From http://msdn.microsoft.com/en-us/library/cc288472(v=vs.85).aspx#search
Clickjacking Defense: Some hackers try to trick users into clicking buttons that appear to perform safe or harmless functions, but instead perform unrelated tasks. Clickjackers embed malicious code or "redress" the user interface by using transparent frames that overlay specific UI elements with misleading text and images. To help prevent clickjacking, Web site owners can send an HTTP response header named X-Frame-Options with HTML pages to restrict how the page may be framed.
X-Frame-Options: Deny
If the X-Frame-Options value contains the token Deny, Internet Explorer 8 prevents the page from rendering if it is contained within a frame. If the value contains the token SameOrigin, Internet Explorer will not render the page if the top level-browsing-context differs from the origin of the page containing the directive. Blocked pages are replaced with a "This content cannot be displayed in a frame" error page.
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