popunder

Pop-up ads or pop-ups are forms of online advertising on the World Wide Web. A pop-up is a graphical user interface (GUI) display area, usually a small window, that suddenly appears ("pops up") in the foreground of the visual interface. The pop-up window containing an advertisement is usually generated by JavaScript that uses cross-site scripting (XSS), sometimes with a secondary payload that uses Adobe Flash. They can also be generated by other vulnerabilities/security holes in browser security.
A variation on the pop-up window, the pop-under advertisement, opens a new browser window under the active window. Pop-unders do not interrupt the user immediately, but appear when the user closes the covering window, making it more difficult to determine which website created them.

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