Ad stacking, a prevalent form of mobile ad fraud, involves layering multiple ads atop one another within a single ad placement. Although only the top ad is visible to users, advertisers are charged for all ads in the stack that receive clicks or impressions. This results in fake engagements, with advertisers paying for interactions that don’t actually occur. Ad stacking is especially problematic for Cost Per Mille campaigns and is viewed as click spam in click-based campaigns. This deceptive practice diverts ad budgets from legitimate advertisers while boosting revenue for participating publishers.