Outlook Repeatedly Prompting for Authentication

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We’ve seen a large number of people posting in various newsgroups, web forums, mailing lists, etc., regarding a sudden change in behavior in Outlook where it starts prompting for authentication on a regular basis. Many of the ones we’ve seen have been Outlook 2007 against SBS 2008 (Exchange 2007).

We’ve found one possible source for this behavior, see if this matches your situation:

If you have installed any of the following updates on the workstations, it changes the authentication mechanism.

970430 Extended Protection for Authentication in the HTTP Protocol Stack (http.sys)
974318 Vulnerabilities in the Internet Authentication service could allow remote code execution
976325 MS09-072: Cumulative security update for Internet Explorer
971737 Description of the update that implements Extended Protection for Authentication in Microsoft Windows HTTP Services (WinHTTP)
973917 Description of the update that implements Extended Protection for Authentication in Internet Information Services (IIS)

These updates can come down automatically and they are on the assumption that you have already applied the latest rollup from Exchange which was released a while back. If you have not, then Outlook will no longer be able to retain its authentication with the Exchange.

You can download it here: You will need to restart the server afterwards

Install Update Rollup 9 for Exchange Server 2007 Service Pack 1 (KB970162)

Exchange Rollup 9 should be coming down via WSUS, if you are using that. If you are not, then it is also an automatic update from Microsoft Update. If you are using another patching mechanism be sure to include Exchange Rollups in the configuration.

Other resources will point out that Exchange SP2 also includes the updates that will address this issue, but as Exchange 2007 SP2 is a challenge to install on an SBS 2008 server, we recommend holding off on installing SP2 on an SBS 2008 server and wait for updated instructions for how to safely install that update.

Hopefully this will help some of you who haven’t found the solution on your own just yet.

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