From labs.bitdefender.com
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.