AI Overview        

Popunder ads—which open in a new browser window hidden behind your current tab—are not automatically dangerous, but they carry significant security and privacy risks depending on the website they come from. 
[1] (https://www.anstrex.com/blog/the-truth-about-popunder-advertising-can-it-harm-your-seo-efforts)
.

The Real Risks

Malware and Adware: Shady ad networks can inject malicious code into popunders, leading to accidental downloads or compromised systems.
.

Phishing Scams: 

Some hidden windows redirect you to fake login pages designed to steal personal or financial details.

 Industry data shows that popunder traffic has high rates of invalid or fraudulent activity compared to standard display ads.
. 

[1] (https://infinityit.co.nz/avoiding-risks-from-popup-ads-to-secure-your-business/), 
[2] (https://maagsoft.com/popup-ads-from-annoyance-to-attack-how-to-stay-safe-online/), [
3] (https://www.youtube.com/watch?v=3Ssn2BwZqOc), 
[4] (https://www.youtube.com/shorts/6bDWj9tDKLA), [
5] (https://it.stonybrook.edu/help/kb/determining-whether-links-in-pop-up-messages-are-safe), 
[6] (https://www.linkedin.com/pulse/ad-formats-push-popunder-fraud-risks-detection-real-numbers-jbwce)

Ends

See also

https://www.bitdefender.com/consumer/support/answer/2574/  or 
https://hitprobe.com/what-is/pop-under-ads