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TUCoPS :: Windows Net Apps :: ciack006.txt

MS TCP Randomness





             __________________________________________________________

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

                             INFORMATION BULLETIN

           Microsoft - Improve TCP Initial Sequence Number Randomness

October 26, 1999 17:00 GMT                                        Number K-006
______________________________________________________________________________
PROBLEM:       The algorithm which generates the initial sequence numbers 
               (ISN) of a TCP packet is not as strong as it should be. 
               Microsoft has discovered that "although not perfectly 
               predictable, certain sequence numbers were used more frequently 
               than others."  This has the effect of reducing the resistance
               to certain types of attacks, such as IP spoofing and session
               hijacking.
PLATFORM:      Microsoft Windows NT 4.0 Workstation and Server. 
DAMAGE:        If an ISN were to be predicted, it increases the risk for 
               certain types of attacks, such as IP address spoofing and 
               session hijacking. 
SOLUTION:      Apply the patch. 
______________________________________________________________________________
VULNERABILITY  Risk is medium. The patch should be loaded on any Windows NT 
ASSESSMENT:    Server which supplies information to the Internet or contains 
               important internal data. 
______________________________________________________________________________


[Start Microsoft Advisory]




Microsoft Security Bulletin (MS99-046)
--------------------------------------

Patch Available to Improve TCP Initial Sequence Number Randomness
Originally Posted: October 22, 1999

Summary
=======
Microsoft has released a patch that significantly improves the randomness of
the TCP initial  sequence numbers (ISNs) generated by the TCP/IP stack in
Microsoft(r) Windows NT(r) 4.0.  Improving the randomness of ISNs eliminates
a class of potential attacks against Windows NT 4.0  systems.

Frequently asked questions regarding this vulnerability can be found
at http://www.microsoft.com/security/bulletins/MS99-046faq.asp.

Issue
=====
The ISNs used in TCP/IP sessions should be as random as possible in order to
prevent attacks such  as IP address spoofing and session hijacking. This
patch improves the randomness of the Windows  NT 4.0 TCP/IP ISN generation,
providing 15 bits of entropy.

Affected Software Versions
==========================
- Microsoft Windows NT 4.0 Workstation
- Microsoft Windows NT 4.0 Server
- Microsoft Windows NT 4.0 Server, Enterprise Edition
- Microsoft Windows NT 4.0 Server, Terminal Server Edition

Patch Availability
==================
- x86:
   http://download.microsoft.com/download/winntsrv40/patch/
   4.0.1381.7014/nt4/en-us/q243835.exe
- Alpha:
   http://download.microsoft.com/download/winntsrv40/patch/
   4.0.1381.7014/alpha/en-us/q243835.exe

NOTE: Line breaks have been inserted into the above URLs for readability.

More Information
================
Please see the following references for more information related to this
issue.
- Microsoft Security Bulletin MS99-046: Frequently Asked Questions,
   http://www.microsoft.com/security/bulletins/MS99-046faq.asp.
- Microsoft Knowledge Base (KB) article Q243835,
   How to Prevent Predictable TCP/IP Initial Sequence Numbers,
   http://support.microsoft.com/support/kb/articles/q243/8/35.asp.
   (Note: It may take 24 hours from the original posting of this
   bulletin for this KB article to be visible.)
- CERT Advisory CA-95.01,
   Topic: IP Spoofing Attacks and Hijacked Terminal Connections,
   http://www.cert.org/advisories/
   CA-95.01.IP.spoofing.attacks.and.hijacked.terminal.connections.html.
   (Note: A line break has been inserted into the above URL for readability)
- Microsoft Security Advisor web site,
   http://www.microsoft.com/security/default.asp.

Obtaining Support on this Issue
===============================
This is a fully supported patch. Information on contacting Microsoft
Technical Support is available at
http://support.microsoft.com/support/contact/default.asp.

Acknowledgments
===============
Microsoft acknowledges National Bank of Kuwait for bringing this issue to
our attention.

Revisions
=========
- October 22, 1999: Bulletin Created.

------------------------------------------------------------------

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[End Microsoft Advisory]


______________________________________________________________________________

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


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