Earlier we talked about some Flappy Bird-related threats. In the course of uncovering their background, we found several third-party app stores that distributed or created similarly dangerous mobile apps.
These third-party app stores target mobile users in Vietnam and inject advertising or even malicious code into popular apps. These apps put the user’s privacy at risk, and may even cause financial loss – the recent Trojanized Flappy Bird app used premium service abuse to profit, and also connected to a command-and-control server in order to receive commands.
This example of a third-party app store imitates the Google Play store and contains various well-known apps that have been Trojanized. Even a fake version of the Google Play app itself is present, but it leads to their own third-party store.
Figure 1. Example of imitated Google Play’s page
The apps in this store contain added advertising code; the profit from these ads goes to the cybercriminals and not the app developers. Among the information sent is the user’s phone number, email address, and device information.
Figure 2. Advertising information
In addition, this advertising module may remotely load code to be executed on the device, effectively acting as a backdoor. This poses a great risk for users.
Figure 3. Backdoor code
Apps with this malicious code are detected as ANDROIDOS_FLEXLEAK.HBT.
Another third-party store was even riskier – this single store contained more than 500 OPFAKE malware variants. One of the malicious Flappy Bird apps was downloaded from this store. Not only do they contain the potentially malicious advertising code, they also abuse premium service numbers in order to get money directly from the user.
Adult apps are also present on this store, with the users having to pay via SMS to use these apps.
Figure 4. Second malicious third-party store
A third app store has similar threats as the other stores mentioned in this post. This one, however, has higher download counts (more than 70,000 downloads).
Figure 5. Third malicious third-party store
These incidents highlight the possible dangers from downloading apps from third-party stores. Users often visit third-party app stores to obtain apps that may be unavailable in official app stores or even pirated apps (like free versions of paid apps). Some users, meanwhile, rely on these sites because of the unavailability of official app stores in their region.
However, visiting these sites can often be a hit-or-miss. Third-party app sites may not be as strict in monitoring and removing malicious apps compared to, say, Google Play. Apps from these third-party sites should be treated as potentially malicious, as a user has no easy way to determine what malicious code was added.
We detect all the apps listed in these stores that contain malicious content or may violate a user’s privacy.