order to circumvent Google Play’s automated detection solutions.
This article is based on joint research we have conducted with Avast and SfyLabs, who have also published their respective blog articles on the topic.
In October and November 2017 we ran into two new campaigns using droppers in the Play Store — the first campaign to drop the banking malware. This second campaign has recently been described on this site; we are adding some additional IoCs at the end of this blog article.
The droppers from the previous campaigns were far more sophisticated, using Accessibility Services to perform clicks in the background and enable app installation from unknown sources. This new dropper does not have such trickery and relies on the user having unknown sources already enabled. If this is not the case, the dropper will fail to install the BankBot malware resulting in no threat to the user. If installation from unknown sources is enabled, the user will be prompted to install the BankBot malware. This malware seems to be pretty much the same as the instance Trend Micro blogged about in September.
Interestingly enough, even though the Tornado FlashLight dropper (com.andrtorn.app) has been removed from Google Play, it is not detected by Google’s Play Protect. The same goes for the malware that is dropped by the dropper (com.vdn.market.plugin.upd). This means the dropper app and malware can still be installed from third-party locations and run without interference, unless the device is running suitable security software.
Detailed analysis
When the dropper is first started, it will check the installed applications against a hardcoded list of 160 apps. We’ve only been able to identify 132 of them, since the package names are not included in the dropper, but just their hashes. The list of targeted packages has remained the same since the campaign described by Trend Micro. If one or more of the targeted apps are installed when the dropper app is closed, it will start the service with dropper functionality.
The dropper will run the same check on device boot and if it succeeds it will also start the service. The service will first request administrator permissions from the user and after obtaining those it will continue to the download routine. The BankBot APK, which is the same for all dropper samples is downloaded from hxxp://138.201.166.31/kjsdf.tmp. The download is only triggered two hours after device administrator rights have been granted to the dropper.
Once the download is completed, the dropper will try to install the APK, using the standard Android mechanism to install applications from outside the Google Play store. Besides requiring unknown sources to be already enabled, this install method requires the user to press a button to continue the installation.
Looking at the name and icon of the package to install, we assume the attackers are trying to make the user think it is a Google Play update. Once the install is finished, the new APK will request device administrator rights and then the attack continues.
If installation from unknown sources is not enabled, Android will show an error message and the installation will fail.
How to prevent a successful attack?
For a user, it can be difficult to figure out whether an app is malicious. First off it is always good only to install applications from the Google Play store, since most malware is still mainly spread through alternative stores. Second, unless you know exactly what you are doing, do not enable ‘unknown sources’. If you are asked to do this by an app or someone you do not trust personally, it is most likely malware-related.
But what if you want to install an app from the Google Play? For the typical user, we recommend using a security solution to catch the already detected malware that has not yet been blocked by Google. Besides installing a security solution, you can check some things yourself to decrease the risk of infection.
First, make sure the app has many users and good reviews. Most malware will not have been in the store for a very long time and will not have lot of users. Then, after you install the app, take note of several things: Most malware will ask to become device administrator (do not give this permission as it can be used to prevent being removed). Other malware may ask for accessibility service permission, which would enable it to simulate user interaction with the device, basically taking over the device. Another indicator is the app icon disappearing from your app drawer after the first time you start the app. The malware does this to hide itself. If this happens to you, it’s probably best to back up your data and do a factory reset to make sure the malware is gone.
