Denial of Service Attack Against WinNT Simple TCP/IP Services

• Microsoft Windows NT Workstation version 4.0
• Microsoft Windows NT Server version 4.0

SYMPTOMS

As your computer is being attacked there may be a jump in bandwidth utilization on a subnet containing Windows NT computers and performance may suffer. A network analyzer shows a large amount of UDP traffic, typically from port 19 (chargen).

CAUSE

A malicious attack may be mounted against Windows NT computers with the Simple TCP/IP Services installed. The attack consists of a flood of UDP datagrams sent to the subnet broadcast address with the destination port set to 19 and a spoofed source IP address. The Windows NT computers running Simple TCP/IP services respond to each broadcast, creating a flood of UDP datagrams.

RESOLUTION

Windows NT TCP/IP, Windows Sockets, and Simple TCP/IP services have been modified to be more attack resistant. Windows Sockets now supports a new socket option, SO_BROADCAST, that can be set to allow the recvfrom() call to pass broadcast datagrams to the application. The default for this option is OFF. Previous implementations passed broadcasts datagrams to any Windows Sockets application that issued a recvfrom() call. Additionally, the chargen service and other Simple TCP/IP services have been modified to drop any datagrams that have the source port equal to the destination port to prevent "looping" attacks.

To resolve this problem, obtain the hotfix below, or wait for the next service pack.

This hotfix has been posted to the following Internet location:

   ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/nt40/
   hotfixes-postSP3/simptcp-fix

STATUS

Microsoft has confirmed this to be a problem in Windows NT version 4.0. A supported fix is now available, but has not been fully regression-tested and should be applied only to systems experiencing this specific problem. Unless you are severely impacted by this specific problem, Microsoft recommends that you wait for the next Service Pack that contains this fix. Contact Microsoft Technical Support for more information.