Browser hijacking is a form of unwanted software that modifies a web browser's settings without a user's permission, to inject unwanted advertising into the user's browser. A browser hijacker may replace the existing home page, error page, or search page with its own. These are generally used to force hits to a particular website, increasing its advertising revenue. Some browser hijacking can be easily reversed, while other instances may be difficult to reverse. Various software packages exist to prevent such modification.