Threat Actors Use Mockups of Popular Apps to Spread Teabot and Flubot Malware on Android

Spreading malware on Android devices is not easy, as the official store can usually (not always) prevent these types of apps from reaching users. But one of Android’s greatest strengths, the ability to sideload apps from non-official sources, is also a weakness.

Using a combination of tricks to persuade users to install apps outside of the official store, criminals spread most of their malware through sideloading. If mobile devices have no security solution installed, malicious apps roam free.

The TeaBot and Flubot are the newest banker trojan families, as multiple security researchers identified them in the early months of 2021. Bitdefender researchers have discovered a batch of new malicious Android applications that impersonate real ones from popular brands but with a malware twist.

TeaBot Strikes Again

TeaBot (also known as ‘Anatsa’) and its working mechanisms are known. According to an early analysis report, the malware can carry out overlay attacks via Android Accessibility Services, intercept messages, perform various keylogging activities, steal Google Authentication codes, and even take full remote control of Android devices.

Criminals welcome the opportunity to spread malware directly from app stores, but that isn’t easy. Instead, they go for the next available method –  imitating top-rated apps in the hopes of tricking at least some users into downloading and installing their malicious versions.

Bitdefender researchers have identified five new malicious Android applications that pack the Teabot banking trojan and impersonate real ones. Two of the apps are mentioned as banking malware on Twitter, and we made the connection to the Anatsa malware.

The fake apps housing the Teabot payload are based on popular apps residing on Google Play, some with as many as 50M+ downloads. It’s no surprise that criminals try to take advantage and weaponize their popularity. The developers of the official apps have no fault in this matter.

The start of this fake malicious Android apps campaign dates to the beginning of December 2020, earlier than previously identified. This was also indicated in a tweet when the original article was published.

The campaign to distribute these apps in the wild remains active. Bitdefender has identified a strange distribution method with attackers using a fake Ad Blocker app that acts as a dropper for the malware. It’s just one new distribution method. We suspect others are used, but they remain unknown for the time being.

We put together a list of all banks targeted by the Teabot, but there’s a caveat: Its operators can adapt it any given time, add more banks or remove support for some. The list is valid right now, but it’s likely to change in the future.

From Bitdefender’s telemetry, we were able to identify two new infection vectors, namely the applications with package names’ com.intensive.sound’ and ‘com.anaconda.brave’, which downloads  Teabot. These are malware dropper applications known for imitating legitimate applications (such as Ad Blocker in our case).

The fake Ad Blocker apps don’t have any of the functionality of the original version. They ask permission to display over other applications, show notifications, and install applications outside of Google Play, after which they hide the icon.

From time to time, the fake apps will show out-of-context ads and will eventually download and attempt to install Teabot, as instructed by the CnC.

This is not a surprise; offering malware download as a service is a fairly common practice in the underground malware industry.

We detect these dropper applications with Bitdefender Mobile Security as Android.Trojan.HiddenApp.AID.

This is the current distribution, with an app simply named MediaPlayer ruling the landscape. But it’s not the only one. As it turns out, the MediaPlayer.apk actually tries to impersonate one of the most famous multimedia players in the Google Play Store, named VLC. Security researchers from Cleafy were the first to identify the malware impersonating the VLC app.

The country distribution of Teabot  is already known and rather interesting, with countries  such as Spain, Italy and Netherland highly targeted.

TeaBot heatmap distribution:

FluBot is not far behind

Flubot (also known as Cabassous) is another banker family quickly gaining popularity. The heatmap shows that this family of banking trojans has been more successful in spreading internationally, predominantly in Germany, Spain, Italy and the UK.

Unlike Teabot, which is sometimes dropped by an app posing as an ad blocker, Flubot operators have a much more direct campaign, using spam SMS as a means of delivery.

While its malicious functionality is not as complex as Teabot’s (which through accessibility logging enables threat actors to monitor the device in real-time), FluBot is still a banker trojan. It steals banking, contact, SMS and other types of private data from infected devices, and has an arsenal of other commands available, including the ability to send an SMS with content provided by the CnC.

Flubot uses that command to spread, through its SMS spamming and worm-like behavior. In our analysis, we have observed over 100 different domains used in the campaign to host the fake APK files. These domains belong to hacked/hijacked sites, where the threat actors injected their malware to spread further. In many cases, these are legitimate websites and domains that criminals have successfully attacked through existing vulnerabilities, allowing them to inject download links for malware.

Some examples of sent SMS:

By looking at hundreds of SMS, we noticed that attackers use templates that only modify the recipient’s name and download link . The malware steals real contact names and phone numbers from the victim’s phone and sends them to servers hosted by the threat actors. The server composes the SMS message, using that real information and sends it back to the malware on infected phones.  Flubot then sends SMS messages directly from the victim’s device.

An observed example of such a template:

Example of Flubot SMS messages:

Like in the case of Teabot, the FluBot operators go after banks and their apps, but the list of targeted apps can change at any time, as commanded by the attackers.

FluBot imitates the following apps, among others:

This is a top 10 list of the countries where Bitdefender’s telemetry identified the largest number of samples, but FluBot’s reach extends to many others including Hungary, Japan, Ireland, Greece, Argentine, Austria, France and others.

Flubot heatmap

The file name distribution of Flubot is seen in this chart.

Indicators of compromise

Applications impersonated by Anatsa/TeaBot

FluBot/Cabassous

The research on FluBot is extensive and approximately 1600 MD5 hashes have been identified by various teams. Here are three for some of the more widely circulated app tainted with the FluBot malware. Unfortunately, there are hundreds of compromised websites and domains, many of which are legitimate. Some of them are now offline, some have fixed the intrusions and deleted the malware, so listing them separately is not a viable solution.

FluBotDroppers

The best way to avoid infection with either of these two threats is never to install apps outside the official store. Also, never tap on links in messages and always be mindful of your Android apps’ permissions. Finally, having a security solution such as Bitdefender Mobile Security installed on Android devices is recommended.