Description of the KnownContentTypes registry entry for Outlook Web Access in Exchange Server 2003 Service Pack 1

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INTRODUCTION

When you install Microsoft Exchange Server 2003 Service Pack 1 (SP1), a new registry entry is added for Microsoft Outlook Web Access. The KnownContentTypes registry entry specifies and therefore restricts the kind of attachment types that Microsoft Internet Explorer may open in the browser when you use Outlook Web Access. This functionality helps prevent unsafe code from running in the browser. The registry entry affects Internet Explorer versions that are equal to or later than 5.0 and that are earlier than version 5.5. By default, in these versions of Internet Explorer, attachments of an unknown MIME content-type automatically run in the browser.

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MORE INFORMATION

When you install Exchange Server 2003 SP1, the KnownContentTypes entry is added to the following registry subkey:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeWEB\OWA\

By default, the value of the KnownContentTypes entry is the following:

application/fractals,application/hta,application/mac-binhex40,application/pkcs10,application/
pkcs7-mime,application/pkix-cert,application/pkix-crl,application/postscript,application/
vnd.ms-pki.certstore,application/vnd.ms-pki.pko,application/vnd.ms-pki.seccat,application/
vnd.ms-pki.stl,application/x-cdf,application/x-gzip,application/x-internet-signup,application/
x-latex,application/x-mix-transfer,application/xml,application/x-mplayer2,application/
x-msdownload,application/x-pkcs12,application/x-pkcs7-certificates,application/x-stuffit,application/
x-tar,application/x-troff-man,application/x-zip-compressed,audio/aiff,audio/basic,audio/mid,audio/
midi,audio/mpeg,audio/wav,audio/x-aiff,audio/x-background,audio/x-midi,audio/x-mpegurl,audio/
x-wav,image/bmp,image/gif,image/jpeg,image/pjpeg,image/png,image/tiff,image/xbm,image/
x-icon,image/x-jg,image/x-png,image/x-wmf,image/x-xbitmap,message/rfc822,text/css,text/
h323,text/html,text/iuls,text/plain,text/scriptlet,text/webviewhtm,text/x-component,text/
xml,text/x-scriptlet,text/x-vcard,video/avi,video/mpeg,video/msvideo,video/x-mpeg,video/
x-mpeg2a,video/x-ms-asf,video/x-ms-asf-plugin,video/x-msvideo

Note The value has been wrapped for readability.

When you try to open a file attachment in Outlook Web Access, the attachment type is checked against the list in the KnownContentTypes entry and against the list in the Accept header. If the attachment type is not in the known list, Outlook Web Access sends back a content-type of application/octet-stream. This signals Internet Explorer not to open the attachment. Instead, Internet Explorer can either display the attachment as raw text or you can save the attachment to your hard disk.

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Behavior during reinstallation of Exchange Server 2003 SP1

If you remove any one or more of the content types in the list, they are re-added if you reinstall Exchange Server 2003 SP1.
If you add any new content types, those types are preserved if you reinstall Exchange Server 2003 SP1.

For additional information about the KnownContentTypes registry entry update in Microsoft Exchange 2000 Server, click the following article number to view the article in the Microsoft Knowledge Base:

833809 (http://support.microsoft.com/kb/833809/ ) An update is available for Exchange 2000 OWA to restrict the opening of certain attachment file types in Internet Explorer 5.x

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