NetBIOS Name Server Protocol Spoofing Vulnerability

A security vulnerability exists in the NetBIOS Name Server (NBNS) protocol implemented in Microsoft® Windows® systems which could be used to cause a machine to refuse to respond to requests for service.

Issue

The NetBIOS Name Server (NBNS) protocol, part of the NetBIOS over TCP/IP (NBT) family of protocols, is implemented in Windows systems as the Windows Internet Name Service (WINS). By design, NBNS allows network peers to assist in managing name conflicts. Also by design, it is an unauthenticated protocol and therefore subject to spoofing. A malicious user could misuse the Name Conflict and Name Release mechanisms to cause another machine to conclude that its name was in conflict. Depending on the scenario, the machine would as a result either be unable to register a name on the network, or would relinquish a name it already had registered. The result in either case would be the same - the machine would not respond requests sent to the conflicted name anymore.

If normal security practices have been followed, and port 137 UDP has been blocked at the firewall, external attacks would not be possible. A patch is available that changes the behavior of Windows systems in order to give administrators additional flexibility in managing their networks. The patch allows administrators to configure a machine to only accept a name conflict datagram in direct response to a name registration attempt, and to configure machines to reject all name release datagrams. This will reduce but not eliminate the threat of spoofing. Customers needing additional protection may wish to consider using IPSec in Windows 2000 to authenticate all sessions on ports 137-139.

Affected Products

• Windows NT/2000

Solution

Software patches are available from the following locations:

• Windows 2000:
  http://www.microsoft.com/Downloads/Release.asp?ReleaseID=23370
• Windows NT 4.0 Workstation, Server, and Server, Enterprise Edition:
  http://www.microsoft.com/Downloads/Release.asp?ReleaseID=22138
• Windows NT 4.0 Server, Terminal Server Edition:
  http://www.microsoft.com/Downloads/Release.asp?ReleaseID=24516

Further Details

Source: Microsoft Corporation

Reference: Microsoft Corporation

Updated: July 27, 2000

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