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Title:      Malformed URL can cause Service Failure in IIS 5.0 and
            Exchange 2000
Date:       01 March 2001
Software:   IIS 5.0 and Exchange 2000
Impact:     Denial of Service
Bulletin:   MS01-014

Microsoft encourages customers to review the Security Bulletin at:
http://www.microsoft.com/technet/security/bulletin/MS01-014.asp.
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Issue:
======
IIS 5.0 contains a flaw affecting the way that an URL is handled if
it has a specific construction and its length is within a very narrow
range of values. If such an URL were repeatedly sent to an affected
system, a confluence of events could cause a memory allocation error
that would result in the failure of the IIS service.

Exchange 2000 is affected by the same vulnerability. To support
web-based mail clients, it introduces the ability to address items on
the store via URLs. This is done in part by using IIS 5.0, and in
part via code that is specific to Exchange 2000. Both pieces of code
contain the flaw, but the effect of exploiting the vulnerability via
either would be the same -- it could be used to cause the IIS service
to fail, but could not be used to attack the Exchange service itself.
That is, successfully attacking an Exchange server via this
vulnerability would disrupt web-based mail clients' use of the
server, but not that of MAPI-based mail clients like Outlook.

Because the flaw occurs in two different code modules, one of which
installs as part of IIS 5.0 and both of which install as part of
Exchange 2000, it is important for Exchange 2000 administrators to
install both the IIS and Exchange patches.

Mitigating Factors:
====================
 - The vulnerability would not enable the attacker to gain any
   administrative control over the server, or to alter any data
   on it.
 - The affected services automatically restart in the event of
   a failure, so an affected system would resume service almost
   immediately.
 - A successful attack against an Exchange server would only
   disrupt web-based mail clients' use of the server. The server
   would continue to be available for MAPI-based clients like
   Outlook.
 - The ISAPI involved in this vulnerability authenticates the
   user prior to servicing the request, so a properly configured
   Exchange server would be at less risk than an IIS server.

Patch Availability:
===================
 - A patch is available to fix this vulnerability. Please read the
   Security Bulletin
   http://www.microsoft.com/technet/security/bulletin/ms01-014.asp
   for information on obtaining this patch.

Acknowledgment:
===============
 - Kevin Kotas of eSecurityOnline.com (http://eSecurityOnline.com)

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