Yes, pop-under browser windows have been extensively abused by porn sites and related networks, primarily for **ad fraud** and **malware distribution**.
Unlike standard pop-ups that appear immediately, **pop-unders** open a new browser window *behind* the user's active tab. Because these windows often remain hidden while the user continues browsing the main site, they are particularly effective for malicious activities that go unnoticed until the user accidentally clicks on the hidden window or closes the browser, revealing it https://www.reddit.com/r/explainlikeimfive/comments/2q807s/eli5_why_do_porn_sites_allow_their_popup_ads_to/.
### How They Are Abused
* **Ad Fraud and "Zombie" Traffic:**
Unscrupulous ad networks and publishers use pop-unders to generate massive amounts of fake traffic. When a user clicks anywhere on a porn site (even unintentionally), the site triggers a pop-under that automatically loads a chain of other websites in the background. These sites, often called "zombie sites," cycle through thousands of page views and ad impressions without any human interaction https://www.reddit.com/r/explainlikeimfive/comments/2q807s/eli5_why_do_porn_sites_allow_their_popup_ads_to/. This traffic is then sold to legitimate advertisers, who pay for views from users who never actually intended to visit those sites. This technique has been traced from porn sites to mainstream platforms, including esports streaming services and news outlets, allowing fraudsters to "launder" traffic so it appears to come from legitimate sources https://www.reddit.com/r/explainlikeimfive/comments/2q807s/eli5_why_do_porn_sites_allow_their_popup_ads_to/.
* **Malware Distribution:**
Pop-unders are a common vector for delivering malware. Once the hidden window opens, it can force the download of malicious software, such as adware, browser hijackers, or ransomware, often disguised as fake video players or system updates https://www.malwarebytes.com/blog/threats/browser-hijacker https://www.pcrisk.com/removal-guides/20278-device-infected-after-visiting-an-adult-website-pop-up-scam-mac. Because the window is hidden, users may not realize their device has been infected until they experience performance issues or see unwanted ads appearing elsewhere https://www.avast.com/c-browser-hijacker-and-how-to-remove-it.
* **Cookie Stuffing and Tracking:**
These hidden windows can also drop tracking cookies onto a user's device without their knowledge. This results in users seeing targeted ads from dating or adult sites even if they never explicitly visited those specific pages, leading to confusion about their browsing history https://www.justanswer.com/computer/837ui-pop-ups-leave-cookies-behind-computer.html.
### Industry Response
Due to the severity of these abuses, major advertising platforms like **Google** have explicitly banned the use of pop-unders in their AdSense programs. Their policy states that ads may not appear on pages loaded as pop-ups or pop-unders, nor on any site that triggers them https://www.reddit.com/r/explainlikeimfive/comments/2q807s/eli5_why_do_porn_sites_allow_their_popup_ads_to/. Security firms like Malwarebytes and Avast consistently identify pop-unders from adult and piracy sites as a primary source of browser hijackers and adware infections https://www.malwarebytes.com/blog/threats/browser-hijacker https://www.avast.com/c-browser-hijacker-and-how-to-remove-it.
To protect yourself, using a browser with robust pop-up blocking (like Brave) and reputable anti-malware software is highly recommended, as these tools can prevent the hidden windows from opening or executing malicious scripts in the first place.
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