Researchers said that a tip from a child led them to discover aggressive adware and exorbitant prices lurking in iOS and Android smartphone apps with a combined 2.4 million downloads from the App Store and Google Play.

Posing as apps for entertainment, wallpaper images, or music downloads, some of the titles served intrusive ads even when an app wasn’t active. To prevent users from uninstalling them, the apps hid their icon, making it hard to identify where the ads were coming from. Other apps charged from $2 to $10 and generated revenue of more than $500,000, according to estimates from SensorTower, a smartphone-app intelligence service.

The apps came to light after a girl found a profile on TikTok that was promoting what appeared to be an abusive app and reported it to Be Safe Online, a project in the Czech Republic that educates children about online safety. Acting on the tip, researchers from security firm Avast found 11 apps, for devices running both iOS and Android, that were engaged in similar scams.