Extensibility




Internet Explorer exposes a set of Component Object Model (COM) interfaces that allows add-ons to extend the functionality of the browser. Extensibility is divided into two types: Browser extensibility and content extensibility. Browser extensibility involves adding context menu entries, toolbars, menu items or Browser Helper Objects (BHO). BHOs are used to extend the feature set of the browser, whereas the other extensibility options are used to expose that feature in the user interface. Content extensibility adds support for non-native content formats. It allows Internet Explorer to handle new file formats and new protocols, e.g. WebM or SPDY. In addition, web pages can integrate widgets known as ActiveX controls which run on Windows only but have vast potentials to extend the content capabilities; Adobe Flash Player and Microsoft Silverlight are examples. Add-ons can be installed either locally, or directly by a web site.

Since malicious add-ons can compromise the security of a system, Internet Explorer implements several safeguards. Internet Explorer 6 with Service Pack 2 and later feature an Add-on Manager for enabling or disabling individual add-ons, complemented by a "No Add-Ons" mode. Starting with Windows Vista, Internet Explorer and its BHOs run with restricted privileges and are isolated from the rest of the system. Internet Explorer 9 introduced a new component – Add-on Performance Advisor. Add-on Performance Advisor shows a notification when one or more of installed add-ons exceed a pre-set performance threshold. The notification appears in the Notification Bar when the user launches the browser. Windows 8 and Windows RT introduce a Metro-style version of Internet Explorer that is entirely sandboxed and does not run add-ons at all. In addition, Windows RT cannot download or install ActiveX controls at all; although existing ones bundled with Windows RT still run in the traditional version of Internet Explorer.

Internet Explorer itself can be hosted by other applications via a set of COM interfaces. This can be used to embed the browser functionality inside a computer program or create Internet Explorer shells.

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