Applications that trick users into paying huge subscription fees raked in over $400 million from the Apple App Store and Google Play Store, according to research done by Avast.

The so-called “fleeceware” takes advantage of a weakness in both app stores: deleting an application tied to a subscription doesn’t cancel the subscription.

What is fleeceware?

The apps aren’t malware in the traditional sense. But they are a scam. “The purpose of these applications is to draw users into a free trial to ‘test’ the app, after which they overcharge them through subscriptions which sometimes run as high as $3,432 per year,” noted Jakub Vávra in an Avast blog post.

He urges users to pay attention when installing “musical instrument apps, palm readers, image editors, camera filters, fortune tellers, QR code and PDF readers, and ‘slime simulators,’” as fleeceware developers frequently release these types of software.

The applications can have very high ratings on the App Store or Google Play because the unscrupulous developers buy positive reviews, according to Vávra. That’s a common practice for malware of all kinds.

Deleting the app doesn’t stop the scam

Deleting an application tried to a subscription doesn’t automatically stop the subscription. That’s because someone might still want to use the service on another device. But scammers depend on this to continue to collect money from people who think getting rid of an app means they’ve escaped its reoccurring subscription fee.

The App Store asks users when deleting software tied to a subscription if they want to keep the subscription. So does Google Play. But there’s a reason why fleeceware applications are often aimed at children: they’re less likely to pay attention to warnings that they’ll be charged a fee later.

In the Avant blog post, Vávra recommends that Apple and Google stop allowing subscription fees to begin immediately after a free trial. “If the user accepts a free trial, the app could require another confirmation before paying money for the actual subscription once the free trial is over,” he said.

iPhone or iPad users can check what subscriptions they’re currently paying for by going to Settings > [Apple ID User Name] > Subscriptions. Get more details in the Cult of Mac guide on how to cancel App Store subscriptions on iPhone or iPad.