Security researchers have once again found Google Play offering malicious apps that have been downloaded by millions of Android users. According to a blog post published Tuesday by antivirus provider Avast, the apps include the Durak card game app and at least two other titles. Combined, those apps have been installed as many as 15 million times. Researcher Filip Chytry wrote:

When you install Durak, it seems to be a completely normal and well working gaming app. This was the same for the other apps, which included an IQ test and a history app. This impression remains until you reboot your device and wait for a couple of days. After a week, you might start to feel there is something wrong with your device. Some of the apps wait up to 30 days until they show their true colors. After 30 days, I guess not many people would know which app is causing abnormal behavior on their phone, right? Each time you unlock your device an ad is presented to you, warning you about a problem, e.g. that your device is infected, out of date or full of porn. This, of course, is a complete lie. You are then asked to take action, however, if you approve you get re-directed to harmful threats on fake pages, like dubious app stores and apps that attempt to send premium SMS behind your back or to apps that simply collect too much of your data for comfort while offering you no additional value.

It's not the first time Google's official Android app bazaar has been found to host malicious apps. In the past, it has offered titles laced with surreptitious remote access trojans, Bitcoin miners, and rogue advertising networks. Three years ago, Google introduced a cloud-based scanner that scours Play for malicious apps, but attackers have been known to bypass it.

Google officials regularly remove apps from Play when they are found to be malicious. At the time this post was being prepared, all three flagged by Avast remained available for download.

There's no way to know for sure an app available in Play isn't malicious. Choosing titles with large numbers of downloads developed by well-known developers is a good strategy, but even then, attackers have been known to create malicious programs that masquerade as popular ones. And given the number of installs for Durak, IQ test, and the history app, millions of installations is no guarantee either.

Update: About six hours after this post went live, a Google spokeswoman e-mailed to say the three apps have been pulled from Play.