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CERT Coordination Center

Denial of Service Attack in NetBIOS Services

Vulnerability Note VU#32650

Original Release Date: 2000-09-26 | Last Revised: 2000-11-29


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

For additional information on securing Windows systems, see the following CERT/CC Tech Tips:

RFC 1001, "Protocol Standard for a NetBIOS Service on a TCP/UDP Transport: Concepts and Methods":

RFC1002, "Protocol Standard for a NetBIOS Service on a TCP/UDP Transport: Detailed Specifications":

Vendor Information


Microsoft Affected

Updated:  November 03, 2000



Vendor Statement

We have not received a statement from the vendor.

Vendor Information

The vendor has not provided us with any further information regarding this vulnerability.


The CERT/CC has no additional comments at this time.

If you have feedback, comments, or additional information about this vulnerability, please send us email.

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

Other Information

CVE IDs: CVE-2000-0673
Severity Metric: 8.10
Date Public: 2000-07-27
Date First Published: 2000-09-26
Date Last Updated: 2000-11-29 16:44 UTC
Document Revision: 8

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