More than 200 "freeware" apps have been discovered in Apple and Google Play app stores that generate millions of dollars in fraudulent revenue for developers.

Have you ever downloaded an app that promises a free subscription but ends up charging you a hefty fee? According to Avast researchers who released their results yesterday (March 24), this is not a sporadic occurrence affecting a small percentage of users.

Rather, freeware is a scourge that ravages the best Android phones and the best iPhone app stores. Avast researchers list 204 freeware apps with an estimated total of over 1 billion downloads and more than $400 million in revenue.

"Freeware" is software that promises something for free but charges hidden high fees. While other types of malicious apps, such as adware and spyware, infiltrate devices to generate ad fraud revenue or steal user data, freeware is cunning in that it entices users to download software before charging them astronomical subscription fees

They are also cunning in that they entice users to download the software before charging them astronomical subscription fees.

They are also more or less legal, although clearly unethical. Subscriptions to many of these apps run into the hundreds of dollars per year, and according to avast, the highest annual fees can exceed $3,000.

There are a variety of mechanisms through which freeware tries to hook subscribers, starting with the lure of a free trial, but most of these apps over- and underestimate expectations about their services.

According to the avast report, such seemingly benign apps "lock users into recurring high subscription fees and generate significant revenue for developers.

Also, users may forget to cancel their free trial, resulting in more expensive fees."

The waters are further muddied by fake reviews that falsely reinforce the legitimacy of rogue apps.

"This tactic undermines the user's ability to make an informed decision about the application," avast states.

Children have also been victimized, frequently downloading seemingly innocuous apps only to have their parents discover extortionate charges on their bank statements "weeks or months later."

Furthermore, avast states that "as part of FreeSware's strategy, it appears to be targeting young adults through playful themes promising 'free installs' or 'free downloads' and catchy ads on popular social networks."

"The company has been working with parents for years.

"By the time parents notice the weekly payments, the freeware may have already drawn down a significant amount of money," the report added.

Avast lists the freeware apps it has found: 134 freeware apps in the Apple App Store and 70 freeware apps in the Google Play Store.

Many of the apps offer horoscopes and palm readings, simple photo filters and music creation effects, lessons, and PDF document scanners/QR code readers. Avast researchers have found that apps often promise a 3-day free trial before starting a subscription.

To measure app profitability, avast used estimates from app industry analyst Sensor Tower. Sensor Tower estimates that Android apps have over 500 million downloads and generate approximately $38.5 million in revenue.

iOS apps made even more money. iOS apps were also downloaded 500 million times, but the estimated revenue was nearly ten times that, which Sensor Tower calculated at $365 million.

Sensor Tower told avast that these apps are "actively advertised on major social networks such as Facebook, Instagram, Snapchat, and TikTok."

We checked several apps from both of avast's lists, and all were still available for download as of this writing. Avast has notified Apple and Google of these apps and asked them to review them for violations of the app store's terms of service.

If you have been duped by a freeware app, your options for getting your money back are limited. Google and Apple are not liable for refunds after a certain period of time.

Even though legitimate companies often refund exorbitant fees out of the goodness of their hearts, for example, if a child's purchase of an in-game item results in a huge credit card bill, the company has no such obligation and people must involve their bank for chargeback processing They may have to.

Fleecewear is a very tricky issue. While it is clearly very unethical, it is a legal gray area. One should also be aware of the resilience of apps, as many of these apps continue to charge the user's credit card even after the app is removed from the user's phone.

You can learn more about how to best manage subscriptions on the Apple and Google support pages. Until then, keep an eye out for anything suspicious happening in the Wild West of mobile apps.

MORE: the best free Android apps