Remember when we learned it was theoretically possible for a webpage — or app — to steal your processor cycles to mine cryptocurrency, potentially draining your battery and cellular data in the process? BuzzFeed reports that ad networks have figured out a similar scam — one that lets lucrative, power-hungry video advertisements hide behind traditional banner ads in Android apps, so users don’t even know they’re there.

According to BuzzFeed, it’s not app developers to blame — they were surprised to find an influx of complaints about why their apps are draining users’ batteries and eating up more than their fair share of data. Instead, the report suggests that the ad networks they’d signed up with had been hijacked by fraudsters within the larger ad business. (BuzzFeed traced the ads to a company called OutStream Media, a subsidiary of Aniview; Aniview says it runs a self-service platform and is not to blame, but BuzzFeed paints a picture of Aniview as a seemingly sketchy organization.)

The scam isn’t just at the expense of consumers, but also ad networks too, as the scammers buy up cheap banner spots and fill them with expensive video ads, profiting in the process.

The technique is a form of “ad stacking” or sometimes “ad stuffing” — though fraudsters can stack or stuff other, non-video kinds of ads too — and ad fraud companies who spoke to BuzzFeed say they see “tens of millions of dollars’ worth” of these bad ads every month.

You can read BuzzFeed’s full investigation here.

Correction, March 22nd at 11:02 AM ET: While we (and BuzzFeed) originally said the bad ads were known as “in-banner video ads,” legitimately purchased in-banner video ads also exist. “Ad stacking” might be a better term for this fraud; thanks to the commenter who pointed this out.