Why is a user prompted for authentication when opening an Office document from a web server?

When Internet Explorer opens an Office document, it spawns the appropriate Office application passing the URL to the document.  The Office application then will attempt to access the document directly from the server.  This is different from other browsers and other file types – most browsers download the file and have the application open it from the local cache.  However, when this is done, if the opened file is changed and saved, the changes are only made to the local copy and not to the server copy.
Client can apply the below workaround to get lessen the impact

1. Leaving the application open –  the user’s can reduce the impact of multiple prompts by leaving the application open after the first access is made.  By closing only the document instead of the application another document using the same application can be opened without prompting for credentials since the executable has already been authenticated.  Documents of different types requiring a different application to edit them will require a prompt for each new application that needs to access the site.

2. Select “remember the password” – The credential prompts can also be less annoying if the user selects to remember the password.  The saving of the password will not eliminate the prompt but will prepopulate it with the information so that only a single click/keystroke is needed to respond.  The site should be added to the Trusted sites zone if this approach is used.

3. Pre-configure the OPTIONS result – The OPTIONS call is not executed if the information has been previously obtained and cached in the Office Protocol Discovery Cache.

Important – It is not recommended to edit the registry key or values directly.  The easiest way to prepopulate the information is to copy and distribute the information after it has been successfully populated on a test machine.  The saved information is temporary since Office clears the cache periodically – by default it will expire after a couple weeks although it can be extended.  An update was created prior to and included in Office 2003 Service Pack 3 (also available in Office 2007) that allows the extension of this time up to 10 years (see KB 916658).  The downside of using this registry entry is that if the Web authoring protocol or server type changes then Office 2003 will still assume the old information is valid.

The Office Protocol Discovery Cache contains sub-key entries for each Web folder that is opened and that has successfully responded to an OPTIONS request.  The registry key used by Office 2003 is:

HKEY_CURRENT_USERSoftwareMicrosoftOffice11.0CommonInternetServer Cache

Note: Office 2007 uses a similar key but with 12.0 instead of 11.0.

The maximum number of cache entries may be set by the MaxCount registry value under the same Server Cache key.  Office removes old entries to make space if the maximum count is reached. If no space can be cleared, the results of the OPTIONS call are not cached.  Pre-configuring the OPTIONS response may not eliminate all prompts; a PROPFIND request can also result in a prompt for authentication.

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