Cybersecurity researchers have identified an ad fraud and malvertising operation called Trapdoor, targeting Android users with 455 malicious applications and 183 command-and-control domains. Users often download these disguised apps, which initiate malvertising campaigns and lead to further downloads of malicious applications. At its peak, Trapdoor generated 659 million bid requests daily, with over 24 million downloads of the associated apps, primarily from the United States. The operation exploits install attribution tools to activate malicious activities only for users acquired through fraudulent ad campaigns, while suppressing such behavior for organic downloads. Trapdoor employs advanced evasion techniques, including obfuscation and impersonation of legitimate software, to avoid detection. Google has removed the identified malicious apps from the Play Store in response to the threat.