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TUCoPS :: Windows Net Apps :: ciacj001.txt

Windows NT Rpc Spoofing DoS Vulnerability




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             __________________________________________________________

                       The U.S. Department of Energy
                    Computer Incident Advisory Capability
                           ___  __ __    _     ___
                          /       |     /_\   /
                          \___  __|__  /   \  \___
             __________________________________________________________

                             INFORMATION BULLETIN

            Windows NT RPC Spoofing Denial of Service Vulnerability

October 2, 1998 21:00 GMT                                         Number J-001
______________________________________________________________________________
PROBLEM:       A vulnerability exist in the way that Windows NT processes bad
               Remote Procedure Call (RPC) datagrams.
PLATFORM:      Microsoft Windows NT Workstation version 4.0
               Microsoft Windows NT Server version 4.0
               Microsoft Windows NT Server version 4.0, Terminal Server 
               Edition
DAMAGE:        If exploited, an attacker could send an RPC datagram to a
               machine and spoof the return address so that the datagram
               appears to have come from another machine. This tricks the
               two servers into erroneously sending RPC error messages to
               each other continuously.
SOLUTION:      Apply available patches.
______________________________________________________________________________
VULNERABILITY  Risk is low. While there have not been any reports of customers
ASSESSMENT:    being adversely affected by these problems, Microsoft is
               releasing a patch to address any risks posed by this issue.
______________________________________________________________________________

[  Start Microsoft Advisory  ]


Microsoft Security Bulletin (MS98-014)

Update available for RPC Spoofing Denial of Service on Windows NT
Originally Posted: September 29, 1998
Last Revised: September 29, 1998

Summary
Microsoft has released a patch that enhances the way that Windows NT processes
bad Remote Procedure Call (RPC) datagrams. This patch prevents a malicious
attack in which an attacker sends an RPC datagram to a server and spoofs the
return address so that the datagram appears to have come from another server.
This tricks the two servers into erroneously sending RPC error messages to
each other. These error messages could cause a temporary loop resulting in
high CPU utilization and network bandwidth until the bad packets are
discarded.

Customers using affected software listed below should evaluate risks to their
systems posed by this issue, then download and apply this patch as appropriate
to their configurations.

Issue
It is possible for a malicious attacker to send spoofed RPC datagrams to UDP
destination port 135 so that it appears as if one RPC server sent bad data to
another RPC server. The second server returns a REJECT packet and the first
server (the spoofed server) replies with another REJECT packet creating a loop
that is not broken until a packet is dropped, which could take a few minutes.
If this spoofed UDP packet is sent to multiple computers, a loop could
possibly be created, consuming processor resources and network bandwidth.
While there have not been any reports of customers being adversely affected by
these problems, Microsoft is releasing a patch to address any risks posed by
this issue.

Affected Software Versions
The following software is affected by this vulnerability:
*  Microsoft Windows NT Workstation version 4.0
*  Microsoft Windows NT Server version 4.0
*  Microsoft Windows NT Server version 4.0, Terminal Server Edition
 
What Microsoft is Doing
On September 29th Microsoft released a patch that changes the behavior of the
RPC service so it can better detect spoofed packets and not respond to them,
preventing an attack of this type from causing any problem.
Microsoft has sent this security bulletin to customers subscribing to the
Microsoft Product Security Notification Service (see
http://www.microsoft.com/security/bulletin.htm for more information about this
free customer service).

Microsoft has published the following Knowledge Base (KB) articles on this
issue:

* Microsoft Knowledge Base (KB) article Q193233, RPCSS.EXE Consumes 100%
  CPU due to RPC-spoofing Attack
  http://support.microsoft.com/support/kb/articles/q193/2/33.asp
 
(Note: It might take 24 hours for the KB article to be visible in the Web-
based Knowledge Base. A copy of the article is also available on the FTP
server in the same directory as the fix.)

What customers should do
Microsoft recommends that customers using affected software listed above
should evaluate risks to their systems posed by this issue, then download and
apply this patch as appropriate to their configuration.

Fix for Windows NT 4.0 x86 version -
* ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/nt40/hotfixes-
postSP3/snk-fix/snk-fixi.exe

Fix for Windows NT 4.0 Alpha version -
* ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/nt40/hotfixes-
postSP3/snk-fix/snk-fixa.exe

Fix for Windows NT Server 4.0, Terminal Server Edition - This fix will be
available shortly. It will be available in the following location: 
*  ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/
NT40TSE/hotfixes-postSP3/Snk-fix/

Additional Details
The following are additional notes about this issue:

* It is relatively easy to detect a malicious attack of this kind by using
  a network analyzer to watch for bad RPC packets. Also, during an attack,
  the RPCSS.EXE service will consume a very large amount of CPU cycles
  (during an active, on-going attack, it will consume 100% of CPU cycles.)
* Systems that are currently in an error loop (sending error messages back
  and forth) will recover by themselves shortly after the attacker has
  stopped sending spoofed packets.
* If two systems are currently in an error loop (sending error messages
  back and forth) momentarily disconnecting one system from the network
  will cause an end to that loop, since UDP packets are connectionless.
* These error packets are addressed to UDP port 135 and can be filtered at
  a corporate firewall to protect against external attackers.


More Information
Please see the following references for more information related to this
issue.

* Microsoft Security Bulletin MS98-014, Update available for RPC Spoofing
  Denial of Service on Windows NT, (the Web posted version of this
  bulletin), http://www.microsoft.com/security/bulletins/ms98-014.htm
* Microsoft Knowledge Base (KB) article Q193233, RPCSS.EXE Consumes 100%
  CPU due to RPC-spoofing Attack
  http://support.microsoft.com/support/kb/articles/q193/2/33.asp


(Note: It might take 24 hours for the KB article to be visible in the Web-
based Knowledge Base. A copy of the article is also available on the FTP
server in the same directory as the fix.)

Acknowledgements
Microsoft wishes to acknowledge the contributions of Internet Security
Systems, Inc. (http://www.iss.net) for reporting this problem to us.

Revisions

September 29, 1998: Bulletin Created

For additional security-related information about Microsoft products, please
visit http://www.microsoft.com/security

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS"
WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER
EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS
FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS
SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT,
INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN
IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY
OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF
LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION
MAY NOT APPLY.


[  End Microsoft Advisory  ]
______________________________________________________________________________

CIAC wishes to acknowledge the contributions of Microsoft for the
information contained in this bulletin.
______________________________________________________________________________


CIAC, the Computer Incident Advisory Capability, is the computer
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