MS99-046: How to Prevent Predictable TCP/IP Initial Sequence Numbers

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.

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: NOTE: This fix was reposted on November 9, 1999 to resolve the following error message encountered while applying the hotfix: 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:If you are running Windows NT 4.0 Service Pack 5: For additional information about how to download Microsoft Support files, click the following article number to view the article in the Microsoft Knowledge Base: Microsoft scanned this file for viruses. Microsoft used the most current virus-detection software that was available on the date that the file was posted. The file is stored on security-enhanced servers that help to 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: The following files are available for download from the Microsoft Download Center:
For additional information about how to download Microsoft Support files, click the following article number to view the article in the Microsoft Knowledge Base:Microsoft scanned this file for viruses. Microsoft used the most current virus-detection software that was available on the date that the file was posted. The file is stored on secure servers that prevent any unauthorized changes to the file.

Windows NT 4.0

Microsoft has confirmed that this problem may cause a degree of security vulnerability in Windows NT 4.0.

Windows NT Server 4.0, Terminal Server Edition

Microsoft has confirmed that this problem may cause a degree of security vulnerability in Windows NT Server 4.0, Terminal Server Edition.

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: For additional security-related information about Microsoft products, please visit the following Microsoft Web site: