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Title:      Deeply-nested OWA Request Can Consume Server CPU
            Availability
Date:       26 September 2001
Software:   Exchange 2000
Impact:     Denial of Service
Bulletin:   MS01-049

Microsoft encourages customers to review the Security Bulletin at:
http://www.microsoft.com/technet/security/bulletin/MS01-049.asp.
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Issue:
======
A security vulnerability exists in Exchange 2000 Outlook Web Access,
because it will accept and process a request for an item in an
authenticated user's mailbox without verifying first that the
folder structure is valid.  An attacker could mount a denial of
service attack by repeatedly levying a request for a non-existent
but deeply nested folder in his own mailbox.

Exploiting the vulnerability wouldn't necessarily affect the OWA
server itself.  The effect of the vulnerability would be to
cause the process servicing the attacker's mailbox to consume most
or all of the CPU availability on the server it was running on.
In may cases, this process would run on the OWA server, 
and thus the effects would be seen there.  However, if the process
servicing the attacker's mailbox ran on a back-end server, the
effect of exploiting the vulnerability would be seen there. 
In any event, the affected server would resume normal service
once the request was handled. 

Mitigating Factors:
====================
 - Only users who could authenticate to the server could exploit
   this vulnerability.
 - The attacker would need to have permissions on at least one 
   mailbox in order to exploit the vulnerability.  
 - The user can only perform this task against mailboxes to which
   they have permission.
 - The vulnerability could not be used to cause the mailbox store
   to fail, or to corrupt mailbox data. 

Patch Availability:
===================
 - A patch is available to fix this vulnerability. Please read the 
   Security Bulletin at
   http://www.microsoft.com/technet/security/bulletin/ms01-049.asp
   for information on obtaining this patch.

Acknowledgment:
===============
 - Joao Gouveia (tharbad@kaotik.org)

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