Researchers say that 14.8 percent of Android users who were targeted with mobile malware or mobile adware last year were left with undeletable files. These range from trojans that can install and run apps without the user s knowledge, to less threatening, but nevertheless intrusive, advertising apps. The number of those affected varies from 1 to 5 percent of users with low-cost devices, and reaches 27 percent in extreme cases. Kaspersky researchers say most devices harbor pre-installed default applications that are also undelisted.
Source: https://threatpost.com/android-users-undeletable-adware/157189/