ICMP Redirect Attack Causes Windows NT Server and Workstation to Hang

Your computer running Windows NT may stop responding (hang) or the computer's performance may degrade drastically when connected to an internetwork segment or to the Internet. Other symptoms may include:

• The Perfmon tool's Processor Utilization object may indicate 100 percent utilization.
• Issuing a ROUTE PRINT command at a command prompt shows that the route table has changed.
• If the computer is disconnected from the network segment or the Internet and then restarted, the problem does not occur again until the computer is reconnected to the network segment or Internet.

This problem is caused by the receipt of multiple Internet Control Message Protocol (ICMP) Redirect packets that are used to change routing table entries. Too many ICMP Redirect packets in a short period of time may cause Windows NT to dedicate an inordinate amount of CPU time to update the routing table and cause the computer to stop responding. In many cases, this scenario indicates a denial of service attack through the use of a program to send multiple ICMP Redirect packets against a specific TCP/IP address.

To resolve this problem, obtain the latest service pack for Windows NT. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:
After you have installed Windows NT 4.0 Service Pack 5, you can enable a Windows NT registry entry that allows your computer to disregard ICMP Redirects. To disable ICMP Redirects:

WARNING: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.

1. Start Registry Editor (Regedt32.exe).
2. Go to the following key:
   HKEY_LOCAL_MACHINE\System\Currentcontrolset\Services \Tcpip\Parameters
   NOTE: The above registry key is one path; it has been wrapped for readability.
3. On the Edit menu, click Add Value, type EnableICMPRedirects, click REG_DWORD in the Data Type box, and then click OK.
4. Type 0, and then click OK. NOTE: Setting this registry entry to a value of 1 enables ICMP Redirects.
5. Quit Registry Editor, and then restart the computer.

Microsoft has confirmed that this is a problem in the Microsoft products that are listed at the beginning of this article. This problem was first corrected in Windows NT version 4.0 Service Pack 5.

For additional information about ICMP Redirect packets, please see the following article in the Microsoft Knowledge Base: