I have enabled Client Certificate Authentication for Exchange 2010 through IIS7 and the users are prompted to choose their User Certificate when they log in, but they are all then presented with the following error message

Request Url: https://<domain_name>:443/owa/ User host address: <server_ip_address> OWA version: 14.1.355.2

Exception Exception type: System.NullReferenceException Exception message: Object reference not set to an instance of an object.

Call stack

Microsoft.Exchange.Clients.Owa.Core.RequestDispatcher.GetUserIdentities(OwaContext owaContext, OwaIdentity& logonIdentity, OwaIdentity& mailboxIdentity, Boolean& isExplicitLogon, Boolean& isAlternateMailbox, ExchangePrincipal& logonExchangePrincipal) Microsoft.Exchange.Clients.Owa.Core.RequestDispatcher.InternalDispatchRequest(OwaContext owaContext) Microsoft.Exchange.Clients.Owa.Core.RequestDispatcher.DispatchRequest(OwaContext owaContext) Microsoft.Exchange.Clients.Owa.Core.OwaRequestEventInspector.OnPostAuthorizeRequest(Object sender, EventArgs e) System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

The method I followed to enable Certificate authentiaction was from this post:


Any ideas? Google isn't being very helpful


The fix was simple, you just need to search for <authentication mode="Form"/> in the web.config file and change it to Windows (capital W)

The authentication type specified in the

C:\Program Files\Microsoft\Exchange Server\V14\ClientAccess\owa\web.config file is incorrect. The correct authentication type is "Windows".

  • What is the OWA authentication mode set to in EMC? Exchange has a tendency to overwrite manually configured values... – Trondh Sep 4 '13 at 10:56
  • This is a little while ago amigo! I cant remember :) – Christian Sep 4 '13 at 20:05

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