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Q238600: Multiple Connection Requests Promote Denial of Service Attack

Article: Q238600
Product(s): Microsoft Windows NT
Version(s): winnt:4.0
Operating System(s): 
Keyword(s): kbnetwork
Last Modified: 08-MAY-2002

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The information in this article applies to:

- Microsoft Windows NT Server version 4.0, Terminal Server Edition 
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SYMPTOMS
========

When a request to open a new terminal connection is received by a Terminal
Server computer, the server undertakes a resource-intensive series of operations
to prepare for the connection. The server performs these operations before
authenticating the request, thereby allow an attacker to mount a denial of
service attack by levying a large number of connection requests and consuming
all memory on the Terminal server.

This vulnerability could be exploited remotely if connection requests are not
filtered. In extreme cases, the server could crash in the face of such an
attack; in other cases, normal processing would return when the attack ceased.
The patch works by causing the server to require authentication before
processing the connection request.

CAUSE
=====

This problem occurs because during the connection setup, there is no control
over CPU resource usage. Simultaneous multiple connection requests can prevent
the server from responding to other connection requests.

RESOLUTION
==========

A supported fix is now available from Microsoft, but it is only intended to
correct the problem described in this article and should be applied only to
systems experiencing this specific problem. This fix may receive additional
testing at a later time, to further ensure product quality. Therefore, if you
are not severely affected by this problem, Microsoft recommends that you wait
for the next Windows NT 4.0 service pack that contains this fix.

To resolve this problem immediately, contact Microsoft Product Support Services
to obtain the fix. For a complete list of Microsoft Product Support Services
phone numbers and information about support costs, please go to the following
address on the World Wide Web:

  http://support.microsoft.com/default.aspx?scid=fh;EN-US;CNTACTMS

NOTE: In special cases, charges that are normally incurred for support calls may
be canceled, if a Microsoft Support Professional determines that a specific
update will resolve your problem. Normal support costs will apply to additional
support questions and issues that do not qualify for the specific update in
question.

The English version of this fix should have the following file attributes or
later:

  Date       Time     Size      File name     Platform
  ----------------------------------------------------
  07/22/99   03:53p   134,928   Termsrv.exe   x86
  07/22/99   03:58p   242,448   Termsrv.exe   Alpha

This hotfix has been posted to the following Internet location as Tsememfxi.exe
(x86) and Tsememfxa.exe (Alpha):

  ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/NT40tse/hotfixes-postSP4/Flood-fix/



WORKAROUND
==========

To work around this problem, you can filter Transmission Control Protocol (TCP)
packets. Terminal Server monitors connection requests on port 3389. If you
create a filter that allows only specific TCP/IP addresses or networks to gain
access to the Terminal server, it may be possible to prevent this condition from
occurring.

For additional information about TCP filters, click the article numbers below to
view the articles in the Microsoft Knowledge Base:

  Q169548 Using Proxy Server with Routing and Remote Access

  Q166371 NT 4.0 Does Not Filter Ports Destined for Remote Segments

  Q187628 Using Telnet to Test Port 3389 Functionality

  Q191146 How to Create a DMZ Network with Proxy Server 2.0

STATUS
======

Microsoft has confirmed this to be a problem in Windows NT Server 4.0, Terminal
Server Edition.

MORE INFORMATION
================

For more information concerning Windows NT and security issues, please visit the
following Microsoft Web site:

  http://www.microsoft.com/security/

Additional query words:

======================================================================
Keywords          : kbnetwork 
Technology        : kbWinNTsearch kbWinNT400search kbWinNTSsearch kbWinNTS400search kbNTTermServ400 kbNTTermServSearch
Version           : winnt:4.0
Hardware          : ALPHA x86
Issue type        : kbbug
Solution Type     : kbfix

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