Vulnerability Note VU#32650

Denial of Service Attack in NetBIOS Services

Original Release date: 26 Sep 2000 | Last revised: 29 Nov 2000


The NetBIOS Name Service (NBNS) provides a means for hostname and address mapping on a NetBIOS-aware network. The NetBIOS over TCP/IP protocols (including NBNS) are described in the Internet Engineering Task Force (IETF) Request for Comments RFC1001 and RFC1002. These protocols do not specify a method for authenticating communications, and as such, machines running NetBIOS services are vulnerable to spoofing attacks.

NetBIOS is a set of defined software interfaces for vendor-independent PC networking and is primarily used on Microsoft Windows computers. NetBIOS is enabled by default on Windows95 and Windows98 machines.


An attacker sending spoofed "Name Release" or "Name Conflict" messages to a victim machine could force the victim to remove its own (legitimate) name from its name table and not respond to (or initiate) other NetBIOS requests. This renders the victim unable to communicate with other NetBIOS hosts, thus resulting in a denial-of-service attack.


An attacker can cause a victim's machine to refuse all NetBIOS network traffic, resulting in a denial of service.


Block NetBIOS services at the the network perimeter. NetBIOS services include

  • NetBIOS Name Service, 137/tcp and 137/udp
  • NetBIOS Datagram Service, 138/tcp and 138/udp
  • NetBIOS Session Service, 139/tcp and 139/udp
    Note that this prevents external hosts from sending NetBIOS Name Service traffic to internal machines, but it does not prevent local users from exploiting this vulnerability.

    Furthermore, the CERT/CC recommends that sites, even those which do not use NetBIOS services, block all ports unless they are explicitly needed.
  • For Windows NT and Windows 2000, apply the patches recommended in Microsoft Security Bulletin MS00-047:
    Note that no patch is being furnished for Win9x systems; Microsoft has publicly stated that patching these systems to disable name conflict resolution would cause more problems than it would help prevent, especially in networks with large numbers of Win9x systems.

Related Documentation

Systems Affected (Learn More)

VendorStatusDate NotifiedDate Updated
MicrosoftAffected-03 Nov 2000
If you are a vendor and your product is affected, let us know.

CVSS Metrics (Learn More)

Group Score Vector
Base N/A N/A
Temporal N/A N/A
Environmental N/A N/A



This document was written by Jeffrey P. Lanza and Chad Dougherty.

Other Information

  • CVE IDs: CVE-2000-0673
  • Date Public: 27 Jul 2000
  • Date First Published: 26 Sep 2000
  • Date Last Updated: 29 Nov 2000
  • Severity Metric: 8.10
  • Document Revision: 8


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