Pop-up ads or pop-ups are often forms of online advertising on the World Wide Web intended to attract web traffic or capture email addresses. Pop-ups are generally new web browser windows to display advertisements. The pop-up window containing an advertisement is usually generated by JavaScript using cross-site scripting (XSS), sometimes with a secondary payload using Adobe Flash, but can also be generated by other vulnerabilities/security holes in browser security. A variation on the pop-up window is the pop-under advertisement, which opens a new browser window hidden under the active window. Pop-unders do not interrupt the user immediately and are not seen until the covering window is closed, making it more difficult to determine which web site opened them.