A PDF creator app potentially served a Trojan to more than 100 million Android users via downloads on the Google Play store.

In summer 2019, the CamScanner – Phone PDF Creator app caught the attention of Kaspersky Lab. The program had generated more than 100 million downloads through the Google Play store, but in July and August, it began to receive negative user reviews suggesting the presence of unwanted features.

Upon a closer look, security researchers discovered that the app used an advertising library that contained a malicious dropper at the time of analysis. This dropper, detected by Kaspersky as Trojan-Dropper.AndroidOS.Necro.n, decrypted and executed malicious code contained within the mutter.zip file in the app’s resources. The dropper then decrypted a configuration file, revealing several locations from which it could download and then execute an additional module as its malicious payload.

After Kaspersky reported its findings to Google, the app was promptly removed from the app marketplace.

Malware Hiding on the Google Play Store

Trojan-Dropper.AndroidOS.Necro.n isn’t the only malware family that’s been found on the Google Play store. In April 2019, for instance, Check Point detected a clicker malware family, dubbed PreAMo, that generated more than 90 million downloads across six apps available on the Play store.

That was just two months before ESET discovered several apps available for download on Google’s official app marketplace that were capable of stealing one-time passwords in SMS-based two-factor authentication (2FA) messages without achieving the proper permissions. And in August 2019, Trend Micro detected adware hidden within 85 photography and gaming apps that had registered a combined total of 8 million downloads on the Google Play store.

How to Defend Against Mobile App Threats

Security professionals can help defend their organizations against threats like Necro.n by following mobile security best practices, which include keeping devices up to date with the latest software patches and restricting app downloads to only trusted developers on official app marketplaces. Companies should also use a unified endpoint management (UEM) tool to monitor all devices for suspicious activity and automatically remediate suspicious behavior.