App developer Johnny Lin last week analyzed the App Store to reveal which apps generate the most revenue. While his search found that some big name services like Dropbox and Evernote were among the leaders, it also discovered that a relatively obscure app from a little-known developer named Ngan Vo Thi Thuy was ranked No. 10 among the top 10 highest-grossing apps in the App Store. According to app-tracking service Sensor Tower, that app, called “Mobile protection :Clean & Security VPN” (sic), generates $80,000 monthly in revenue.

Intrigued about what made that app so widely used, Lin, who wrote about his findings on Medium, did some research about the app and also used it. He found that it engages in what he calls “duplicitous behavior” by asking users to share their contacts with it. As he continued to use the app, Lin discovered that it wanted to charge him $99.99 for a seven-day subscription to its virus scanner. If he had agreed—something many iOS users seem to be doing—he would have paid $400 a month for access to the app’s virus scanner.

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The app, which is still available in the App Store, describes itself as a security program. However, Lin notes that as a VPN, or virtual private network, it can also serve to funnel Internet traffic. In other words, users are sending all of their Internet traffic through the app.

While that isn’t necessarily a bad thing, there are many similar apps in the App Store from better-known sources that cost less than $100 per week. Still, it’s important to note that as of this writing, Mobile protection :Clean & Security VPN has a three-and-a-half star rating from users. Some of those users say the app makes the iPhone “work better” and “faster.” Others “highly” recommended the app. The authenticity of those reviews could not be verified.

Lin, who was suspicious of the app, then examined how it and similar programs attract users and generate so much revenue. He said that the apps use Apple’s App Store search ads to move to the top of the search results when users input relevant queries.

“Turns out, scammers are abusing Apple’s relatively new and immature App Store Search Ads product,” Lin wrote. He added that the “ads look almost indistinguishable from real results, and some ads take up the entire search result’s first page.”

Apple (AAPL) unveiled the search ads at last year’s Worldwide Developers Conference (WWDC). They are clearly marked with a blue “ad” tag, but have a similar design to the apps listed in the search results, which could confuse some users.

Apple did not respond to a Fortune request for comment about the report. The developer of Mobile protection :Clean & Security VPN could not be found for comment.

For its part, Apple has long marketed apps in its App Store as being well-vetted. All apps submitted to the store are checked by Apple to ensure they’re safe, useful, and protect user information. Of course, Mobile protection :Clean & Security VPN and other apps Lin is concerned about, made it through that vetting process to get into the App Store.

Whatever the case, it’s clear Apple’s App Store is a potential money-maker for developers. At its annual WWDC event last week in San Jose, Calif., Apple said that it had paid $70 billion to developers in earnings for people buying their apps and from other revenue generated through the store since its debut in 2008.