Users who access their mailboxes through an Exchange 2003 front-end server and Outlook Web Access might get connected to another user's mailbox if that other mailbox is (1) hosted on the same back-end mailbox server and (2) if that mailbox has been recently accessed by its owner. Attackers seeking to exploit this vulnerability could not predict which mailbox they might become connected to. The vulnerability causes random and unreliable access to mailboxes and is specifically limited to mailboxes that have recently been accessed through OWA.

By default, Kerberos authentication is used as the HTTP authentication method between Exchange Server 2003 front-end and back-end Exchange servers. This behavior manifests itself only in deployments where OWA is used in an Exchange front-end/back-end server configuration and Kerberos has been disabled as an authentication method for OWA communication between the front-end and back-end Exchange servers.

This vulnerability is exposed if the Web site that is running the Exchange Server 2003 programs on the Exchange back-end server has been configured not to negotiate Kerberos authentication, causing OWA to fall back to using NTLM authentication. The only known way that this vulnerability can be exposed is by a change in the default configuration of Internet Information Services 6.0 on the Exchange back-end server. This vulnerability cannot be exposed by a routine fallback to NTLM because of a problem with Kerberos authentication. This configuration change may occur when Microsoft Windows SharePoint Services (WSS) 2.0 is installed on a Windows Server 2003 server that also functions as an Exchange Server 2003 back-end.

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