Theoretically, it is possible. No antivirus can recognize every single malicious program. Security software only detects objects included in its algorithms - and nothing more. Solutions featuring neural networks and heuristic engines are not an exclusion, for they also work following certain settings. And if hackers detour those settings, they can potentially create undetectable malware. It can be adware, ransomware, or whatever. The good news is that it is unlikely that someone would get busy creating such an elusive example of adware. The existing adware is constantly installed and removed by thousands of users, and the agencies that spread it still get their reward.