DOCUMENT:Q243835 09-MAY-2002 [winnt] TITLE :MS99-046: How to Prevent Predictable TCP/IP Initial Sequence Num PRODUCT :Microsoft Windows NT PROD/VER::4.0,4.5 OPER/SYS: KEYWORDS:kbnetwork kbSecurity kbWinNT400PreSP7Fix kbgraphxlinkcritical KbSECVulnerability ====================================================================== ------------------------------------------------------------------------------- The information in this article applies to: - Microsoft Windows NT Server version 4.0, Terminal Server Edition - Microsoft Windows NT Workstation version 4.0 - Microsoft Windows NT Server version 4.0 - Microsoft Windows NT Server, Enterprise Edition version 4.0 - Microsoft BackOffice Server versions 4.0, 4.5 - Microsoft BackOffice Small Business Server versions 4.0, 4.5 ------------------------------------------------------------------------------- SYMPTOMS ======== When you use Microsoft TCP/IP on a computer running Windows NT 4.0, you may be able to predict a session's initial sequence numbers (ISN). Someone can use this information to attack a computer by posing as a trusted user. RESOLUTION ========== Windows NT Server or Workstation 4.0 ------------------------------------ To resolve this problem, obtain the individual package referenced below or obtain the Windows NT 4.0 Security Rollup Package. For additional information on the SRP, click the article number below to view the article in the Microsoft Knowledge Base: Q299444 Post-Windows NT 4.0 Service Pack 6a Security Rollup Package (SRP) NOTE: This fix was reposted on November 9, 1999 to resolve the following error message encountered while applying the hotfix: Hotfix setup has detected that the service pack version of the system installed is newer than the update you are applying to it. The following files are available for download from the Microsoft Download Center. Click the file names below to download the appropriate file: If you are running Windows NT 4.0 Service Pack 6a: x86: Microsoft Download Center: DownloadDownload Q243835i.exe now (http://download.microsoft.com/download/winntsrv40/Patch/TCP-SP6/NT4/EN-US/Q243835i.exe) Alpha: Microsoft Download Center: DownloadDownload Q243835a.exe now (http://download.microsoft.com/download/winntsrv40/Patch/TCP-SP6/NT4/EN-US/Q243835a.exe) If you are running Windows NT 4.0 Service Pack 5: x86: Microsoft Download Center: DownloadDownload Q243835sp5i.exe now (http://download.microsoft.com/download/winntsrv40/Patch/TCP-SP5/NT4/EN-US/Q243835sp5i.exe) Alpha: Microsoft Download Center: DownloadDownload Q243835sp5a.exe now (http://download.microsoft.com/download/winntsrv40/Patch/TCP-SP5/NT4/EN-US/Q243835sp5a.exe) For additional information about how to download Microsoft Support files, click the article number below to view the article in the Microsoft Knowledge Base: Q119591 How to Obtain Microsoft Support Files from Online Services Microsoft used the most current virus detection software available on the date of posting to scan this file for viruses. After it is posted, the file is housed on secure servers that prevent any unauthorized changes to the file. Windows NT Server 4.0, Terminal Server Edition ---------------------------------------------- To resolve this problem, either obtain the hotfix referenced in this section or the Windows NT Server 4.0, Terminal Server Edition, Security Rollup Package (SRP). For additional information about the SRP, click the article number below to view the article in the Microsoft Knowledge Base: Q317636 Windows NT Server 4.0, Terminal Server Edition, Security Rollup Package The following files are available for download from the Microsoft Download Center: x86:DownloadDownload Q243835i.exe now (http://download.microsoft.com/download/winntterminal/Patch/Q243835/NT4ALPHA/EN-US/Q243835i.exe) Alpha:DownloadDownload Q243835a.exe now (http://download.microsoft.com/download/winntterminal/Patch/Q243835/NT4ALPHA/EN-US/Q243835a.exe) For additional information about how to download Microsoft Support files, click the article number below to view the article in the Microsoft Knowledge Base: Q119591 How to Obtain Microsoft Support Files from Online Services Microsoft used the most current virus detection software available on the date of posting to scan this file for viruses. After it is posted, the file is housed on secure servers that prevent any unauthorized changes to the file. STATUS ====== Windows NT 4.0 -------------- Microsoft has confirmed that this problem could result in some degree of security vulnerability in Windows NT 4.0. Windows NT Server 4.0, Terminal Server Edition ---------------------------------------------- Microsoft has confirmed that this problem could result in some degree of security vulnerability in Windows NT Server 4.0, Terminal Server Edition. MORE INFORMATION ================ In the TCP/IP protocol suite, data is exchanged in the form of packets. Because of the way TCP/IP performs routing, the path taken by packets can change dynamically throughout a session. This means that if Host A and B are communicating and Host A sends two packets, they could potentially take different paths to Host B, and could arrive out of order. Host B must therefore have a way to re-assemble the packets (containing the data in octets) in correct order. TCP/IP provides a way to do this using sequence numbers. Starting from an initial sequence number (ISN) that each host provides as part of the session setup, each octet is assigned a sequence number that is one larger than that of its predecessor. For example, if Host B provides 123 as the initial sequence number for its session with Host A, Host A's first octet is numbered 123, the second is numbered 124, the third is numbered 125, etc. If someone attempts to launch an attack against another computer and the attacker can predict how a host selects ISNs, it's possible to conduct two types of attacks, known as IP address spoofing and session hijacking. The more random the ISNs are, the more difficult it is to carry out these attacks. For related information on this problem, please visit the following Microsoft Web site: http://www.microsoft.com/technet/security/bulletin/MS99-046.asp For additional security-related information about Microsoft products, please visit the following Microsoft Web site: http://www.microsoft.com/technet/security/ Additional query words: security_patch kbtsesrp ====================================================================== Keywords : kbnetwork kbSecurity kbWinNT400PreSP7Fix kbgraphxlinkcritical KbSECVulnerability Technology : kbWinNTsearch kbWinNTWsearch kbWinNTW400 kbWinNTW400search kbWinNT400search kbWinNTSsearch kbWinNTSEntSearch kbWinNTSEnt400 kbWinNTS400search kbWinNTS400 kbNTTermServ400 kbNTTermServSearch kbAudDeveloper kbBackOfficeSearch kbSBServSearch kbBackOfficeServ400 kbBackOfficeServ450 kbSBServ400 kbSBServ450 Version : :4.0,4.5 Hardware : ALPHA x86 Issue type : kbbug Solution Type : kbfix ============================================================================= 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. Copyright Microsoft Corporation 2002.