Ads are a necessary component of modern technology, but they come with drawbacks even when they aren’t being used to scam people. Some ads in Android apps have been scamming people, though. An investigation by ad analysis firm Protected Media, and Buzzfeed News has found that a popular ad network was exploited by fraudsters to push hidden video ads in apps, which subsequently drained batteries on the phones displaying them.

Protected Media detected the fraudulent ads on Android phones, but the scheme doesn’t necessarily have anything to do with Android itself. The affected apps aren’t even to blame. Developers often use banner ads to monetize free apps, and that’s how the scammers get into your phone.

The sketchy ads were mostly running on Twitter’s MoPub platform. Each ad used a generic ad image featuring a well-known company like McDonald’s or Disney. The ads showed up in a large number of apps, but the report didn’t include exact numbers.

While the user of an app might just see a single static banner ad image, the ads would include several layers of auto-playing video ads behind that. These were hidden from the user, but the videos “played” to register as an impression. The advertiser would pay the middleman for an ad no one ever saw, and the user would be none the wiser. Well, they might have been if it wasn’t for the battery drain.

Investigators traced the scam to a company called Aniview, which denies any involvement. It says an unknown third-party associated with a now-shuttered subsidiary hijacked its tools. Whatever the situation, Aniview tracking code and video technology were discovered in the fraudulent ads. Even though the videos weren’t visible on the screen, they were still rendered by the phone. As you can imagine, playing three videos in the background takes a toll on battery performance. Users understandably blame the apps and leave negative reviews.

This particular scam has been exposed, but a recurrence is almost certain thanks to the way ads are sold. With so many layers in ad networks and advertisers, there are too many opportunities for someone to slip malicious code into an ad. There have been numerous examples of ads engaging in fraud and even distributing malware. Draining your battery isn’t nice, but at least the scam didn’t include hacking attempts.

Now read: