Ad stacking
Ad stacking refers to a type of mobile ad fraud in which the fraudsters "stack" or hide ads beneath the primary ad that is displayed to users.
The reason that fraudsters use ad stacking is that while only the top ad will be seen by the user, advertisers that have multiple ads stacked beneath will still have to pay for the fake impressions.
For example, if a user clicks or views the top ad and there are other ads stacked beneath, this means that a click or impression will be reported for every ad in the stack. Ultimately, by stacking ads, fraudsters are stealing ad budgets from advertisers and increasing the ad revenue for publishers that may be involved in the scheme.