Over the last several years, popular applications such as Microsoft Internet Explorer and Netscape Navigator have become prime targets of attacks. These applications are targeted because their function is to process unauthenticated network data that often carry active content. The processing is done either by helper applications, or by the web browser itself. In both cases the software is often too complex to be bug free. To make matters worse, the underlying operating system can do very little to protect the users against such attacks since the software is running with the user's privileges.