An adware campaign supported by as many as 182 mobile apps managed to push advertisements to millions of users on Google Play and third-party Android store. The software would function as expected by the user, for at least 30 minutes; then the malicious code would kick in blasting full-screen advertisements that could not be closed immediately. Trend Micro discovered the campaign in mid-June this year and alerted Google. Only eight of the apps had been installed more than nine million times before they were finally removed.
Source: https://www.bleepingcomputer.com/news/security/android-apps-with-millions-of-installs-deceptively-pushed-ads/