A Web browser plug-in is a software component that interacts with a Web browser to provide capabilities or perform functions not otherwise included in the browser. It provides a way for certain types of content to be displayed or executed through a Web browser.
Attackers sometimes create malicious plug-ins that act as spyware. When installed in a browser, these plug-ins can monitor all use of the browser, such as which websites and pages a user visits, and report the use to an external party. Because plug-ins are loaded automatically when a Web browser is started, they provide an easy way to monitor Web activity on a system. Some malicious Web browser plug-ins are spyware dialers, which use modems to dial phone numbers without the user's permission or knowledge. Many of the dialers are configured to call numbers that have high per-minute charges, while others make nuisance calls to numbers such as emergency services (i.e., "911").