Downloading interesting, informative, or entertaining "content" from a remote site to a user's local machine is central to the activity of Web browsing (or "net surfing"). The content getting the most attention from Web users and the greatest concern from security experts is executable content, code to be executed on the local machine on download. This executable content may provide live audio of a conference in progress, a jazz tune, three-dimensional (3-D) animation effects, or hostile code that destroys the local file system. Executable code is authored using one or more Web-related programming or scripting languages designed specifically for the production of platform-independent executable content. Languages in this category include JAVA and ActiveX. Executable content is called an "applet" in JAVA and a "control panel" in ActiveX. Web-related programming languages pose new security challenges and concerns because code is downloaded, installed, and run on a user's machine without a review of source code (the recommended practice for secure use of publicly available software). These activities can be triggered by following any hypertext link or opening any page while browsing. A user may not even be aware that code has been downloaded and executed. Some Web-related programming languages, most notably JAVA, have built-in security features, but security experts are concerned about the adequacy of these features. As executable content makes Web browsing even more alluring, further research in software engineering and greater user awareness will be necessary to counter security risks. Presently, the security of executable content depends upon the correctness of multiple vendors' implementations, the inherent security of platform-independent "virtual machines," and the safety of the source code that is executed. In the foreseeable future, users need to be educated about the risks so they can make informed choices about where to place their trust.
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