A massive ad fraud and click fraud operation dubbed SlopAds ran a cluster of 224 apps, collectively attracting 38 million downloads across 228 countries and territories.
"These apps deliver their fraud payload using steganography and create hidden WebViews to navigate to threat actor-owned cashout sites, generating fraudulent ad impressions and clicks," HUMAN's Satori Threat Intelligence and Research Team said in a report shared with The Hacker News.
The name "SlopAds" is a nod to the likely mass-produced nature of the apps and the use of artificial intelligence (AI)-themed services like StableDiffusion, AIGuide, and ChatGLM hosted by the threat actor on the command-and-control (C2) server.
The company said the campaign accounted for 2.3 billion bid requests a day at its peak, with traffic from SlopAds apps mainly originating from the U.S. (30%), India (10%), and Brazil (7%). Google has since removed all the offending apps from the Play Store, effectively disrupting the threat.
What makes the activity stand out is that when a SlopAds-associated app is downloaded, it queries a mobile marketing attribution SDK to check if it was downloaded directly from the Play Store (i.e., organically) or if it was the result of a user clicking on an ad that redirected them to the Play Store listing (i.e., non-organically).
The fraudulent behavior is initiated only in scenarios where the app was downloaded following an ad click, causing it to download the ad fraud module, FatModule, from the C2 server. On the other hand, if it was originally installed, the app behaves as advertised on the app store page.
"From developing and publishing apps that only commit fraud under certain circumstances to adding layer upon layer of obfuscation, SlopAds reinforces the notion that threats to the digital advertising ecosystem are only growing in sophistication," HUMAN researchers said.
"This tactic creates a more complete feedback loop for the threat actors, triggering fraud only if they have reason to believe the device isn't being examined by security researchers. It blends malicious traffic into legitimate campaign data, complicating detection."
The FatModule is delivered by means of four PNG image files that conceal the APK, which is then decrypted and reassembled to gather device and browser information, as well as conduct ad fraud using hidden WebViews.
"One cashout mechanism for SlopAds is through HTML5 (H5) game and news websites owned by the threat actors," HUMAN researchers said. "These game sites show ads frequently, and since the WebView in which the sites are loaded is hidden, the sites can monetize numerous ad impressions and clicks before the WebView closes."
Domains promoting SlopAds apps have been found to link back to another domain, ad2[.]cc, which serves as the Tier-2 C2 server. In all, an estimated 300 domains advertising such apps have been identified.
The development comes a little over two months after HUMAN flagged another set of 352 Android apps as part of an ad fraud scheme codenamed IconAds.
"SlopAds highlights the evolving sophistication of mobile ad fraud, including stealthy, conditional fraud execution and rapid scaling capabilities," Gavin Reid, CISO at HUMAN, said.
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