Security and ActiveX controls overview
ActiveX controls, also known as add-ons, plug-ins, browser extensions, or Browser Helper Objects, can improve your site visitor's experience by providing multimedia or interactive content. Examples of ActiveX controls include the browser plug-ins for Apple QuickTime, Adobe Reader, and Real Network RealPlayer. ActiveX controls are used extensively on the Internet, and generally make web browsing more enjoyable. In some cases, these controls can be used to collect information from your computer in ways you might not approve of, possibly damage information on your computer, install software on your computer without your consent, or allow someone else to control your computer remotely. Given these risks, you should only install ActiveX controls or add-ons if you completely trust the publisher and the website offering it.
Microsoft Expression Web supports the ActiveX controls that run Silverlight Movie, Silverlight XAML, Windows Media, and Flash SWF files. To support any other type of ActiveX control or an unsigned ActiveX control, you must enable Expression Web to run the control on your computer. You should be cautious about running ActiveX controls on your computer, even if they have a valid digital signature. Some ActiveX controls can cause your computer to stop responding or display content you don't want, such as pop-up ads.
An ActiveX control that is unsigned is a control that does not have a valid digital signature. A digital signature helps to verify the identity of the publisher of an ActiveX control and indicates that the file has not been tampered with since it was signed. By default and for your security, Expression Web does not run unsigned ActiveX controls and does not prompt you for permission to run them. If you want to run an unsigned ActiveX control, you can set security options to enable Expression Web to prompt you for that permission.
See also
Concepts
Run unknown or unsigned ActiveX controls
Set security options for ActiveX controls
Insert and edit Flash (.swf), Windows Media, and other media files