||Many popup windows contain advertising, but they are increasingly being used as a way to attack computers. Some popup windows are crafted to look like legitimate system message boxes or Web sites, and can trick users into going to phony Web sites, including sites used for phishing, or authorizing changes to their computers, among other malicious actions. For example, a popup window may tell a user that the computer is infected with spyware and to click on OK to disinfect it. By clicking on OK, the user unwittingly permits spyware or other types of malware to be installed on the computer.
To control popup windows, various third-party popup blocking utilities were created. Most Web browsers also offer popup blocking capabilities. Generally, these utilities and features prevent popup windows from opening, and indicate to the user that a popup window was blocked. If the user did not want the window to be blocked, he can then choose to permit that particular popup window or to permit all popup windows from a trusted Web site.