Outlook 2007/2010 on Windows XP Cannot Connect to Availability Service with Exchange 2007/2010

I ran in to an interesting issue recently with some users reporting that they could not see Free/Busy information. Upon further discovery, I found out that they couldn’t use the Out-of-Office Assistant either. The immediately tipped me off that we had some sort of issue Availability/AutoDiscover.

We narrowed it down to our users using Windows XP and Outlook 2007. Our Windows 7 Users with Outlook 2010 and our Windows XP users with Outlook 2003 were unaffected.

This coincided with our deployment of some additional Client Access Servers in a new site. It seemed like it had to be related, but I couldn’t immediately think of how.

All of our Exchange 2010 AutoDiscover tests were successful, and the Best Practices Analyzer didn’t reveal anything telling. The Application Log was clear as well.

So, if our Exchange 2010 infrastructure was testing out fine, and there really wasn’t anything apparently wrong with the client PCs either, then what could be the problem?

The answer came when I tried to browse directly to the /owa virtual directory on one of the new client access servers via HTTPS. I got “Page cannot be displayed” and nothing else, even though it worked fine from Windows 7.

It turns out there is a “gotcha” when creating CSR’s with Windows Server 2008 R2.
 
When the original SSL certificate request was created for the new Client Access Servers, it was created using Windows Server 2008 R2 using the non-default “Legacy Key” custom request. This causes the private key to be stored in Microsoft’s Legacy Cryptographic API framework.
 
However, Internet Information Services, under Windows Server 2008 R2, will try to process the request using its brand new Cryptographic Next Generation (CNG) framework… which works with the Legacy Key, but has some limitations. Specifically, it will only support two AES cipher suites:

  1. TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA
  2. TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA

…and those two AES cipher’s are not supported by Windows XP by default.
 
Therefore, an SSL certificate that is the product of a Legacy Key CSR will result in an SSL failure between Windows XP and Server 2008 R2.
 
To fix it, you need to generate a new CSR using the default “CNG Key” (which supports numerous ciphers under Server 2008 R2) instead of “Legacy Key” and issue the certificate with your favorite signing authority. Then just apply it to your Client Access Servers and you should be good to go. Don’t forget to assign the IIS services to the new certificate using the Exchange Management Console/Shell.
 
Be careful though, not everything in your Exchange 2010 technology stack is compatible with a CNG-based certificate request. I know that because I ran into issues when I chose the default selection “CNG Key” for a certificate on Threat Management Gateway 2010 and found that it was not compatible.

So, having already been through troubleshooting that during the deployment of TMG, “Legacy Key” seemed like the safe (and logical) choice when generating CSR’s for use with our Client Access Servers.

Had I not done that though, I wouldn’t have had this problem, and I wouldn’t have written this article… which means you would still be looking for a solution.

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